8 physical science sample lessons

8/19/2019 8 Physical Science Sample Lessons

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Grade 8Physical Science

Oak Meadow Coursebook

Oak Meadow, Inc.Post Office Box 1346

Brattleboro, Vermont 05302-1346oakmeadow.com

Item #b085010


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Contents Grade 8 Physical Science

Lesson 4: Energy ............................................................. 39 Types of Energy

Potential Energy and Kinetic Energy

Energy Can Change Form

Used Energy and Heat Energy

Lesson 5: Thermodynamics and Conservationof Energy ..................................................................... 49 The Laws of Thermodynamics

First law of Thermodynamics and Conservation of Energy

Pendulums and Conservation of Energy

Lesson 6: Force ............................................................... 59Different Kinds of Force

Force and Motion

Resultants

Lesson 7: Force of Gravity ............................................. 69Newton’s Law of Gravity

Mass, Weight, and Gravity

Units of Weight and Mass

Center of Gravity

Lesson 8: The Laws of Motion ...................................... 83Friction

Minimizing FrictionProjectiles

Lesson 9: More Motion ................................................. 93 Velocity

Acceleration

Newton’s Second Law of Motion

Newton’s Third Law of Motion

Lesson 10: Work and Power ....................................... 103 Work

Power Unit of Measurement for Power


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Grade 8 Physical Science Contents

Lesson 11: Machines .................................................... 113Inclined Planes and Mechanical Advantage

Ramps

Wedges

Levers The Laws of Thermodynamics and Machines

Friction, Gravity and Machines

Lesson 12: More Machines ......................................... 127 Wheels and Axles

Gears

Pulleys

Lesson 13: Waves as Moving Energy .......................... 137 Wave Parts, Period and Frequency

Wave Velocity Transverse and Longitudinal Waves

Wave Interference and Reflection

Waves You Can’t See

Lesson 14: Sound ......................................................... 149How Does Sound Travel?

Loudness

Pitch

The Speed of Sound

Lesson 15: More Sound ............................................... 161 Transmission, Absorption, and Reflection of Sound Waves

Acoustics Inside Buildings

Noise Pollution

Lesson 16: Light ............................................................ 173Light Waves

Illumination

Reflection

Lesson 17: Opaque Materials and Shadows ............. 181 Transparent Materials and RefractionLight Technology


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Lesson 18: Color ........................................................... 191 Visible Spectrum

Refraction and Dispersion of Light

Blue Skies and Red Sunsets

The Electromagnetic Spectrum

Lesson 19: Lenses ......................................................... 205Converging and Diverging Light Rays

How Eyes Work

Creating Images

Wearing Glasses

Perspective

Cameras

Lesson 20: Electricity .................................................... 217 What causes Electricity Laws of Electrical Change

Static Electricity

Current Electricity

Conductors and Insulators

Circuits and Switches

Lesson 21: Batteries ..................................................... 231 Wet Cells

Dry cells

Lesson 22: Electric Circuits and MeasuringElectricity .................................................................. 237Series Circuits

Parallel Circuits

Measuring Electricity

Lesson 23: Resistance and Ohm’s Law ...................... 247Ohm’s Law

Resistors and Circuits

Resistors and Series Circuits

Resistors and Parallel Circuits


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Grade 8 Physical Science Contents

Lesson 24: Home Electricity ........................................ 257Overload

Short Circuits

Electric Shocks

Electric meters

Lesson 25: Magnetism ................................................. 267Magnets and Poles

Magnetism and Atoms

Creating Magnets

Magnetic Fields

The Earth’s Magnetism

Lesson 26: Magnetism and Electricity ........................ 279Making a Magnet with Electricity

Making Electricity from MagnetismDirect Current and Alternating Current

Lesson 27: Matter ......................................................... 291Element Names and Symbols

Atomic Mass

Electron Shells

Lesson 28: Mixtures, Compounds and Molecules .... 305Molecules

Oxidation

Photosynthesis

Lesson 29: Solutions .................................................... 313 Types of Solutions

Solubility and Concentration

Lesson 30: Heat, Temperature and Pressure ............. 323Heat On the Move

Thermal Expansion and Contraction

Temperature

Pressure


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Lesson 31: Aerodynamics and Flight .......................... 341Aerodynamic Forces

Bernoulli’s Principle and Lift

Airplanes

Supersonic Aircraft

Lesson 32: Modern Machines ..................................... 353Microwaves

Television

Satellites

Compact Discs and Long-Playing Records

Lesson 33: Cars ............................................................ 367Hydraulics

Hybrid Cars

Lesson 34: Energy Use in Our World ......................... 375Fossil Fuels

Hydroelectric Power

Electricity from Steam

Geothermal Energy

Wind Power

Biomass

Solar Energy

Nuclear Energy

Lesson 35: Energy Problems ....................................... 395Energy and Food

Chemicals

Pollutants

Air Pollution

The Greenhouse Effect

Acid Rain

Organic Compounds and Ozone Depletion

Thermal Pollution

Lesson 36: Final Review ............................................... 415


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Scientific Method

Grade 8

3Science describes what we know about our world. We learn about the

world by observing what is happening all around us. We observe through

all our senses: we watch, we listen, we feel, we smell, we taste, and we use

our intuition. Then we reach conclusions about what it all means. This is

how we make sense out of the world.

Observing and exploring nature and the workings of our earth is largelya matter of being receptive to what lies all around us. This does not take

special training; look at any small child and you’ll see that he/she observes

many things that many of us don’t notice. As we said in lesson 1, our

species has survived because we pay attention to novel events. Careful

observation is the basis of scientific inquiry.

In this lesson, you will learn about the classic “scientific method”. This

is an organized way of testing observed phenomena, useful in science

courses and in cer tain research applications. However, it is not the only

way that scientific progress is made! Scientists observe the world likechildren do: exploring every corner, every new thing. It is observation and

questioning that is scientific inquiry, and this can come about in many

different ways. Sometimes you cannot create experiments around the

observed phenomena. If a shower of meteors falls to the earth, how can

you devise an experiment to test that they are meteors? You can’t recreate

it, but you can observe carefully, and it can open your eyes to new

possibilities and new things to observe. This is the way science works.

We need to be constantly aware. In fact, most scientific discoveries happen

completely by surprise. However, even the surprises aren’t surprising,

because scientists expect them!


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ScientificMethod

(continued)

Lesson 3 Grade 8 Physical Science

We are all scientists. We ask questions, we guess what the answer will be,

we watch to see what happens. Our minds record the results and then we

decide what the results mean. We take this knowledge and use it throughout

our lives as we decide what to do and how to do it. In the scientific method,

observations are made about the world, and then experiments areconducted to explain the observation. How the experiment is designed and

then conducted is important, because only then can we get an accurate

explanation for the observation. If the experiment is not controlled, then

it will not give us a reliable explanation. We will next look at the different

things which can make an experiment controlled or uncontrolled, and

therefore more or less reliable.

Variable and Constant Factors

When we make observations about the

world, it is important to understand

what possible variable factors there may be

in what we are observing. Factors is a term

that describes all the possible parts of an

observation or experiment. A variable factor

is something that can be varied or changed.

Factors that do not change are called con-

stant (or determinate) factors.

Let’s say that using the observation above

about the ice cream, you decide to figureout why the ice cream is soft sometimes and

really hard other times. You have thought

about it and come up with four of the

variable factors listed above: temperature,

placement, type of ice cream, and length of

time in freezer. To determine which variable is causing the ice cream

to be hard or soft at different times, you decide to conduct a series of

experiments to explain your observation.


