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CHAPTER 17 – FORCE, ENERGY AND POWER IN ACTION

CONCEPT MAP MECHANICAL (AND HEAT) ENERGY IN ACTION

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FORCE (F)

PARALLELOGRAM OF FORCES

RESULTANT FORCE

LEVERS

MOMENT (M)= F × distance from pivot

PRINCIPLE OF MOMENTS clockwise M = anticlockwise M

NEWTONS

MASS (m)

ACCELERATION (a)a = F/m or F = ma

COMPONENT FORCEat angle θº = F × cosθ

WEIGHT

REACTION FORCE

has a property

GRAVITY

has a property

INERTIA

is a

STATIONARY OR AT CONSTANT VELOCITY

causes

is tendency to remaincausesapplied in

SI unit isovercomes

used to define

JOULES

WATTS

MECHANICAL ENERGY= force × distance

POWER= energy ÷ time

does work moving

combined by

calculates

SI unit is

SI unit is

in unit time

has

always opposed by

have turning effect called

KINETIC ENERGY = ½ mv2

POTENTIAL ENERGY = mghhas kinds

g = 9.8 m/s2

due to Earth’s gravity

HEAT ENERGY4.2 kJ heats 1 kg water by 1ºC

can be converted to

Note: For some students, BS&T is likely to be a terminal science course; for others it may be a stepping stone to the study of science at High School. Parts of this chapter are aimed at the latter and may be considered ‘difficult’. However the level of treatment is always basic and teachers are advised to use their own judgement as regards what is or is not suitable for their own students. To save space, key definitions and formulae have not been placed in text boxes but are highlighted in blue instead.

17.1 MASS, FORCE, ENERGY AND POWER

Aims: To remind students about some concepts from earlier chapters, specifically force, mass and energy. To introduce students to the contents of this chapter.

Activities: Review work on forces from Chapter 10, focussing on the units of force, friction, the distinction

between mass and weight, centre of gravity and levers (Modules 10.2 to 10.5, and 10.9). Use demonstrations wherever possible and make sure that students understand the contents of the first three paragraphs of the text in this module.

Review work on energy from Chapter 14, focussing on the different kinds of energy and their inter-conversion (Modules 14.2 and 14.3) and the conservation and degradation of energy (Module 14.4). Use demonstrations wherever possible.

Provide students with a brief overview of the remaining contents of this chapter along the lines outlined in the last paragraph of the text in this module.

17.2 TURNING FORCES AND THE PRINCIPLE OF MOMENTS

Aims: To introduce students to the turning effect of a force and to the measurement of this as a moment. To introduce students to the principle of moments and show them how to perform simple calculations

based on this.

Activities: Revise the idea of levers (Module 10.9) and focus on the idea that a force acting on a lever has a turning

effect. Discuss the example of a spanner turning a nut and other examples that are familiar to your students. Establish the idea of moments and how to measure them.

Demonstrate clockwise and anticlockwise moments and the balancing of these using simple beam balances of whatever kind are available. If possible allow students to experiment with their own balances. They should measure clockwise and anticlockwise moments and confirm that these are equal when the beam balances. Make sure they understand the word equilibrium as a balance of opposing forces or moments.

Use the example of the wheelbarrow in the module (or use the model on the right) to show how the principle of moments can be used to perform simple calculations. Add two or three additional examples of your own.

Use the questions at the end of the module for homework or small-group work in class. Check students’ answers and their method of calculation for question 3. Be encouraging but take the opportunity to correct any misunderstandings.

Answers: Q1. (i) A clockwise moment is the turning effect of a force in the same direction as the hands of a clock

(that is, to the right as viewed from the pivot). The moment is equal to the magnitude of the force multiplied by the perpendicular distance from the force and the pivot. (ii) Equilibrium means balance; two forces (or two moments) are in equilibrium when they are equal in size but opposite in direction – so that they cancel each other out.

Q2. The turning effect of a spanner can be increased by increasing the force applied to the spanner, or by increasing the length of the spanner. (If a nut is rusted on and you can not move it with your spanner, first soak the nut in oil to lubricate it; allow time for the oil to soak in then try again. If you still cannot

1 kg

ruler

spring balance

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move it, find a long piece of pipe that will fit over the spanner, then push on the end of the pipe. If the pipe is long and strong enough the nut will turn – or the bolt will break!).

Q3. To balance the see-saw the moment of the boy must equal the moment of his sister. The moment of the boy is 20 × 2.5 = 50 Nm. The moment of the girl is 22 × d where d is the distance of the girl from the pivot. If these moments are equal, 22 × d = 50, so d = 50/22 = 2.27m. To balance the boy, his sister must sit 2.27 m from the pivot.

17.3 COMBINING AND RESOLVING FORCES

Aims: To enable students to distinguish between scalar and vector quantities and to identify force as a vector. To show students how forces acting at a point can be combined using a parallelogram of forces. To show students how to resolve a force into two components at right angles.

Activities: Discuss the difference between scalar and vector quantities using force as an example of the latter. Use

the example of tugs berthing a ship to bring out the importance of both the direction and magnitude of a force and of how forces can be combined to perform complex tasks.

If possible, demonstrate a resultant force as follows. Hook together three spring balances into a key ring representing the point P in the diagram in the module. Two students pull at different angles representing F1 and F2, while a third student pulls against them in the opposite direction against the resultant force R. The students should not pull too hard so all three spring balances give proper readings.

