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5 – 19 Do Now Problems1. Define electric current and give the associated units

2. What is the formula associated with Ohm’s law?

3. Differentiate between the terms voltage source and potential difference

Homework Read & outline 34.5 – 34.11

Water vs. Electric Current

There are many analogous properties between water and electric current

Characteristics such as

•Pressure•Volume

•Flow

PressureIn an electrical system pressure is measured in

A higher voltage pushes the charges through the wire with greater force

In a water system we

measure water

pressure infoot lbs or PSI (pounds per square in)

Volts

Pressure

How a water tower works:1. Pump station2. Reservoir3. Water user

Voltage• Voltage is a measure of electric

potential energy, just like height is a measure of gravitational potential energy.

• Voltage is measured in volts (V). Using a voltmeter

• A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

Voltage

• Since 1 joule per second is a watt (power), you can interpret voltage as

• measuring the available electrical power per amp of current that flows.

Voltage Source

A pump is a source of fluid pressure

difference

A battery is a source of electrical

potential difference

Voltage & Batteries• The positive end of a 1.5 volt

battery is 1.5 volts higher than the negative end.

• If you connect batteries positive-to-negative, each battery adds 1.5 volts to the total.

• Three batteries make 4.5 volts.

• Each unit of current coming out of the positive end of the three-battery stack has 4.5 joules of energy.

Voltage source

Charges do NOT flow unless there is potential difference

The battery or source is represented by an escalator which raises charges to a higher level of energy.

A voltage source is needed to provide a sustained potential difference i.e. batteries or generators

VOLTAGE causes current

VOLTAGE causes current

Current and voltage• A battery uses chemical energy to create a

voltage difference between its two terminals.

• In a battery, chemical reactions provide the energy to pump the current from low voltage to high voltage.

• A fully charged battery adds energy proportional to its voltage.

Volume

In our water system volume is in

The Electrical equivalent is a

Coulomb is a unit of charge, consists of

6.3 x 1018 electrons (a big number indeed)

cubic feet (ft3) or liters Coulomb

Electric Charge

• A building up at the negative terminal of a cell/battery which then flows from this terminal as negative charges repel one another.

Flow• Fluid flowing through a system of pipes can

behave in many respects like electricity flowing in a circuit

• The main difference is that water will fill any space whereas electricity will 'fill' only a conductor

Flow• Charge can only flow when there is a

potential difference or a difference in voltage across the ends of a conductor

Current is a flow of charge

Current• Electric current is measured in units called

amperes, or amps (A) for short.

• One amp is a flow of a certain quantity of electricity in one second.

• The amount of electric current entering a circuit always equals the amount exiting the circuit.

Electric Current

What is need for current to flow?

• Electrons have a charge of –1.610-19 Coulombs

– one amp = 61018 electrons per second

By convention, electrical current flows from high voltage to low voltage

In other words, a positive current flows from higher to lower electrical potential

NO POTENTIAL DIFFERENCE = NO FLOW OF CURRENT

Recap• Relate water pressure and electric pressure.

• Units for electric pressure?

• Relate water volume to electric volume.

• Units for electric volume?

• Relate water flow and electric flow.

• Units for electric flow?

• Condition needed for electric flow.

5 – Do Now Problems

1. What condition is necessary for the sustained flow of charge in a conductor?

2. Differentiate between AC and DC.

3. What is voltage?

HW. Pg. 545 - 546 Review questions

How does current move through a circuit?

Conventional current

• Conventional current was proposed by Ben Franklin in the 1700’s.

• Scientists later discovered that the particles that carry electricity in a wire actually travel from negative to positive.

• Today, we still use Franklin’s definition.

Current Getting it Started

A potential difference is established between two points and some charges are released

The positive terminal of a battery will attract electrons charges

They will be acted on by the electrical force and start to move

Voltage source

Charges do NOT flow unless there is potential difference

The battery or source is represented by an escalator which raises charges to a higher level of energy.

A voltage source is needed to provide a sustained potential difference i.e. batteries or generators

VOLTAGE causes current

VOLTAGE causes current

Current & flow of chargesElectrical circuits, consist of current in the wire moving from higher to lower electrical potential

Current Electricity Example• The continuous flow of charge

in a complete circuit.

What does a battery do?

• A battery uses chemical energy to move charges.

• If you connect a circuit with a battery the charges flow out of the battery carrying energy.

Effects of Electric Current on the Body

Shock examples

Electrical Resistance and Ohm’s LawKey Question:

How are voltage, current, and resistance related?

Electrical resistance

• Resistance measures how difficult it is for current to flow.

Electrical Resistance• The total amount of electrical resistance in a circuit

determines the amount of current that in the circuit for a given voltage.

• The more resistance the circuit has, the less current that flows.

The ohm• Resistance is measured in ().

• One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.

Ohm’s Law The formulae

The resistance of electrical devices

• The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω).

• Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.