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ScientificMethod

(continued)

Grade 8 Physical Science Lesson 3

Examples of Variable Factors

Let’s suppose that we have made the observation that

sometimes the ice cream in the freezer is really hard andsometimes it is a little soft. What are some of the variable

factors that could explain this?

• The temperature of the freezer • The placement of the ice cream in the freezer • The type of ice cream• How long the ice cream has been in the freezer • How many times the door has been opened

• How much ice cream is lef the container

Ice Cream Experiment #1

Let’s say you first decide that you think the most important factor is the

placement in the freezer. In order to test this, you put some ice cream in

a certain spot in the freezer and then after a while you go and test it for

hardness. It seems pretty hard. The next day, when you go to test the ice

cream again, you realize that someone ate it all, and there is another kind

of ice cream right in the same spot. Since it is in the same place in the

freezer, you do another hardness test. It is pretty soft. Uh-oh!

When you think about why the ice cream was soft the second time, you

come up with several possible reasons:

a. The ice cream was a different kind, so that might be why it was

soft the second time

b. Maybe the ice cream had not been in the freezer for very long.

Maybe it was just put in there after sitting in the car on the

way home from the grocery store.

c. Maybe the temperature changed in the freezer.

Your hardness tests of the ice cream didn’t really prove anything because

you still don’t really know why the ice cream is soft sometimes and hard


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ScientificMethod

(continued)


at other times. After you did the experiment, you don’t know if it has to do

with placement in the freezer any more than you did before. The problem

was that there were too many variable factors in your original experiment.

This is an example of an uncontrolled experiment - there was not enough

control over the variables to find an explanation for the observation. If youreally want to find out what causes the ice cream to be harder or softer at

different times, you will need to limit the variables.

This brings us to an important rule about experiments: only one variable

factor allowed in each experiment! The only way you can figure out why

something is happening is to limit the variable factors to one. Each

experiment should only have one variable factor.

Limiting Variables

Scientists often simplify the world in order to study just one

or two things. This is known as limiting the variables. Think

of it like this: if you had an allergic reaction to something

that you ate one day, you would probably not be able, at

first, to figure out which food it was that gave you the allergic

reaction. Each food is a variable. The way to figure it out

would be to make a list of all the foods that you ate on that

day (a list of all the variables), and then each day eat only one

of them at a time. In this way you isolate each food (eachvariable) until you figure out which one is making you react.

This is the process of limiting the variables.

How would this work with the ice cream hardness question? Let’s redesign

the experiment to make all the factors constant except one; the variable

factor will be the placement of the ice cream in the freezer.

Variables and Constants

• Only one variable factor in each experiment.• All other factors should be constant.


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ScientificMethod

(continued)


Ice Cream Experiment #2

This time you need to make sure that all other factors stay constant (stay

the same) throughout the experiment. Now you will have to decide what

constants you will need to have in your experiment to make sure only one

factor is variable. Let’s say you come up with these constant factors:

a. The temperature of the freezer. You discuss with your family

that no one is to touch the freezer control for a couple of days

while you conduct the experiment.

b. The type and amount of ice cream. You buy three containers of

the same ice cream, all in the same size container , and you ask

that no one in your family eat any of it, or move it, for the next

several days. (Why three containers? Keep reading!)

c. How long the ice cream has been in the freezer. You place eachof the three ice cream containers in the freezer at the same time

and you make a note of the time you put them into the freezer.

You decide that the variable factor you will test is the location of the ice

cream in the freezer. You are going to vary this factor by placing three

identical containers of the same type of ice cream in three different places

in your freezer. You then conduct the experiment by checking the hardness

in each of the three containers on a set schedule - every six hours, for

example — and you write your results down each time. As you do the

experiment, you are careful not to change the location of any of the threecontainers.

Now let’s look at your results. If the results were that the ice cream in one

of the containers was soft and the ice cream in the other two containers

was hard, then the placement of the ice cream in the freezer affects the

hardness of the ice cream! If the results were that the ice cream in all of

the containers was equally hard or soft in all locations, then the placement

of the ice cream in the freezer is not the variable that affects the hardness

and softness of the ice cream. You will have to design another experiment

that has a different factor as a variable, and where the placement of the

ice cream in the freezer is a constant.

As we have seen, the experiment must be controlled so as not to have too

many variable factors.


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ScientificMethod

(continued)


1. Take some time to make an observation around your home.

Perhaps you notice that your cat naps in different places at different

times of day. Or maybe you see that the temperature on one side of

your house generally feels colder than on the other. Then make a list

of variable factors that you might consider if you were to design anexperiment. After each variable you list, explain how you might

control that variable to make it a constant in your experiment.

Controlled Versus Uncontrolled Environments

The environment in which an experiment is conducted has an effect on

the outcome of a scientific experiment. It is important to control the

environment (the variables), or you will not get an accurate explanation

for your observation or question.

A controlled environment is an environment where there is only one, orat most, a few variable factors. Most scientists, when they are working

to explain an observation they have made, strive to design and conduct

experiments in a controlled environment and to limit the variable factors

to as few as possible. An example of a controlled environment is a science

laboratory where the scientist can control the temperature, the humidity,

and the materials that are used.

An uncontrolled environment is an environment where there are many

variable factors. Some kinds of observations cannot be reduced to

experiments that can be conducted in a laboratory with controlledvariable factors. For example, when dealing with a global environmental

issue such as the effect of ozone depletion (which we will discuss more

thoroughly in Lesson 35), it is impossible to create a controlled environment

to examine this problem. Since ozone depletion occurs on a very large

scale - the Earth - it cannot be made to fit into a laboratory. The best that

can be done is to study certain pieces of it.

Sometimes it is impossible to isolate variables. Other times the variables

work together, and isolating them doesn’t give you an accurate assessment.

This has been the case when studying the human body. Scientists have

isolated different organs and studied them individually and made conclusions,

only to find later that each organ is quite connected to the whole body/

mind system. They interact with the system in many complex ways, and

controlled systematic study of each separately isn’t quite so simple!


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ScientificMethod

(continued)


The Scientific Method

The scientific method is one procedure that all scientists use in trying to

understand the earth and all that occurs. You will use this method throughout

the rest of this course and many times throughout your life. The scientific

method is a series of steps that ask, “Why does something happen?” “Can

I figure out why it happened?” “Did I figure out what happened?” and “What

did I learn from this?” The steps of the scientific method are as follows:

1. Observation: Identify a problem or a question. This is called the

observation.

2. Hypothesis: Make a guess about the answer to the question, based

on what you know already. This is called the hypothesis, otherwise

known as an “educated guess.”

3. Experiment: Figure out an experiment to test your hypothesis. Try tocontrol the experiment or procedure in order to have as few variable

factors as possible. Describe your experiment and the specific steps.

Do the experiment. List all the variables that you can figure out.

4. Results: Describe what happened when you did your experiment.

What happened is called the results. Sometimes results can be

presented in a chart or graph form.

5. Conclusion: Review your original question (Step 1) and your hypothesis

(Step 2).

Compare your hypothesis with what actually happened (Step 4).

• Did what you think would happen actually happen?• Did something unexpected happen?• Describe the variables and which ones may have impacted your

results.

• Consider possible explanations for what happened in yourexperiment.

• Try to come up with an explanation for your results. This iscalled the conclusion. The ultimate goal of experimenting is tofind scientific truths, or principles, that are true in any situation.

This is called a theory, and theories are formed after much

experimentation with consistent results.