Demonstrate the use of the parallelogram of forces to find the resultant of two forces acting at a point. Use a large diagram, drawn to scale, and involve students if practicable. Give them examples to practice themselves and assist them to do these correctly.

Discuss the resolution of forces using the example given in the module. Give them additional examples to practice themselves and assist them to do these correctly. Students going on to study science at a higher level should learn the cosine rule.

Use the questions at the end of the module for homework or small-group work in class. Check students’ methods as well as their answers. The answer to question 3 represents an important principle and should be thoroughly discussed. Be encouraging but take the opportunity to correct any misunderstandings.

Answers: Q1. Approximately 370 N at a compass bearing of 107º (or E 17º S). Q2. The horizontal component of the force dragging the box is:

100 × cos30º = 86.6 N Q3. We choose the second component at right angles to the first so that

one component is completely independent of the other. (A force has zero component at right angles to itself, or to put it another way, cos90º = 0).

17.4 INERTIA AND NEWTON’S 1st LAW OF MOTION

Aims: To introduce students to the notion of inertia and to Newton’s first law of motion.

Activities: Introduce the idea of inertia and demonstrate the activities described in the first part of the module. If

possible students should try these and similar activities themselves. Emphasise that inertia is almost the same thing as mass – the more the mass the more inertia and the greater the tendency to keep doing whatever it is doing (standing still or moving in a straight line) - you could call it the laziness of mass!

Tell students something about Newton (he is mentioned briefly in Module 1.9 but you can supplement this from library or internet sources).

Introduce Newton’s first law. This often appears to be counter-intuitive because our experience suggests that force is needed to keep motion going so remind students about friction (Module 10.3) and use the notes in the module to help them understand that the force is needed to overcome friction, not to keep the mass moving. Roll a marble or ball on a smooth surface to show how well motion keeps going when friction is minimised. Take the opportunity to make clear the distinction between speed and velocity.

250N

500N

N

SE

107º

370N

3

Use the questions at the end of the module for homework or small-group work in class. Discuss students’ answers and be encouraging, but take the opportunity to correct any misunderstandings.

Answers: Q1. (i) Weight is the force that gravity exerts on any object. (ii) Mass is the amount of stuff in any

object. (iii) Inertia is the tendency of any object to resist being moved, or if it is already moving, to have its motion slowed down, speeded up or changed in direction. (iv) Friction is a force that opposes motion between solid surfaces and fluid layers sliding against one another. (v) Air resistance is (mostly) the frictional force between the air and an object moving through it.

Q2. When the card is flicked it moves sideways fast. The force of friction between the card and the coin tries to drag the coin sideways with the card. Because of inertia, the coin is slow to get moving and the card overtakes it. When the coin is no longer supported by the card the force of gravity pulls it down and it falls into the glass. (If the card is moved slowly, the coin moves with the card of course!).

Q3. Students’ answers will vary: look for the idea,, in their own words, that mass seems to be ‘lazy’ and that force is needed to make it move or change the way it is already moving.

17.5 ACCELERATION AND NEWTON’S 2nd LAW OF MOTION

Aims: To introduce students to the notion of acceleration and to Newton’s second law of motion.

Activities: Introduce the idea of acceleration as change in velocity (speed or direction) drawing on students’ own

experiences as far as possible. Go on to the measurement of acceleration as rate of change of velocity, introducing SI units and simple calculations as given in the text box in the module.

Use the suspended buckets from the last module to demonstrate Newton’s 2nd law. (Give the empty bucket a gentle then a strong push to show that the acceleration produced increases with the force. Then give the empty and full buckets equal pushes to show that the acceleration decreases with the mass or inertia of the object). Formalise this and make sure they understand the examples given in the module then go on to define the newton as the SI unit of force.

If possible, repeat the ‘Pisa experiment’ by dropping a large heavy object and a small dense object such as a coin or key (that will not be affected by air resistance) from a high place at the same time. Students should observe that they hit the ground at the same time. Establish the constancy and value of the acceleration due to gravity using the approach given in the module. (You can add that most of Galileo’s experiments involved rolling balls down slopes and he probably never dropped anything from the tower at Pisa. Also that the Apollo 15 team dropped a hammer and a feather on the moon – where there is no air resistance – and they hit the ground together! Students could research these events as a project).


Answers: Q1. (i) A vector quantity is a quantity that has direction as well as magnitude. (ii) Acceleration is any

change of velocity – speeding up, slowing down or changing direction. (iii) A newton is the force needed to accelerate a mass of 1 kg at 1m/s2. (iv) Gravity is a force of attraction exerted by any mass on any other mass; in particular the pull of the Earth and other heavenly bodies on all objects near them.

Q2. (i) In the formula F = ma, substitute the mass of 1 kg for m, substitute the acceleration due to gravity 9.8 m/s2 for a, and use F for the force of gravity pulling on the mass. This gives F = 1 × 9.8 = 9.8 newtons, so the force of gravity pulling on a 1 kg mass is 9.8 N (see also Module 10.3). (ii) The usual name for the force of gravity pulling down on any object is the weight of the object.