Ohm Quiz

Ohm plate

Ohm on the range

Broken Ohm

Ice cream Ohm

Ohm sweet Ohm Ohm Alone

Practice Quiz

1. If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12 volt battery?

12 V / 100000 = = 0.00012 A

2. If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24 volt battery, how much current would you draw?

= 24 V / 1000 = 0.024 ASee

slide 18

More Practice ProblemsI = V/R

I = 110 V / 2200 Ώ

I = .05 A

R = V/I

R = 110 V/ .5A

R = 220 Ώ

V = IR

V = 1.2 A (100 Ώ)

V = 120 V

Electric Resistance1. Length

as the electrons will have more distance to move and will therefore be bumping around more.

resistance varies with the length of a conductor

Ex: The length of a conductor is similar to the length of a hallway.  A shorter hallway would allow people to move through at a higher rate than a longer one. 

2. Cross-sectional area

resistance varies inversely with the cross sectional area

If the area doubles then the resistance is halved.

The wider the pipe, the easier it is for water to flow through…………………………………………………

Type of material3. Type of material

good conductors have low resistance and many free electrons

electrical conductance is measured in siemens (S). the inverse of the resistance

Temperature4. Temperature

most materials experience an increase in resistance as their temperature increases

experience a decrease in resistance as their temperature decreases

due to the kinetic theory of matter…………………………………………………

…………………………………………………

Electric Power

                                                               

Power  = Volts x Amps 

The work done by an electric current moving through a circuit is given by              

1 watt = 1 volt x 1 amp

W = V I t

Power EquationsPower (P)

Rate at which work is performedMeasured in watts (W)

•Power• P = V I P = (I R) I = I 2 R

• Decided to power homes with DC power– DC means direct current: just like what batteries

deliver

Getting Power to Our HomesThomas Edison's GE company held the patents on the DC generators being used in the early days

• Do we want power plants close to our home?

• Probably NOT…so we need to have– an ability to “ship” electricity across states

• So power lines are long– Therefore resistance no longer negligible

Getting Power to Our Homes

Power Dissipated in an Electricity Distribution System

120 WattLight bulb

12 Volt Connection Box

Power Plant on Colorado River

150 miles

• Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 W/m 2105 m

= 20 Ohms• We can figure out the current required by a single bulb

using P = VI so I = P/V

=120 Watts/12 Volts

= 10 Amps

Power Dissipated in an Electricity Distribution System

120 WattLight bulb

12 Volt Connection Box

Power Plant on Colorado River

150 miles

• Power in transmission line is P = I 2R = 102 20 = 2,000 Watts!!

• “Efficiency” is έ = 120 Watts/4120 Watts = 0.3%

The Tradeoff• The big problem is the high current

through the (fixed resistance) transmission lines

• Need less current• The square in I 2R that has the most effect

• Our appliances needs a certain amount of power

• I = P /V less current means higher voltage

The Solution• The Solution is high voltage transmission• I = 120 Watts/12 kV = 0.01 Amps for one

bulb, giving P = I 2R = (0.01)220 = 2010-4 Watts, so

P = 0.002 Watts dissipated in transmission line• Efficiency in this case is • έ = 120 Watts/120.004 = 99.996%

The Tradeoff

• But having high voltage in each household is a recipe for disaster– sparks every time you plug something in– risk of fire– not cat-friendly

• We need a way to step-up/step-down voltage at will– We can’t do this with DC, so go to AC

Basic Terminology• Direct Current (DC) – charge

flows in one direction.– Batteries– Solar Panels

Basic Terminology• Alternating Current (AC)

– Magnitude & direction of current flow periodically change

– Each sequence called a cycle– Frequency is cycles per second

(Hz)

AC Circuits

• An AC voltage source reverses the positive and negative terminals many times per second.

• The current flows one direction and then the other then the other…..

• This changing of polarity or cycling is called frequency, measured in cycles per second (hertz).

Why AC?• AC is like a battery where the terminals

exchange sign periodically!

• approx. 60 cycles per sec in household current

• AC sloshes back and forth in the wires

• Although net electron flow over one cycle is zero, useful work can still be done!

• Imagine sawing (back & forth), or rubbing hands together to generate heat

DC (direct current) and AC (alternating current)

I

Batteries produce DC Household/power plant: AC

DC vs AC

DC•Electrons flow constantly•Electrons flow in only one direction•Batteries

AC•Electrons flow in short burst•Electrons switch directions (60 times a second)•House current

AC

Nikola Tesla invented & George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power

RESISTOR COLOR CODES

The resistor code is a color code. Around each resistor are colored stripes. Each color stands for a number. You can tell how much resistance by looking at the numbers.

•Gold or silver tells you how accurate the resistor is

•gold is (+) Or (–) 5% silver is (+) Or (–) 10%

Black = 0 Yellow = 4 Gray = 8

Brown = 1 Green = 5 White = 9

Red = 2 Blue = 6

Orange = 3 Violet = 7

RESISTOR COLOR CODESThis is how to crack the code.

Write the numbers for the first two colors. The third color tells you how many zeros to add.

Resistance is measured in ohms.

This resistor has a resistance of…..

1000 ohms

Resistors

• Electrical components called resistors can be used to control current.

• Resistors have striped color codes to record their "values" (writing on them is difficult).

Resistors

• Used to control current or potential

difference in

a circuit.

Pop Quiz

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