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ScientificMethod

(continued)


Remember: Control the Variables

It is often difficult to create a completely controlled experiment, but

scientists do their best to control as many factors as possible. It is always

important to remember that your observations and/or experiment may

have variable factors that are affecting your results, so in your role as a

scientist you need to control as many factors as you can.

You have now been introduced to the scientific method. Learn the steps

well, as you will use the scientific method in almost every lesson in this

course, and in all your future science courses.

Before we go on, however, let’s review some related concepts. In lesson 1,

you learned that scientific observations must be measurable, repeatable,

and that we strive for objective analysis. Remember to apply these principles

when using the scientific method. Whenever you use the scientific method

for a controlled experiment, it should be written clearly such that somebody

else can read your experiment and repeat exactly what you did. You need

to document your method precisely! This allows other scientists to verify

your results, and it is how scientific theories are proven.

Scientific Experiments Should Be Repeatable By OtherScientists

A repeatable experiment doesn’t mean that the same results will be obtained

again! If the experiment is repeatable, it means you’ve documented your

method very well, and others can try it. If the results are repeatable, thenwe have learned a new scientific truth, and a theory can be formed!

Keep in mind that there is a difference between controlled and uncontrolled

experiments. If your experiment is not controlled, it will not have the feature

of objective analysis, nor will it be repeatable. It is very important to always

consider what variables there are in your experiment. Try to limit the number

of variables so you can figure out what you are actually measuring.

Memorize the Scientific MethodIt is very important to learn the scientific method, as you will

use it in each lesson of this course. Whenever you are asked

for observations, conclusions, or a hypothesis, refer to the

format presented here when you prepare your answer.


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(continued)


Using the Scientific Method

Let’s look at an example of the scientific method in action. Pretend that

you are washing the dishes in the sink one day, and you notice something

about them. This is how the scientific method would be used to make a

conclusion about your observation:

Scientific Method Experiment #1:

1. Observation: You have noticed that some objects sink when put in

water, and that others float. You decide to test several items to see

if you can figure why certain things sink and others don’t.

2. Hypothesis: There are several variables that you need to identify, so

that you can test one of them at a time. Some variables that might

affect whether an object sinks or floats are shape, size, weight, and

density. You decide to test density (which is mass per unit volume). You need to state your hypothesis quite specifically: “Objects that are

the same shape and size, but different densities, will act differently

in water. Objects that are less dense will float, and the more dense

objects will sink. Wood will float and clay will sink.”

3. Experiment: Now you need to clearly document your method,

identifying how you will control each variable: “I will take a small

block of wood and a lump of clay. I will form the clay to be the exact

shape and size as the block of wood. I will put each of them in a sink

with water in it and observe whether they sink or float. Both areexposed to the exact same conditions in the room and the water.

The only difference is the material they are made of.”

4. Results: Write your results in detail: “The block of wood floated and

the clay block sank.”

5. Conclusion: First review your original observation (that some objects

sink and others float), and your hypothesis. Your results indicate that

what you predicted did actually happen. But what is your conclusion?

Basically, all you can conclude from this is that wood floats and clay sinks.

You would like to make the theory that objects that are less dense will

float and those that are more dense will sink. As you think about it more,

though, you wonder whether this is always true. “Less dense” and “more

dense” are vague terms. Less dense than what? What about ships that sail


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ScientificMethod

(continued)


on the ocean? They are metal and quite dense, but they don’t sink. Will clay

always sink, no matter what shape it’s in? There are many more questions

raised by this experiment than answers obtained! This is the way science works!

Your experiment is an important start. Information was learned, and now

further testing can be done. You see that you need to clarify your hypothesiseven more, perhaps adding that those objects that are more dense than

water will sink, and those less dense than water will float. But that still

raises the question about the ships that float. Uh-oh, maybe there is more

than one variable that determines whether an object will float! There could

be variables that you haven’t thought of yet.

It’s important to remain inquisitive and keep questioning. You need to ask

yourself if your conclusion is always true. Consider all the variables you’ve

come up with, any experience you might have with any of them, and raise a

new question to test. You conclude that further experimentation is needed.

2. Now it’s your turn:

Scientific Method Experiment #2

You are to design an experiment that

tests whether the shape of an object has

an effect on whether it sinks or floats.

We’ll get you started with the observation:

Observation: Light things float and heavy things sink. But some heavy

things, such as ships, also float. Why is that?

Now you design the rest of the experiment to test this. You can use clay

as your heavy object since it is easy to change the shape of. Write your

hypothesis and how you will conduct the experiment. Clearly state the

variables involved and how you will keep all but the shape constant. Do

the experiment, write your results, and form a conclusion based on your

hypothesis and results. Finally, write what other questions might come up,

and ideas you have on further testing the variables that affect whether an

object floats or sinks. Is there more than one variable involved, and might

they work together?

Ask yourself:

• Is this always true? • Consider the variables


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ScientificMethod

(continued)


Introduction to Mass and Matter

When you look around you, almost everything

you see is matter. What is matter? It is the stuff

all around you! Matter is anything with mass that

takes up space. For example, when you pick up a

pencil, you can feel that it is made up of matter.

This matter takes up space. You are matter, your

desk is matter, your dog is matter, even the air

around you is matter. All of this matter has mass,

and the mass is what we measure.

So what is mass? In lesson 2, you were introduced to the concept of mass

and how it is similar, but different, from weight. When scientists measure

the amount of something, they use the term mass instead of weight. Mass

is the actual quantity of matter an object contains, whereas weight is themeasure of heaviness. Weight has to do with gravity, while mass doesn’t.

You have the same amount of mass whether you are on earth, in outer

space, or on the moon. But your weight in each of those places is quite

different, because of the difference in gravity. Here on earth, weight and

mass will be the same, which is why we can convert kilograms to lbs. That

conversion factor (2.2lb/1kg) wouldn’t be the same on the moon! You will

learn more about the difference between mass and weight later. Just know

that when something feels heavy to you, you are feeling its weight, which

is a combination of the amount of mass in the object and the ef fect ofgravity on that mass.

3. Are you matter? Remember, matter has mass and takes up space.

Design an experiment to prove that you are matter. The experiment

must demonstrate that you have mass and that you take up space.

It must also be measurable; your experiment should provide

measurements of your mass and space. You do not have to carry

out this experiment; just explain it clearly.

Matter, Molecules and Atoms

4. Write down three examples of matter in liquid phase and three ex-

amples of matter in solid phase. Write down two examples of matter

in the gas phase.

The Scientific Method

1. Observation

2. Hypothesis

3. Experiment

4. Results

5. Conclusion


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ScientificMethod

(continued)


Now that we have reviewed matter, and seen its different phases, let’s

look more closely at the different substances that make up matter. Most

substances are made up of several other substances. Water is made up of

substances called hydrogen and oxygen. Salt is made up of sodium and

chlorine. Water can be broken down into hydrogen and oxygen; salt canbe broken down into chlorine and sodium. Hydrogen, oxygen, chlorine

and sodium are called elements. Elements are the building blocks of all

matter, and cannot be broken down further.

Various elements join together

in different combinations to

make all matter.

But what makes each element

different from one another?

As we have seen, elements are the basic buildings blocks of all matter. Buteach element is made of a different kind of atom. For example, the element

“hydrogen” is made of a hydrogen atom, and the element “oxygen” is

made of oxygen atoms. Likewise, the elements “sodium” and “chlorine”

are made of a sodium atom and a chlorine atom, respectively. Atoms are

very small. Millions and millions and millions of atoms could fit on the

head of a pin.

ELEMENTS

• Cannot be broken down• Building blocks of all matter


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(continued)


Atoms of different types join together to make

matter. Hydrogen and oxygen join together

to make water. In the case of water, two

hydrogen atoms join with one oxygen atom.