Q3. (i) The start velocity of the object is 0. After one second its velocity is 10 m/s straight down. The average speed for the first second is (10 – 0)/2 = 5 m/s, so in 1 second it falls 1 × 5 = 5m. (ii) The start velocity of the object is 0. After two seconds its velocity is 20 m/s straight down. Its average speed is (20 – 0)/2 = 10 m/s, so in 2 seconds it falls 2 × 10 = 20m. (iii) The start velocity of the object is 0. After seven seconds its velocity is 70 m/s straight down. Its average speed is (70 – 0)/2 = 35 m/s, so in 7 seconds it falls 7 × 35 = 245m.

17.6 ACTION, REACTION AND NEWTON’S 3rd LAW OF MOTION

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Aims: To introduce students to the notion of action and reaction and to Newton’s third law of motion.

Activities: Introduce the idea of action, reaction and Newton’s 3rd law. Add to the examples given in the module

from students’ own experiences. Emphasise that if there is no reaction there can be no force (for example it would be impossible to walk, or drive a vehicle, on a completely frictionless surface).

Demonstrate the rocket effect with a toy balloon and add examples that students may be familiar with (for example the recoil of a gun when it is fired, and the recoil of the end a hose when a strong flow of water is turned on). Discuss rockets and show them pictures of rockets from magazines or the internet. Encourage interested students to undertake follow-up research.

Show pictures of jet planes from magazines or the internet and discuss the working of the jet engine using the information provided in the module.


Answers: Q1. Action refers to the force exerted by any object on any other object; reaction refers to the equal and

opposite force which the second object always exerts back on the first object. No force can act if there is no reaction.

Q2. The expanding gases from the combustion of the fuel rotate a turbine as they exit the back of the engine. This rotating turbine turns the shaft that the intake turbine is attached to, so the intake turbine rotates too and sucks in air. The electric starter-motor is no longer needed and the engine will keep going as long as it has fuel.

17.7 MEASURING ENERGY AND POWER

Aims: To introduce students to the notion that mechanical energy can be quantified as the product of force

applied and distance moved, and to reintroduce the joule (previously defined in relation to food in Module 16.2) as the SI unit for measuring energy converted (or work done if this is the term preferred locally).

To develop and quantify the idea of mechanical energy as either kinetic energy or potential energy (previously introduced in Module 14.2).

To provide students with the simple algebraic equations for kinetic and potential energy and give them experience in applying these in simple cases.

To introduce the notion of power as energy transferred in unit time, to introduce the watt as the SI unit of power, and to given students experience of calculating power in simple cases.

Activities: Review thoroughly basic ideas regarding energy as covered in Modules 14.1 to 14.4; also the idea of

food energy as covered in Module 16.2. Demonstrate lifting masses of 100g (a tomato) and 1 kg (a 1l bottle full of water) to a height of 1 m and

let students try this for themselves. Go through the text of the first half of the module and establish that they are transferring, respectively, 1 and 10 J of energy to these masses. Through discussion, extend the idea of energy transferred in relation to experiences that your own students are familiar with.

Discuss kinetic and potential energy stressing their close association with mass. Drop a ball to demonstrate the conversion of potential to kinetic energy and roll a marble to hit another to show the transfer of kinetic energy from one object to another. Discuss locally appropriate examples (for example, the idea of kinetic energy being transferred to other objects and/or converted to other forms of energy could be extended by reference to colliding billiard balls, motor vehicle crashes, and so on).

Introduce the notion of power and emphasise the importance of the time factor. Students could compete in power challenges where they are timed running up stairs (see the answer to question 2 below) or performing any repetitive task involving force and movement.


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Answers: Q1. Energy is the capacity to do work and power is the capacity to deliver energy fast!

Energy transferred can be calculated by multiplying the force applied and the distance moved (energy = force × distance). The SI unit of energy is the joule which is the energy transferred when a force of 1 newton moves a distance of 1 metre.Power can be calculated by dividing the energy transferred by the time taken to transfer it (power = energy ÷ time). The SI unit of power is the watt which is the power delivered when 1 joule of energy is transferred in 1 second.

Q2. (i) The athlete converts the chemical energy stored in his or her food into mechanical energy; first to kinetic energy while running up the steps and finally to the potential energy of his or her mass at the top of the steps. (ii) The amount of energy converted is the force multiplied by the distance moved against the force. In this case the force is the downward pull of gravity on the athlete which is his or her weight (48 kg or 480 N), and the distance moved upwards against the force of gravity which is 16 m. So the energy converted = 480 × 16 = 7680 J (or 7.68 kJ).(ii) The potential energy of the athlete is given by the formula mgh, where m is his or her mass of 48 kg, g in the acceleration due to gravity of 10 m/s2, and h is the height above ground of 16 m. So the potential energy of the athlete = 48 × 10 × 16 = 7680 J (or 7.68 kJ). [All the food energy ‘burnt’ by the athlete in climbing the stairs has been converted to potential energy].(iii) The power of the athlete is the energy converted (7680 J) divided by the time taken to convert it (15 seconds). So the power of the athlete = 7680 ÷ 15 = 512 W (or 0.512 kW). [Students might be interested to know that this is same as the power of five 100 watt light bulbs!].

17.8 MEASURING HEAT ENERGY

Aims: To consolidate and extend what students already know about heat energy from previous modules [see

Modules 7.1 to 7.6 (heat), Modules 9.2 to 3 (kinetic theory) and Modules 14.2 to 5 (energy conversions and thermodynamics)].