When they join together, they make a moleculeof water. A molecule is made of several atoms

of different types joined together to make

to make something like water, salt, skin, hair

and air. Billions of molecules of water make

up a cup of water that you drink.

When scientists draw a picture of how the

different atoms join together to form a molecule, they use illustrations like

the ones here. As you can see, symbols are used to indicate the different

kind of atom, such as the symbol “H” to indicate a hydrogen atom, and thesymbol “O” to indicate an oxygen atom. The next image shows how the

atoms making up the molecule for salt are symbolized.

As you have seen, all matter is made of molecules, and how the molecules

are formed will create different types of matter. Molecules are made up of

atoms of different elements. But what are atoms made of? Atoms are made

up of atomic particles. There are several kinds

of particles which make an atom. These

particles are called protons, electrons, and

neutrons. The protons and neutrons make

up the core, or inner part, of the atom. The

core of an atom is called its nucleus. Electrons

move around the outside of the atom.

Electrons can move from atom to atom.

In Lessons 20-27 you will learn more about

atomic particles and how moving electrons

create electricity.

5. Define the following terms:

Matter Solid

Liquid

Gas

Element

Water molecule

Salt molecule


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Notes


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Energy

Grade 8

4Go outside and look around you. You can see and hear activity all around

you. The wind rustles the leaves on the trees. You feel the heat of the sun.

You see cars and bicycles move by. People are moving, talking, working

and playing. All of these things require energy for them to happen. We

use energy to keep ourselves alive. We use it to work and to make our

work easier. Energy runs all living things; energy runs us and our machines. We live in a sea of energy; energy is all around us.

Energy is the capacity for movement and change. It produces changes in

matter. You get energy from the sun and from the food you eat that stores

the sun’s energy. In fact, most of the energy on earth comes from this one

source — the sun. Your body uses energy every time it does anything. Energy

is needed to make anything move, even the smallest cell. And whenever

anything moves, energy is used.

Most of this course is about energy

and the different forms that it takes. This lesson is an overview of the types

of energy that we will be studying in

more detail throughout the course.

Types of Energy

There are many different types of energy. All of them concern some type

of motion. Everything has at least one type of energy and many things

have several different types of energy. We will discuss some of the most

common ones in turn.

Thermal or heat energy is the energy in moving molecules. All things

contain some heat energy. Rub your hand on your arm and it will become

warm. Adding heat energy to anything makes its molecules move faster.

When you boil water, the water molecules move faster; they move so fast

Energy • Runs all living things• Is everywhere


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Energy

(continued)


that some molecules begin to leave the container as the water boils away

and evaporates. Heat can turn a solid into a liquid and a liquid into a gas.

With each of these transitions, the molecules are able to move about more

and more freely.

Light energy comes from the sun to Earth in the form of light waves. We

cannot see these waves, but they are very much like ocean waves. (We will

learn more about light waves in Lessons 16 and 17.) Light waves travel in a

straight direction which is described as a ray of light. Anything that gives

off light has light energy. Plants grow by using light energy. Photography is

an excellent example of the ability of light to cause change. Light can form

an image on photographic film by changing the state of the silver coatingon the film.

Electrons are one of the types of atomic particles we looked at in Lesson

3. Electrical energy is the energy that is in moving electrons. Light bulbs,

radios, and appliances use this type of energy. Electrical energy can turn

a motor, and it can transfer your ideas onto a magnetized tape in a tape

recorder or onto a magnetized disc in a computer. It can send your voice

thousands of miles through a telephone system. You will learn more about

electrical energy in future lessons.

Chemical energy is energy that is stored in chemicals. It is released in

chemical reactions or whenever two or more chemicals interact. Chemical

energy heats your home when you burn coal, wood, gas or oil. It is in bat-

teries and changes to electrical energy when the battery is used. Chemical

energy is what your body runs on when you digest food.


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Energy

(continued)


Some Types of Energy

Heat or Thermal Mechanical or Motion

Light Gravitational Electrical

Sound Chemical Atomic

Mechanical energy is often defined as “the ability to do work.” It is the

energy that is in moving things, or things that have the potential to move

if they weren’t being held back by something. Wind, falling rocks, and

moving water all have mechanical energy. So do all machines that move,

and so do you, when you are running across a field or swimming in a pool.

The rock that is about to fall (but isn’t yet) also has mechanical energy,

just as you do when you are standing on a diving board. You will learn

more about mechanical energy—and the related ideas of work, power,

and force—in Lessons 6 through 10

Gravitational energy is a type of mechanical energy. Gravity is the force

of attraction between two objects and it exists between any two objects

in the universe. The Earth’s huge size makes it easy for you to feel its

gravitational energy but there is also a gravitational force between you

and everything around you—it’s just too small for you to feel it. You will

learn more about gravity and gravitational energy in Lesson 7.

Sound energy is energy caused by vibrating objects. The object vibrating

causes the air to vibrate, and the sound wave travels through the air to

our ears. Have you ever felt the house shake from a really loud thundercrack? That is sound waves causing

the house to vibrate. This is sound

energy. So is the music you hear from

your CD, and the sound of a kettle of

hot water whistling on the stove. We

will discuss more about sound energy

and sound waves in Lessons 14 and 15.

Atomic or nuclear energy is the energy

that is stored in the nucleus (nucleusis another word for center or core)

of an atom. The sun produces light

and heat from atomic energy. The

destructive power of atomic energy


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Energy

(continued)


is easy to see in pictures of Hiroshima and Nagasaki after the explosion of

the atomic bombs in World War II. We will learn more about atomic energy

in Lessons 34 and 35.

1. Sit quietly alone in a room for a while. Listen and watch carefully.

After a time, you will start to hear and see signs of energy around you— perhaps a family member walks by in the hall, or a bicyclist goes by

on the street. Perhaps you can feel air coming through a vent, or see a

curtain moving in the breeze. Write down the signs of energy you find,

and describe which type of energy it is.

2. For each of the eight types of energy just discussed, write down

an example of how or where this energy type occurs. Describe how

your example shows that type of energy.

3. For the following story, list the types of energy present. Your answers

should include at least one of each of the eight energy types you have

learned about:

Pat and her friend Kevin rode to the park on their bicycles ( a ).

The sun was shining brightly ( b ) and by the time they got there,

they were hot and tired ( c ). They were also hungry, so they

pulled out two sandwiches and ate them ( d ); soon they felt

much better. They sat on the swings for a while, swinging back and

forth ( e ) and talking ( f ). After awhile, they decided to

listen to some music on their portable radio ( g ) but soon

realized that their batteries were low ( h ), so they rode homeand listened to the stereo ( i ) at Pat’s house.

Potential Energy and Kinetic Energy

All types of energy can be divided into two states: kinetic energy and

potential energy. The word kinetic means moving. Kinetic energy is energy

of motion. When you are bouncing a ball, it has kinetic energy because

it is moving. When we refer to kinetic energy, we are usually referring to

energies where visible movement occurs. But even sound and electrical

energy involve movement (of atoms and molecules, and electrons,respectively), so they could technically fall under this category.

What about when you aren’t bouncing the ball and it just sits on a shelf

in your room? Then the ball has potential energy—stored up energy that

is waiting to be released. If the ball got a chance, it would roll off the shelf


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Energy

(continued)


and fall to the floor. We can say that the ball has gravitational potential

energy (which is actually a type of mechanical energy that you just read

about). Potential energy is stored energy. Energy does not have to show

itself in order to exist; it exists even when you can’t see it.

Potential energy often exists as a result of the position of an object.Gravitational potential energy is everywhere. Birds have it when they are

in the air or in trees. Trees have it; if disturbed they fall down. You have

it as you hold yourself up. When you are tired, what do you do? You lie

down to lower your potential energy!

Potential energy is energy that is just waiting to happen!

Though gravitational potential energy is quite common, you can see

potential energy in other places. There is electrical potential energy; we

call it voltage. It is stored electrical energy. When it is released it’s not anobject that moves, but an electrical charge (electrons). Electrical potential

energy is also found in batteries. This energy is released when you turn on

the portable CD player or the flashlight. There is also chemical potential

energy stored in molecules. This can be released by a chemical reaction.

When you open a jack-in-the-box, the

potential energy of the coiled spring

inside is released. If you stretch a rubber

band, it has potential energy until it is

released and snaps back to its normalposition. This is called elastic potential

energy, which is a form of mechanical

energy.

Another example of potential energy is a

pile driver. (A “pile” is another word for

a big post, such as the piles which hold

up a pier.) A pile driver is a machine

with a huge weight on the end of a

heavy metal cord. The huge weight israised up high and then dropped. As

it falls, it goes faster and faster and by

the time it hits its target, it has enough

energy to drive a huge post into the


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Energy

(continued)


ground. It acquired that

energy because as it was raised

against gravity, the amount of

gravitational potential energy

increased. The potential energywas then changed into kinetic

energy (energy of motion) when

it was released.

Any system wants to get to the

lowest potential energy possible.

That’s why things fall down! If

you leave the lights of your car

on, the electrical energy will

flow from the battery, eventuallygetting to the point where the

battery has no electrical

potential energy! In other

words, you have a dead battery.

If you wind up a spring loaded

toy, it will release and unwind at

the first possible chance. When you think about whether something has

potential energy, think about whether that thing will move on its own if

whatever is holding it in place is removed. A kitchen appliance that plugs

into a wall doesn’t have potential energy of its own, as it needs to haveenergy added to it to run. (OK, it does have some gravitational potential

energy because it’s sitting on the shelf!). A ball sitting on the ground

doesn’t have potential energy unless it’s at the top of a hill.

4. Think of and write down two different examples of potential energy

(energy waiting to happen). The examples should be different from

the ones listed in this lesson describing potential energy. Then describe

what can happen to create kinetic energy in each of your examples.

5. Take a rubber band, and stretch it between the thumb and index

finger of one hand. Hold it there for as long as you can. At what state

is the energy in the rubber band? As your hand gets tired, what state of

energy is it fighting against? Release the rubber band from your finger.

At what state is the energy in the rubber band as it is released?


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Energy

(continued)


6. Choose one of the following:

a. Think of a person standing still at the end of a springboard,

getting ready to make a dive into a pool. When the diver is ready,

he will make several jumps on the springboard, and then dive into

the water. Describe each step of his dive, identifying the pointswhere the amount of potential and kinetic energy change.

b. Think of a rollercoaster standing still as passengers board. Then

it starts up, climbing to a high point before beginning its first

descent. As it goes along the track it goes up and down, around

turns, and perhaps around loops several times. Describe an

imaginary roller coaster ride, identifying the points where the

amount of potential and kinetic energy change.

Energy Can Change Form Think about what happens when you strike a match. You are holding a match

in one hand and the match box with a striker on it in the other. You, the

match, the box and all their components have potential energy. You strike the

match on the box, changing your potential energy into mechanical energy

as you move. The chemical in the match head sparks (chemical energy) and

the match head explodes into flame (heat energy) and makes a “whooshing”

sound (sound energy). The chemical energy in the burning match continually

changes into heat and light. You blow the match out (mechanical energy

again) and the match gives off smoke (chemical energy) as it cools.

This is just one example of how a simple action can produce many energy

changes. There are many examples of changing forms of energy all around

us all of the time. Our muscles are continually changing the chemical energy

that we derive from the food we eat into mechanical energy as we move. A

CD player turns electrical energy into mechanical energy (motor that plays

the CD) and sound energy (the music that you hear) and heat energy (heat

is released from the back of the CD player).

Take some time to consider how energy can change form. For example,

think about chemical energy. The chemical energy in food changes form inyour body to give you the energy to move (mechanical energy). What energy

was changed in order to give energy to the food? The sun’s atomic and

light energy was transformed for food to grow. The food stored the sun’s

energy and released it into your body in the form of chemical energy.


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Energy

(continued)


Used Energy and Heat Energy

When energy is used, there is one thing that always happens: heat energy

is produced. No matter which type of energy is used, heat is produced. To

put it another way, whenever energy changes form, heat is produced. To

understand this, let’s look at some examples.

Here is an example of used mechanical energy making heat. Have you ever

bent a thin piece of metal back and forth to break it in half? What happens

to the metal as you bend it back and forth? The mechanical energy that

you are supplying is transferred to the stress point, which becomes warm

until it snaps. (The snap is sound energy.) You can feel the heat from the

broken metal. The heat is slowly released into the air until the metal cools.

When electrical energy is used, it also releases heat. Electrical energy is

used to operate a CD player, stereo, television, or video tape player. If you

ever looked at the back of any of these appliances you would see a grill

covering a vent through which heat is released. When you buy a new

appliance and set it up, the instructions will tell you to place the appliance

away from the wall with enough room for the heat in the back to escape.

Heat is produced whenever you use electrical energy to operate one of

these appliances.

Using chemical energy also releases heat. If you are running, your muscles

use a lot of chemical energy through the food you eat to keep you moving.

The activity of running warms your body. You actually radiate much of this

heat out and away from you, warming the air around you (even though

you may not notice that you’re doing this). As your body uses chemical

energy, heat is released. Have you ever been in a room with a lot of people

dancing or playing an active game? The room warms up with all of the

heat being given off from the moving bodies as they use chemical energy.

Whenever energy changes form, some of it is always changed into heat

energy and released. Scientists and engineers try very hard to minimize

this loss of heat because it is considered “wasted” energy. Imagine that

you like to eat ice cream, but you only like it when it is frozen. You can’t

stand to eat melted ice cream, but every time you eat a bowl of frozen icecream some of it melts. Some of the ice cream is wasted because it changes

into a form that you won’t eat. Has the wasted ice cream disappeared?

No, it has just changed into a form that is not useful to you.


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Energy

(continued)


Even when a machine is supposed to produce heat - like a toaster oven —

some of the electrical energy that could be put into making the inside of

the toaster oven hot escapes to the air around it. In every use or change

of energy, some is “lost” as heat.

When energy is used, heat energy is always produced.

This problem of escaping heat energy is termed by engineers as a problem

of efficiency. If all of the electrical energy were used by a CD player in order

to produce music, the CD player would be considered 100% efficient. This

is not possible however, as some energy is always converted to heat energy

and wasted. Some machines are more efficient than others in using the

energy put into them. A highly efficient machine is one that uses most of the

energy that is put into it and releases very little as heat energy. Efficiency is

an important factor to consider when you are deciding which model of anappliance to buy. When an appliance is labeled as “energy efficient,” what

it is referring to is how efficient the appliance is at converting the electrical

energy or fuel it runs on into the work the appliance is designed to do, while

eleasing a minimal amount of heat.

Why can’t we have 100% efficient machines? Why is heat energy always

released when energy is used? Does energy disappear? Is energy created?

In the next Lesson, we will learn about the Laws of Thermodynamics and

get the answers to these questions.

7. Examine some appliances around your house. Find where the heatis released. Write down what appliances you looked at and what you

found.