To introduce a simple notion of absolute zero. To inform students that 4.2 kJ of energy raises the temperature of 1 kg of water by 1 ºC and help them

to understand how this can be used to measure heat energy. To introduce a simple notion of the specific heat capacity of other substances.

Activities: Remind students of what they learnt about heat in Modules 7.1 to 7.6, about the kinetic theory in

Modules 9.2 and 9.3, and about heat as energy in Modules 4.2 to 4.5. Take time for re-familiarisation. Establish the idea that heat is the internal kinetic energy of a substance due to the motion of its particles

and that rising temperature can lead to expansion, change of state and chemical change including decomposition and/or combustion. Marbles on a try can be used to demonstrate the kinetic model. (Tilt the tray two ways so the marbles form a stationery pattern in one corner to represent absolute zero; to represent a solid at room temperature shake the tray gently so the marbles rattle without disturbing the pattern; for a liquid shake a bit harder so the marbles start to move around past one another but still remain in the corner of the tray; for a gas remove most of the marbles and shake more vigorously with the tray horizontal so the marbles shoot around everywhere).

Explain how heat energy can be measured following the approach outlined in the text and go on to define specific heat (or specific heat capacity – use whichever term is appropriate locally). As a demonstration, measure the heat given out by any available small stove or heater in one minute as follows. Place a litre (1 kg) of water in a pan and take its temperature, heat it on the stove or heater for exactly 1 minute. Measure the rise of temperature and multiply that by 4.2 to get the heat supplied by the stove or heater in kilojoules per minute.

Emphasise that water has an unusually high specific heat meaning that it absorbs a lot of heat for a small rise of temperature. If possible demonstrate quenching a red hot iron rod in a bucket of water and show that the water does not warm up all that much (use a thermometer if possible otherwise, with caution, your hand). Question 3 at the end of the module provides an opportunity to think more about

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this. You could encourage students to speculate about how the high specific heat of water affects the Earth’s climate (this will be considered in Chapter 18).

Summarise how we use heat domestically and industrially, emphasising the second law of thermodynamics and the low efficiency of most of the technology that uses heat..


Answers: Q1. (i) Convection is a process that distributes heat in fluids; hot fluid expands so it becomes less dense

than cooler fluid and it rises, setting up a circulation that distributes heat throughout the fluid. (ii) Absolute zero is the temperature at which particles stop moving and have no kinetic energy; this happens at a temperature of – 273.15 ºC. (iii) The specific heat of a substance is the heat required to raise the temperature of 1 kg of the substance by 1 ºC.

Q2. Heat energy is the kinetic energy of the particles in any matter. Heat may have the following effects on matter: rise in temperature, expansion (which leads to convection in fluids), change of state (from solid to liquid to gas), decomposition of some substances, other chemical reactions including burning.

Q3. (i) Because it takes a lot more heat to warm up water than it did to warm up the iron. So when the iron loses all its heat, this doesn’t warm up the water all that much.(ii) [Many students will find this question difficult without help but it is good to let them try first. As a hint, tell them that they will have to use the specific heats of iron and water that are given in the module. After they have struggled with the problem you can lead them through the following solution].

Any heat (joules) gained by the water comes from the iron which must have lost all those joules.

Let the final temperature of the water be t ºC. The rise in temperature of the water is (t – 25) degrees, and the fall in temperature of the iron is (1000 – t) degrees. [This is the key step, Now its quite easy!]

Heat gained by water is 4.2 × mass of water × rise of temperature = 4.2 × 20 × (t – 25)

Heat lost by iron is specific heat of iron × mass of iron × fall of temperature = 0.45 × 1 × (1000 – t)

Heat gained by water = heat lost by iron so 4.2 × 20 × (t – 25) = 0.45 × 1 × (1000 – t)So 84(t – 25) = 0.45(1000 – t) So 84t – 2100 = 450 – 0.45t So 84t + 0.45t = 450 + 2100So 84.45t = 2550So the final temperature t = 2550/84.45 = 30.2 ºC

17.9 STATIC ELECTRICITY

Aims: To make students aware that electrical phenomena are associated with the displacement or flow of the

electrons that are normally located in the outer orbits of atoms. To introduce students to the basic phenomena of static electricity and to the explanation of some

familiar everyday effects caused by static electricity.

Activities: Remind students of what they already know about atoms (Chapter 13). Discuss how outermost electrons

may become displaced in insulators or flow in conductors, thus giving rise to electrical phenomena. Demonstrate the basic phenomena of static electricity as described in the first two paragraphs of the

module under ‘static electricity’ (you can use a simple sling of paper to suspend a charged plastic strip). Involve students and let them try some activities themselves. Establish that like charges repel, opposite charges attract and that moisture inhibits static electricity. (If you live in the humid tropics it may be impossible to demonstrate electrostatic phenomena unless you dry everything thoroughly in advance).

CONCEPT MAP ELECTRICAL ENERGY IN ACTION

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If possible show them a Van de Graaff generator, for example in a local high school or science centre. This always has an impact!