Energy

Kinetic energy Potential energy Efficiency


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Notes


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Resistance and

Ohm’s Law

Grade 8

23 We have described how

electric current moves

through a medium, and that

conductors, such as metal,

are media that let electric

current move through themeasily. Other materials,

called insulators, prevent

the movement of electricity.

Resistance is the measure

of how easy or hard it is

for an electric current to

flow through a particular

medium. Conductors have

low resistance while

insulators have high resistance.

Wires can be made of different metals. We have already described

how aluminum and copper can be good conductors as both have low

resistance (although copper is used predominantly in home construction

now due to the fire danger associated with aging aluminum wiring).

However, sometimes metals with a high resistance are used for special

purposes. Nichrome is a metal that is also a conductor, but it has higher

resistance than aluminum or copper. When a large electrical current goes

through nichrome, it gets very hot. Toasters and electric heaters use wire

coils made of nichrome or a metal with similar properties inside themin order to create heat. Tungsten, which is used as the filament in many

light bulbs, is another metal that is a conductor but has a high resistance.

When current passes through a tungsten wire, it gets so hot that it glows

white-hot.


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Resistance andOhm’s Law

(continued)


The amount of resistance a wire has depends on four things: the length

of the wire, how thick it is, what it is made of, and its temperature. The

longer a wire is, the greater its resistance, as the electric current has to

pass through more media. The thinner the wire, the greater its resistance

as well, as it is harder for the electrical current to move along. As we haveseen, the composition of the wire (what it is made of), also determines

resistance. And the warmer a wire is, the greater its resistance.

Let’s look at an analogy. Picture a large elevated tank of water. The height

of the tank determines the potential energy of the water, similar to the

electric potential (voltage) in a circuit. If we want to have water flow out

of the tank (current), we need to have a hose or pipe for it to flow through.

The amount of water flow we get depends a lot on the size of the hose,

as well as on the height of the tank. It is the same with electric current.

A higher voltage can allow a higher current. Also, a larger “pipe” has lessresistance, and a greater current can result. In this way, as we are about to

learn, the current depends on the voltage and the resistance of the material.

Now think about that light bulb again. As you learned in the last lesson, the

tungsten filament is surrounded by a special gas that keeps it from burning

up too quickly. Nevertheless, little by little the tungsten filament evaporates.

As this happens, the wire gets thinner and thinner. This causes the resistance

to be even greater. A greater resistance will cause more heating up, and the

hotter it gets, the more the resistance increases! Gradually, the filament

wears out. Have you noticed that a light bulb usually burns out right when

it is turned on? It glows really bright and then — pow! A cool light bulb

has less resistance, which allows more current to flow. This is more than

the thin filament can take, and when it heats up, the resistance increases

until — pow — out it goes. All this happens in a second!

Superconductors

Some materials are called superconductors because they lose all of their

resistance at low temperatures. Mercury is a good conductor at ordinary

temperatures. It becomes a superconductor at 270 degrees below

zero Celsius. If scientists could find a way to make a superconductor

at normal temperatures, the cost of moving electricity from a power

plant to your house would decrease dramatically!


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(continued)


1. Which wire has greater resistance: a light plugged into an extension

cord or a light plugged directly into a wall?

2. Compare the thickness and the length of the cords for several

appliances in your house, including the cords for small appliances

such as table lamps and coffee maker, larger appliances like TV’sand microwaves, and big appliances such as refrigerators, freezers,

and air conditioning units. List the cords in order of thickness, from

thickest to thinnest, and note the length. Which appliances need the

most electricity to operate? How does the length and thickness of the

cord compare with the demand for electricity each appliance has?

3. Now compare these household cords to the cord bringing electricity

into your house (if you can see it) and the overhead power lines

in your neighborhood. If the power lines in your neighborhood are

buried, try to find an area to observe where they are above ground.Answer the following questions. Are the power lines thicker than the

cords used for the appliances? Why or why not? Are the electrical

wires carrying electricity around your neighborhood underground or

above-ground? What are the advantages and disadvantages of each

of these methods?

4. If you were in charge of designing a wire to carry electricity across

your state or province, which of the following properties would be

most important for your wire to have? Should it be thick or thin,

buried underground or installed out in the sun? What material wouldyou choose and why?

Ohm’s Law

Ohm:

The unit of measurement for resistance.

How is resistance measured? Remember, electric potential is measured in

units of volts (physicists use the symbol “V” for this). The amount of electric

current is measured in units of amperes (symbol “I”). Resistance is measured

in units of ohms (symbol “R”). The definition of an ohm is the resistance atwhich one volt of electric potential allows one ampere of current to flow.

An example of an ohm value is a flashlight bulb. It has the resistance of

1 ohm, meaning that one ampere of current flows through it at one volt.

A 60-watt light bulb has the resistance of about 200 ohms.


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(continued)


George Ohm (1787–1854)

German Physicist, Mathematician

George Ohm was a professorof mathematics at Jesuits

College in Cologne (Germany)

for ten years. In 1827, Ohm

wrote a pamphlet outlining

his discovery of the law later

named after him. Ohm’s

law states that the current flowing through a conductor

is directly proportional to the voltage, and inversely

proportional to the resistance. This was a major statement

with far reaching implications. His work had a great impact

on the theory and applications of current electricity. Sadly,

it was coldly received and he was so deeply hurt he resigned

his teaching position. It wasn’t until 1841 that his work

began to be recognized. At that time he was awarded the

prestigious Copley Medal of the Royal Society of London.

Ohm’s Law:

Electromotive Force = Resistance x Electric current The voltage, amperage (number of amperes) and resistance are related

to each other by a rule known as Ohm’s Law. Ohm’s Law states that the

current (I) in a circuit depends on the difference in electric potential

across the circuit (V), and the resistance of the material (R). Specifically,

Ohm’s Law states that the current in a circuit is equal to the voltage

difference divided by the resistance. This is how it looks:

Amperes (I) = Volts (V) ÷ Ohms (R)

We can use Ohm’s Law to calculate the current in a wire when the resis-

tance and the voltage are known. By inverting the equation, we can also

find the voltage or the resistance if the other two elements are known.

Other ways of writing this equation are as follows:

V = I x R or R = V ÷ I


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(continued)


For example, if a 4-ohm wire is connected to a 12-volt battery, what would

the current be? We know the value for V is 12, and the value for R is 4.

I = V ÷ R

I = 12 volts ÷ 4 ohms

I = 3 amperes

Ohm’s Law:

V = R x I

I = V ÷ R

R = V ÷ I

5. Assume your toaster has a resistance of 10 ohms, and it is plugged

into your house electricity of 120 volts. What is the current in the wirewhen your toaster is plugged in and on? Show your calculation.

Resistors and Circuits

In the last lesson you learned about two types

of circuits: a series circuit and a parallel circuit.

To review, a series circuit is one in which there

is only one path for the electricity to follow

and a parallel circuit is one in which there are

several different paths for the electricity to

follow. As you have also learned, you can

design a series circuit by connecting a wire

from one dry cell terminal to a light socket,

and then connect another wire from the

light socket to the other dry cell terminal.

The electricity would then flow from the dry cell, through the light socket

and back to the batter y, lighting up the bulb. The light bulb uses the

electrical energy carried in the electric current in order to operate.

A resistor is what scientists call any object, such as an appliance or

machine, that is connected in a closed circuit. A resistor uses theelectrical current running through the circuit to operate. In the series

circuit described above, the light bulb is an example of a resistor.

Every time you turn on a lamp or appliance in your home, it operates

because it is a resistor.


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(continued)


Resistors and Series Circuits

When a number of resistors are connected so that

the entire current flows through one resistor after

the other, the result is a series circuit. This is true

in the previous example of Christmas lights. Each

bulb on the string of lights is a resistor. The electrical

current flows through each bulb, one after the other.

The total resistance on a series circuit can be determined. When several

resistors are linked together in a series circuit, the resistance (in ohms) of

each resistor is added together with all of the other resistors. Together they

equal the total resistance of the circuit. For example, the ohms of each

bulb in a string of fairy lights is added together to find the total resistance

of the entire string of lights.

In the last lesson you learned that in a series circuit, the current remains

the same everywhere in the circuit. When you add more resistance to the

circuit by adding resistors, and the total voltage (determined by the power

source) stays the same, then by Ohm’s Law, the current will decrease. So if

you want to determine the amperage of a series circuit, you must first add

up all the resistances in the circuit.

In a series circuit, the voltage will drop as it goes through each resistor.

The more resistors you have, the less voltage difference there will be across

each one. All the individual voltage drops added together will add up to

the total voltage of the circuit, which is determined by the power source.

Let’s look at an example. Let’s say you have a series circuit connected to a

6-volt battery. You add to the circuit, one at a time, a fan with resistance

of 3 ohms, a light bulb with resistance of 1 ohm, and an electric clock

with resistance of 2 ohms.

First you attach the clock which has R = 2 ohms. If you used an ammeter

to measure the current flowing through the wire, what would you expect

the measurement of the current (in amperes) to be?

I = V ÷ R


I = 3 amperes


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(continued)


Next you attach the light bulb which has R = 1 ohm, and the fan which

has R =3 ohms. Again you use an ammeter to measure the current flowing

through the wire. Remember that no matter where you attach the ammeter,

the current will be the same. What would you expect the measurement of

the current to be? To calculate, you need to add up each resistor to findthe total resistance in the circuit.

I = V ÷ R

I = 6 volts ÷ 2 ohm (clock) + 1 ohm (light bulb) + 3 ohm (fan)


I = 1 ampere

The reason the current is lower when you put all three objects on the

circuit than when you only had the clock on the circuit is because of the

greater resistance in the circuit. According to the formula, if the voltagestays the same, and the resistance increases, the current has to decrease!

So what about the voltage? As the electric current leaves the negative

terminal of the battery, it is at its greatest potential. The potential (voltage)

drops after each resistor, but the total voltage change is 6 volts. It is at

its lowest potential as it reaches the positive terminal of the battery. The

chemical activity in the battery then raises the moving charge back to a

high potential as it moves into the circuit again.

What would happen if we were to increase the voltage? Let’s say we had

our three resistors, and a total resistance of 6 ohms. We want our current

to be higher than 1 ampere, so we have to increase the voltage of the circuit.

We switch to a 12 volt battery. In reality, we have to be careful that each

of our resistors can handle the increased voltage and current! Now our

formula looks like this:


I = 2 amperes

IN A SERIES CIRCUIT:

The voltage drops across each resistor in the circuit. The current

remains the same anywhere in the circuit. The individual resistances

add up to the total resistance of the circuit.


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(continued)


Resistors and Parallel Circuits

Now let’s look at parallel circuits and the resistors on them. As you have

learned, a parallel circuit is one in which there are several different paths

for the electricity to follow. Each resistor is wired separately from the others.

In a parallel circuit, the current divides and a part of it passes through

each resistor. Then the separate currents reunite to complete the circuit.

This way if any resistor is open or disconnected, current will still be able

to flow through the other resistors and the circuit will rema n closed. The

sum of the currents through each path of the circuit is equal to the total

current that leaves the source

In a parallel circuit, the total resistance is not equal to the sum of the

resistors as in a series circuit. In fact, it is less than any of the individual

branches of the circuit! If you think about it, the more pathways the

electricity has to travel, the less the total resistance will be The totalresistance can be found by using the following equation:

Here is an example of how the equation works. Let’s reconfigure the series

circuit (the one that had the fan with 3 ohms, the bulb with 1 ohm, and

the clock with 2 ohms) from a series circuit to a parallel circuit.

If we use 6 as a common denominator, this can be rewritten:

Simplify the equation to get:

1 1

1

1

R = R1 + R2 + R3

1=

1 (fan) +

1(bulb) +

1 (clock)

R R R R

1=

1 ohms +

1ohm +

1 ohms

R 3 1 2

R(11) = 6

R = 6/11 ohms (or 0.545 ohms)

1=

2 ohms +

6ohm +

3 ohms

R 6 6 6

1=

11 ohm

R 6


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(continued)


Can you see that this total resistance is less than the resistance of each of

the individual resistors? Compare the total resistance in the series circuit

to the total resistance in the parallel circuit.

Series circuit: R = 6 ohms

Parallel circuit: R = 0.545 ohms

Since there is a lower resistance in a parallel circuit when more resistors

are added, according to Ohm’s Law, the total current will increase.

Okay, so what about the voltage? In a parallel circuit, the voltage is the

same anywhere in the circuit. This is quite different from a series circuit!

IN A PARALLEL CIRCUIT:

The voltage is the same anywhere in the circuit. The current differs

throughout the circuit, and the total current is equal to the sum ofthe currents through each path. The total resistance decreases as you

add more resistors to the

6. Which kind of circuit has less resistance, a parallel circuit or a series

circuit? Would you say this makes it more or less energy efficient?

7. Answer the following questions, showing your calculations if needed:

a. You have a 10 volt parallel circuit, with 2 resistors on it. What is

the voltage across the first resistor? Across the second?

b. You have the same two resistors on a 10 volt series circuit. Will

the voltage going into the second resistor be more, less, or the

same as that going into the first resistor? Exact numbers aren’t

needed!

c. You have a series circuit with a current of 6 amps and three

resistors on it, with resistances of 10 ohms, 5 ohms, and 6 ohms,

respectively. What is the voltage of this circuit? Show your

calculation.


resistance

superconductor

ohm

resistor


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Notes


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Home Electricity

Grade 8

24 The examples of electric circuits that we have looked at

so far mostly involved small amounts of electricity in

appliances. There are electrical circuits in your house,

too. Your house is part of a larger circuit that is

connected to the power supply of a power company.

The amount of electricity flowing through your home atany moment depends on the number of appliances that

are all working at the same time. The total amount of

electricity that flows through your house is determined

by the amount of current being used (the sum of thecurrent being used by

each appliance) and the voltage of your home circuit. Within each home,

there are usually smaller circuits which are wired to the larger circuit

running into the home. These smaller circuits control certain areas of the

home. For instance, there may be a circuit for the upstairs bedrooms,

another for the kitchen, and another for the outdoor lights.

When there are a lot of appliances turned on at the same time in your

house, a lot of electric current flows through the wires of your house.

When large amounts of current pass through a wire, the wire heats up.

If it gets too hot, it can cause materials nearby to heat up and catch fire.

It is therefore very important that wires in your house carry only as much

electrical current as they can safely carry without getting overheated.

Overload

Suppose that you are using a 150-watt light bulb in a bathroom that has

an electrical outlet on the same circuit. It is winter time and you are heatingthe room with a 3600-watt space heater plugged into the outlet. You plug

your 1800-watt hair dryer into the outlet and begin to dry your hair and

all of a sudden everything turns off. You are left in the dark with a wet

head and cold feet. This is an example of what is called overload.


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Let’s figure out what happened to better understand overload. (Take a

moment to review amperes, volts, and watts from Lesson 22 if you need to.)