Get students to try picking up small scraps of paper with a plastic comb, ball-point pen or similar object that has been charged by rubbing briskly with a dry, hairy woollen or similar material. Help them to understand that this occurs because of the opposite charges induced in the paper scraps. Once they have understood this, get them try again with scraps of aluminium foil. These are attracted to the charged plastic in the same way and for the same reason, however they are immediately repelled again. Encourage them think about why this happens but do not be in a hurry to tell them (the answer is explained under ‘Answers’ below).

Discuss peoples’ everyday encounters with static electricity, focusing on the students own experiences. Use the questions at the end of the module for homework or small-group work in class. Discuss

students’ answers and be encouraging, but take the opportunity to correct any misunderstandings.

Answers: Q1. The scraps of foil are attracted then immediately repelled. The attraction is due to the opposite

charge induced by the charged plastic. Let us assume that the plastic has a negative charge, that is to say an excess of electrons. The foil is a conductor so as soon as it touches the plastic the electrons flow into it, quickly neutralising the induced positive charge and giving the foil a negative charge. The foil now has the same charge as the plastic and is repelled.

ELECTRICAL ENERGY

STATIC ELECTRICITY CURRENT ELECTRICITY

CHARGED INSULATOR ELECTRONS CONDUCTING CIRCUITS SERIES & PARALLEL

+

OPPOSITE INDUCED CHARGE

ELECTRIC SPARKS AND LIGNTNING

–CIRCUIT DIAGRAMS

CURRENT (I)

VOLTAGE (V)

RESISTANCE (R)

manifests as

manifests as

has kinds

attract

repel causes

caused by transfer of is flow of flows in

represented by

causes

have potential energy

VOLTMETER

measured by

manifests as

causes resisted by

AMMETER

measured by

OHM’S LAW R = V/I

ELECTROMAGNETIC EFFECT

causes related by

ELECTRIC MOTOR GENERATOR

MAINS ELECTRICITYDOMESTIC ELECTRICAL POWER

applied in

makesdelivers

8

Q2. (i) Static electricity refers to the build up of static (unmoving) electric charges on surfaces due to the displacement of electrons. (ii) Current electricity refers to the flow of electricity (electrons) in a conductor around a circuit.

Q3. Plastic CDs and DVDs often become charged when they are played or when the surface is cleaned by rubbing it on cloth. The charged plastic induces opposite charges on the surface of dust particles, hairs etc and these are then attracted to the disk.

Q4. If the plastics are dry, then any friction involved in handling them will tend to give them an electric charge. The cellulose acetate will get a positive charge while the polythene sleeve will get a negative one. The cover and the sleeve will therefore attract one another and will cling together when you try to removed the book from the sleeve.

17.10 CURRENT ELECTRICITY AND CIRCUITS

Note: For students who will not study science beyond this point, Modules 17.10 to 17.13 could be omitted or treated in a less mathematical way.

Aims: To remind students of the basic facts underlying current electricity (Modules 4.2 to 4.7) and update this

using their knowledge about electrons (Module 13.5). To remind students how to draw and interpret circuit diagrams (Module 4.7) and extend their repertoire

of symbols.

Activities: Review basic knowledge about simple circuits and establish the essential facts as outlined in the text of

the module. Illustrate the discussion with demonstrations or student activities using cells, wires, bulbs, and switches; also a simple torch (flashlight) which can be taken apart to show how it works. (Note that switches vary a lot so the switch may differ from the one shown in the text of the module.)

Discuss the drawing and interpretation of circuit diagrams, emphasising the importance of following the flow of electrons around all branches of the circuit – they can be traced by a finger. For the ac circuit tell them they can trace the current in either direction and that they will learn more about ac and parallel circuits in coming modules. Emphasise that the current must flow through the various components in the correct sequence. Students should practise connecting circuits from circuit diagrams, and also drawing circuit diagrams from actual circuits.


Answers:

Q1. (i) Electrical conductors are materials (mostly metals) that allow electricity to pass through them. (ii) Electrical insulators are materials that do not allow electricity to pass through them. (iii) Electrons are negatively charged sub-atomic particles that orbit the nucleus of atoms and carry the electrical charge in electric currents. (iv) An electric current is a flow of electric charge through a conductor. (v) A fuse is a safety device in an electric circuit that melts and breaks the circuit if the current is dangerously high. (vi) A switch is a device in an electric circuit for completing or breaking the circuit to start or stop the flow of electric current. (vii) AC stands for alternating current; it is a current that reverses direction many times every second. (AC was previously mentioned in Module 14.8 and will be discussed further in Modules 17.15 and 16).

Q2. An electric current flows only when there is a source of electrical energy connected to a complete conducting circuit.

Q3. A 1.5 volt cell (Leclanché cell) is contained in a zinc case which acts as the negative terminal. The case is filled with an electrolyte consisting of a paste of ammonium chloride and manganese dioxide. The electrolyte surrounds a graphite (carbon) rod which emerges from the top of the cell and acts as the positive terminal. (See diagram right and Module 4.6).

17.11 MEASURING ELECTRIC CURRENT AND VOLTAGE

plastic seal

zinc case

graphite rod

electrolyteNH4Cl + MnO2

negative terminal

positive terminal

9

Aims: To provide students with basic information about electric currents and their measurement in amperes

with an ammeter. To provide students with basic information about voltage/potential difference and its measurement in

volts with a voltmeter.