Remember that there is a relationship between amperes, volts, and watts

that can help us. The rule tells us that amperes equals watts divided by volts,

or I = W ÷ V. We know that the circuit in your home is a 120-volt circuit, which is standard

in homes in the U.S. We also know the wattage of each appliance. Now let’s

determine how much current (amperes) is being drawn by each appliance.

The light draws 150 watts on a 120-volt circuit.

I = 150 watts ÷ 120 volts

I = 1.25 amperes

The light uses 1.25 amperes of electricity.

The heater draws 3600 watts on a 120-volt circuit.


I = 30 amperes

The hair dryer draws 1800 watts on a 12-volt circuit.


I = 15 amperes

When you turned on all three appliances, the total usage was 46.25

amperes at the same time. Since these appliances were all being used inone small room (the bathroom), they were probably all wired to a single,

smaller circuit within your home. But why did they all go out?

To prevent wires from overheating, safety devices are installed in homes to

limit the amount of electricity that can flow through the wires. These devices

are called fuses and circuit breakers. In order to understand the overload

problem you experienced in the bathroom, you need to learn about these.

As you now know, electric power is brought into your house in one large

power line from the street. Inside your house it is divided up into several

different circuits. Most houses have about 5 to 15 circuits. Most housecircuits are wired for 120-volts. 240-volt circuits are used for particular

circuits that run major appliances like electric clothes dryers, water heaters,

well pumps, ceramic kilns, or electric stoves that require large amounts of

electricity.


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All houses have either a fuse

box or a circuit breaker box,

but usually not both. Fuses

are inserted into the electric

wiring of your house. Theyare located in the fuse box,

which is located at the point

where the power entering

your house is divided up into

the different circuits. A fuse

is made of a strip of wire

that has high resistance but

melts at a relatively low

temperature. The fuse is

placed somewhere in the cir-

cuit and if the current gets too high, the wire in the fuse melts and

immediately opens the circuit, stopping the flow of electricity. The circuit

is “blown” and electricity can no longer complete the circuit.

A circuit breaker is a similar type of safety device that is part of the electric

wiring of your house. Circuit breakers are located in a circuit breaker box

that is usually placed somewhere convenient in your house, such as on a

wall just inside an attached garage, also at the point where the main line

is divided up into different circuits. A circuit breaker works much the same

way as a fuse. A circuit breaker is a switch with a gap in it. Because of thegap, no current flows through the switch under normal loads. The heat of

a large electrical overload (caused by too many appliances operating at

the same time) causes a bimetallic (two

metal) strip within the circuit breaker

to bend. When it bends, the metal strip

becomes disconnected from the circuit.

Electricity will arc, or jump, across the gap

and activate the switch which immediately

opens the circuit. When the metal strip

cools sufficiently to be safely connected,

the strip returns to its normal position.

While a fuse must be replaced if it is

blown, the circuit breaker only has to be


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reset and moved back into the “on” position after the overloading situation

has been located and corrected. You must manually push the circuit breaker

back to allow the electricity to flow again.

The difference between circuit breakers and fuses is that while a fuse has to

be replaced when it is “blown,” a circuit breaker continues to function overand over again. A circuit breaker can also be manually switched to open a

circuit so that you can make electrical repairs without the danger of electricity

flowing through the circuit (and you!). Most homes nowadays are set up

with circuit breaker boxes instead of fuse boxes. Some individual appliances

might have their own fuse for additional protection for that appliance.

1. Locate the fuse box or circuit breaker box in your own home or

building. Ask your parents where it is located, and examine it closely.

a. Make a sketch which includes all the fuses or circuit breakers

in the box. Usually they will be labeled (such as “Living Room”

or “Refrigerator” or “Central Air” or “Bedr oom”). Copy these

labels. If the fuses or circuit breakers are numbered instead, there

is usually a list nearby to tell you what number goes with what

room or section of your home.

b. On each fuse or circuit breaker there will be a number like 10, 15,

20 or 40. Make a note of this number . This is the number of

amperes allowed for that circuit.

c. Add up the total amper es allowed for your house. This is thetotal of all the circuits for your home.

d. What is the ampere limit for your bedroom circuit? For your

bathroom circuit?

e. Make a list of everything that is plugged into the outlets of one room

or section of your home (one circuit). Examine each item carefully

and determine how many amperes each item uses. Calculate

the amper es using the formula if necessary. Compare the total

amperes used with the total available amperes on that circuit.

If the wires in a circuit carry too much current, it is possible for the wiresin the circuit to build up an excess amount of heat. If the wires become hot

enough, the insulation around the wire can burn off and then the exposed

wire may ignite whatever is touching it. This is how electrical fires start.

Fuses and circuit breakers prevent dangerous overloading of a circuit by


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building in an intentional weak spot in the circuit. The weak spot is the

fuse or circuit breaker.

2. Now let’s return to the question of why everything went off when

the light, a heater, and a hair dryer were all running in the bathroom

at the same time. After all, the light usually stays on just fine wheneveryou use the bathroom. Why would it go off now? Answer this question

assuming the circuit breaker or fuse for the bathroom has a 40-ampere

capacity. Use the terms “circuit,” and “overload.” Then think of at

least two solutions to prevent this from happening again.

3. Assume that you have plugged into your bedroom outlets a 3600-

watt heater, a 300-watt tape recorder, and two 150-watt lamps.

You turn them all on and then you plug in a hair dryer (1600 watts)

and a vacuum cleaner (200 watts) and you dance around the room

vacuuming and drying your hair with your music blasting! Would youblow the fuse or trip the circuit breaker to your bedroom or not?

Use the ampere rating for your room that you found in Assignment 1.

Calculate the total amperes used, showing your work.

Short Circuits

Sometimes you may find that a fuse or circuit breaker may blow even though

the flow of electric current does not exceed the fuse or circuit breaker rating

(the number of amperes that the circuit is designed to carry). This is usually

caused by a short circuit.In order to understand short circuits you must remember that all wires going

into appliances are actually made up of two wires — one wire to bring the

electricity into the appliance (into the resistor) and one to carry it back out.

Look closely at a wire on a small

appliance or lamp in your home.

You will see that it has a ditch down

the middle. This ditch is where the

insulator separates out the two

wires. In an electric circuit, electricity

always moves in a circle, from the

electric energy source through a

wire to a resistor (appliance) and

then back through the second wire

to the electrical source again.


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A short circuit is caused when the insulation

around one of the two wires is worn or

frayed, allowing the two wires to touch.

When this happens there is no resistance

between them. The path then taken by the current is shortened; instead ofgoing through the resistor, the current goes from one wire into the other

wire, and back to the energy source. This is why it is called a short circuit.

Think of it as heading out to the library, only to decide to turn around and

go home again, never getting to your destination. Your trip was shortened.

Short circuits are a problem because of the lack of resistance between the

wires and because the current never reaches a resistor. According to Ohm’s

Law (V= R x I) large amounts of electrical current may flow through the circuit.

The voltage (V) of your house is probably 120 volts; if the resistance (R) is

very small, then the current (I) must get very big for the equation to staybalanced. The resistors on a circuit are important to keep the current from

getting too high.

Here is an example. Let’s see what happens to your hair dryer if it short

circuits. Suppose the cord to your hair dryer was frayed and the dryer

short-circuited. Assuming your hair dryer uses 1800 watts, let’s first look

at the amount of current flowing through the cord before it was frayed.

Remember, amperes = watts ÷ volts.


I = 15 amperes

The flow of current into the hair dryer is 15 amperes. But we need to know

the resistance of the

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