Activities: Discuss the basic information about electric currents in the first paragraph of the module, then

demonstrate the use of an ammeter in a simple basic series circuit. If possible allow students to practise using an ammeter themselves, stressing the importance of getting the polarity correct.

Discuss the basic information about voltage/potential difference in the first paragraph under this heading in the module, then demonstrate the use of an voltmeter in a simple basic series circuit. If possible allow students to practise using an voltmeter themselves, stressing the importance of getting the polarity correct.


Answers: Q1. (i) An amp or ampere is the SI unit for measuring electric current (rate of flow of electric charge) in

a conductor. (ii) An ammeter is a device for measuring electric current (in amps). (iii) A volt is the SI unit for measuring voltage (potential difference), that is the difference in potential energy of electrons between two points. (iv) Cells are connected in series when the top (positive pole) of one cell is connected to the bottom (negative pole) of the next.

Q2. (i) The ammeter can in inserted anywhere in the upper loop of the circuit between X and Y so that the current flowing through the ammeter is the same current that is flowing through the lamp L2. (The ammeter could be placed between X and the dimmer, between the dimmer and L2, between L2 and S2, or between S2 and Y).(ii) The voltmeter must be placed as shown; it contacts the circuit at points adjacent to, and on each side of, L2.

17.12 ELECTRICAL RESISTANCE AND OHMS LAW

Aims: To make students familiar with the concept of electrical resistance and teach them about Ohm’s law. To define resistance, and the ohm as the SI unit of resistance. To introduce the formula V/I = R and give students experience in using it. To make students aware of how the resistance of a wire is effected by its length, thickness and material.

Activities: Discuss with students the ideas in the first paragraph of the module stressing the analogy between

resistance and friction. Students should try the practical activity shown in the text-box in the module or you should demonstrate it (see Modules 4.2 to 4.7 with the associated Teachers’ Guide and Equipment List for practical hints on setting up circuits, making good connections, improvising and so on). Establish clearly that the current goes up when the voltage goes up.

Define resistance, introduce the ohm as its SI unit and establish the formula R = V/I. Use the results from your own experiment, or the typical results given in the text box, to calculate the resistance of a standard resistor or a piece of resistance wire. Show them how the formula can be transposed to facilitate the calculation of voltage, current or resistance from the other two.

Discuss the factors which affect the resistance of a wire and establish that short and thick mean low resistance while long and thin mean high resistance (the analogy of water flowing through pipes may be useful). If possible demonstrate whatever parts of this may be possible, stressing the need to ‘control variables’ (for example if you are investigating the effect of length, all the wires you test must be of the same material and have the same thickness).

230 v

L1

L2 dimmer

S1

S2

X Y

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Review and discuss the information about the resistance of different metals in the last paragraph of the module.

Answers: Q1. (i) Potential difference is another name for voltage; it is the difference in the potential energy of

electrons between two points. (ii) Electric current is a flow of electric charge through a conductor (the charge is usually carried by negatively charged electrons). (iii) Electrical resistance is the opposition of an electrical conductor to carrying an electric current. Resistance can be measured as the ratio of the voltage applied to a conductor to the current produced. (iv) On ohm is the resistance of a conductor when a voltage of 1 volt causes a current of 1 amp to flow through it.

Q2. (i) The thin nichrome wire has a higher resistance than a thicker one of the same length. (ii) The long copper wire has a higher resistance than a shorter one of the same thickness. (iii) The constantan wire has a higher resistance than a tungsten one of the same length and thickness.

17.13 SERIES AND PARALLEL CIRCUITS

Aims: To enable students to distinguish between simple series and parallel circuits and the describe how

electric currents flow around each. To enable students to understand the following advantages of connecting devices in parallel: (i) the full

voltage of the power source is delivered to each device, (ii) the devices can be switched on and off independently.

Activities: Lead students through the descriptions of, and information about, series and parallel circuits presented

in the text of the module. If possible, demonstrate the circuits illustrated. [Use two 1.5 volt cells in a battery clip, two bulbs from a two-cell torch and any insulated connecting wire. This is worth doing even if you have no ammeters and voltmeters. If you have one ammeter and one voltmeter, they can be moved to different parts of the circuit to show current and voltage at those points].


Answers: Q1. A series circuit has a single loop and the electric current flows through several components one

after another. A parallel circuit has two or more loops connected to the same power source so that separate electric currents flow through each component.

Q2.(i) In the series circuit, when the switch S is closed (switched on) electrons flow from the negative terminal of the battery through the torch bulbs L1 and L2, then through switch S and back to the positive terminal of the battery. (Because of the combined resistance of L1 and L2, the flow of electrons is only half what is needed for the bulbs to light up normally). (ii) In a parallel circuit, when switches S1 and S2 are closed (switched on), electrons flow from the negative terminal of the battery to the point X and there the flow divides in two. Half the electrons flow through the torch bulb L1 and the switch S1, and the other half flow through the other loop through L2 and S2. At Y the two streams of electrons join up and return to the positive pole of the battery. (Because each loop has only the normal resistance of one bulb, the flow of electrons in enough for the bulbs to light up normally).

Q3. (i) For the series circuit the current is the same at all points in the circuit. This current depends on the total resistance of the circuit which is 6 ohms for L1, plus 12 ohms for R = 18 ohms. Using I = V/R the current in all parts of the circuit will be 3/18 = 0.167 amps. Using V = IR, the voltage across L1 is 0.167 × 6 = 1.0 volt; the voltage across R is 0.167 × 12 = 2.0 volts.

L1

R

S1

S2

3.0 V0.75 A

0.25 A

0.5 A

Parallel circuit with voltages and currents after both switches are closed

3.0 V

3.0 V

0.75 A

Y X

– +

11

(ii) In the pa (ii) In the parallel circuit, the loops are independent and both are supplied with the full voltage of the battery. The current in first loop with L1 is unchanged. In the second loop using I = V/R the current is 3/12 = 0.25 amps. The current out from and back to the battery is the sum of the currents in each loop which is 0.5 + 0.25 = 0.75 amps.

17.14 ELECTROMAGNETISM

Aims: To make students aware of the magnetic effect of an electric current and to introduce them to

electromagnets and some of their uses. To take the opportunity to warn students about short circuits and their dangers. To make students aware (without any detail) of the motor effect and its use in electric motors, and of

electromagnetic induction and its use in electricity generators.

Activities: Review students’ prior knowledge about magnets and magnetic fields (Modules 4.9 to 4.12). Demonstrate, or help students to carry out themselves, the activity described in the text box in the

module to show that an electric current has a magnetic field . Insist that the wires are touched to the cell only quite briefly and take the opportunity to warn students about short circuits and their danger. (Note that the deflection of the compass can be reversed by running the wire over the compass instead of under it, and also by reversing the cell).

Demonstrate a simple, home made electromagnet and allow students to make and test their own if possible. Use large iron nails or any available soft iron rods or bars. Encourage students to pick up magnetic materials such as paper clips or nails with their electromagnets. Help them to identify the polarity of their magnets and check this using the repulsion of a like pole in a compass. Discuss some familiar uses of electromagnets and show them a working bell or buzzer of possible. Encourage students to make their own bells, buzzers (just a bell without the bell!) and relays.

Review with students the notes in the module on the motor effect and electromagnetic induction. The diagram on the left can be used in explaining both. If you have a suitable large magnet, you can easily demonstrate both effects with a loop of wire. For the motor effect, ask a student to hold the loop between the poles of the magnet while you touch the ends to a 1.5 volt dry cell. Observe that the loop twists and tries to rotate. For the induction effect, connect the loop to an ammeter, then sharply twist the loop between the poles of the magnet (if there is no result, try reversing the direction of your twist or the connections to the ammeter!). If appropriate, refer students to Module 14.8 where they can learn how the motor effect is used in a real electric motor.


Answers: Q1. The deflection of the compass can be reversed (i) by laying the wire over the compass instead of

under it, (ii) by reversing the cell. Q2. (i) A magnetic field is an area in which magnetic forces can be detected. (ii) A magnetic pole is the

end part of a magnet where the magnetic forces are strongest. (iii) An electromagnet is a temporary magnet based on a soft iron core surrounded by a coil of insulated wire connected to a source of electricity; it can be switched on and off. (iv) Electromagnetic induction is the induction of an electric current in a conductor when it moves in a magnetic field.

Q3. Alnico is an alloy of iron with aluminium, nickel and cobalt; it is used for making strong permanent magnets.

Q4. A relay based on the electric bell circuit is shown on the right. When the electromagnet is connected to a cell, the spring is attracted to it and

L1 R S

0.167 A

0.167 A

0.167 A

2.0 V1.0 V

3.0 V

Series circuit with voltages and currents measured after the switch is closed

– +

–+

spring

12

switches the light on. When the electromagnet is disconnected, the spring springs back and turns the light off.

17.15 MAINS ELECTRICITY

Aims: To provide students with basic information about (i) the main processes involved in the generation,

transmission and distribution of mains electricity, (ii) the nature of the mains electricity supply, (iii) the general arrangement of domestic lighting and power circuits.

To inform students of the functions of live, neutral and earth wires and enable them to identify each from its colour coding and know its proper position in a plug or power point (socket).

To make students aware of the main safety issues associated with mains electricity in the home and provide basic information about how these are dealt with both in general and locally.

Activities: Review Module 14.9 about power stations, then discuss the working of the generator using the basic

diagram in the module. Stress that this is an application of electromagnetic induction. If possible, show them working or broken generators – an old alternator from a car would be suitable. Make sure students know that the output at the power station is a high voltage alternating current.

Discuss transmission and distribution along the lines presented in the module, stressing the reasons for using transformers at both ends. If possible visit nearby power stations, pylons and sub-stations. If you do this, encourage students to look at the very high grade insulators that have to be used.

As regards the domestic supply, details vary from country to country. Focus on local systems and practices and refer to students own relevant experiences. Make sure they know the local system of colours for live, neutral and earth wires. Show them local wires and plugs and how the plugs are wired. If a qualified electrician is available to help, you could look at the ‘box’ located where the power supply enters a building and identify the fuses or trip switches (circuit breakers) for the different circuits. If not, try to obtain samples of the real thing to show them.

Stress the safety issues as summarised in the module, focusing again on local systems and practices, and on students’ own experiences. If possible, get a qualified electrician to demonstrate the action of fuses and/or trip switches (circuit breakers).


Answers: Q1. (i) Slip rings are rotating metal rings in a generator. They conduct the electricity produced in the

coils to graphite brushes connected to the output terminals. (ii) Transformers are devices that step-up or step-down the voltage of an alternating electric current. (iii) Lighting circuits are the electrical circuits that deliver mains electricity to the lights in homes (and elsewhere). (iv) A power point (or socket) is a fitting on the wall of a building that delivers mains electricity for various applications; the power is accessed by a power plug that fits into, and makes electrical connection with, the power point. (v) An earth wire is a wire that is electrically connected to the ground (earth) and therefore has a voltage of zero. (vi) A fuse is a safety device in an electric circuit that melts and breaks the circuit if the current exceeds a given limit. (vii) A trip switch (or circuit breaker) is a safety device in an electric circuit that switches off the circuit if the current exceeds a given limit.

Q2. Answers will vary in different countries; if you are not certain, consult a qualified local electrician. Q3. Lighting circuits typically use a 5 (or 8) amp fuse, power circuits a 15 (or 20) amp fuse, and electric

cookers a 30 amp fuse. (This information is provided in the illustration entitled ‘Fuse wires’). Q4. Transformers are used (i) to step-up the voltage of electricity from the generator so that it can be

carried for long distances with minimum power loss, then (ii) to step-down the voltage to make it suitable for various users. The efficiency of basic transmission is 93% or more. (In the module, it is noted that losses are less than 7%).

17.16 USING AND MEASRUING ELECTRIC POWER

Aims: To familiarise students with the power ratings of common domestic appliances.

13

To inform students of the relationship between voltage, current and power in electrical appliances and to give them experience in using this relationship to make simple calculations regarding appliances.

To inform students about the units (kilowatt hours) which electricity companies use to measure the consumption of mains electricity and give them experience in estimating the cost of running different appliances.

Activities: Discuss with students the range of household (and other) electrical appliances they are likely to be

familiar with. Have examples of as many as possible to show them (you could ask students in advance to bring these). Get them to look at the labels and identify the power rating (and voltage) of each.

Introduce the relationship Power (watts) = Voltage (volts) × Current (amps) and its alternative representations in the formulae P = VI or I = P/V. Remind them that watts are joules per second. (For students who want to know how we get joules out of volts and amps, see the textbox at the end of the module).

Give students practice in calculating the current carried by typical local domestic appliances using I = P/V, and discuss what fuses or trip switches might be appropriate. Remind students also that the current in the main power circuit will be the sum of the currents in each parallel branch!

Go on to introduce kilowatt hours as the units used by electricity companies to charge for electrical energy. Show students a local electricity metre and make them familiar with it. Establish that 1 kWh = 3.6 MJ and inform them what the local tariff is for one kWh..

Use the questions at the end of the module for homework or small-group work in class. Discuss students’ answers and be encouraging, but take the opportunity to correct any misunderstandings. Question 2 could lead to valuable follow up work. In addition to discussing the fusing needs associated with different appliances, you could ask students to work out the cost of using each device for one hour.

Answers: Q1. The symbol ‘~’ stands for ‘ac’ or alternating electric current. (ii) An ac adaptor is a transformer and

rectifier that converts high voltage ac mains electricity to low voltage ‘dc’ or direct current. (iii) A heating element is the part of an electric heating appliance that gets hot; it is made of resistance wire, usually in the form of a coil. (iv) A power circuit is the circuit in a building to which and number of power points are connected in parallel. (v) An electricity metre is a metre that measures the amount of mains electricity used by a consumer in kilowatt hours; it is usually supplied by the electricity company. (vi) A kilowatt hour is the energy converted when a device with a power of 1 kW is used for one hour; this is equal to 3.6 MJ of energy.

Q2. The list will depend on the appliances used locally. The table on the next page includes some typical values, and shows how to estimate running costs for follow-up work. (Note that appliances with heaters are the expensive ones to run!).

Q3. Given that the voltage (V) is 230 volts, and that the current (I) has already been calculated as 10 amps, we can use R = V/I. So the resistance of the heating element is 230/10 = 23 ohms.

Q4. The first law of thermodynamics states that when energy is changed from one form to another the total quantity of energy remains the same (Module 14.4). So we can convert energy from one form to another but we cannot use it in the sense of destroying it – it is still there in a different form (although as the second law of thermodynamics tells us, some energy is always wasted as unwanted heat).

Appliance(rated for use with 230 volts ac)

Typical power rating (watts)

Current (amps)

Cost per hour = power (kW) × tariff (t)

RadioLight bulb (with filament)Light bulb (fluorescent tube)FanTVComputerRefrigeratorWashing machine

540 to 10011 to 4010 to 5050 to 15020 to 200100 to 500500

0.020.18 to 0.430.05 to 0.180.04 to 0.220.22 to 0.650.09 to 0.870.4 to 2.22.2

0.005 × t0.04 to 0.1 × t0.01 to 0.04 × t0.01 to 0.05 × t0.05 to 0.15 × t0.02 to 0.2 × t0.1 to 0.5 × t0.5 × t

14

Hair dryerAir conditionerHeater or heating ring on cooker

1000 to 15001000 to 25001000 to 2500

4.3 to 6.54.3 to 10.94.3 to 10.9

1.0 to 1.5 × t1.0 to 2.5 × t1.0 to 2.5 × t

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