24-Oct-10

Chapter 23Electric Current

ResistanceOhm’s Law

Circuits

Lecture 25

24-Oct-10

Electric Current (I)Just as water current is flow of water molecules,

electric current is the flow of electric charge. In circuits, electrons make up the flow of charge.Unit of Current: 1 Ampere = 1 Coulomb/s=1A=1Amp

ON

OFF

Direct (Electric) Current -- DCDirect Current• Current always flows in same general direction• Charge moves due to a voltage difference

produced by a battery, DC generator, or solar cell.• Amount of current may vary, but current never

reverses direction.

Alternating Current (AC)

• Current reverses direction every so often. In US power system current goes through a set of two reversals 60 times per second (60Hz.)

• Charge moves due to a alternating voltage difference produced by an ac generator (alternator).

• Commercial electric power throughout the world uses ac. As we will see later, “transformers” can step ac from an alternator up to very high voltage for transmission over great distances with small heat losses, then step it back down to lower voltage for safety where energy is consumed.

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Direct & Alternating CurrentDirect current (DC) is

current that flows in only one direction.

Alternating current (AC) is current that flows back and forth with alternating direction.

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AmmeterAmmeter measures electrical current.

Battery

CU

RR

EN

T

Ammeter

Ammeter

Current increases as the voltage increases.

Due to charge conservation, same current into and out of light bulb.

Light Bulb

Ammeter readings the same.

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Voltage SourcesCharges flow only when they are “pushed” or “driven.” A

sustained current requires a suitable pumping device to provide a difference in electric potential (a voltage).

Simple Chemical Battery Simple Mechanical Generator

Lemon

Copper

Aluminum

Chemical BatteryBatteries separate

positive and negative charges by using a chemical reaction.

Chemical potential energy is converted into electric potential energy.

Rechargeable BatteryEventually the battery’s chemicals are consumed

unless the reaction can be reversed by passing a current into the battery.

Automobile battery is recharged while the gasoline engine is running since the engine powers a generator that produces a recharging current.

Starting the car

Engine running

Electric CurrentA battery uses chemical reactions to produce a

potential difference between its terminals. It causes current to flow through the flashlight bulb similar to the way the person lifting the water causes the water to flow through the

paddle wheel.

Direction of Electric CurrentThe direction of current flow – from the positive terminal to the negative one – was decided before it was realized that electrons are negatively charged. Therefore, we show current flowing around a circuit in the direction a positive chargewould move; electrons move the other way.

Battery

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Electrical Resistance (R)Current depends not only on the

voltage but also on the electrical resistance of whatever the current is flowing through. For a “wire” --

The wider the wire is, the less the resistance.

A short wire offers less resistance than a long wire.

Metals offer less resistance than other materials.

More water flows through a thick hose than through a thin one connected to a city's water system (same water pressure).

Jumper cables use thick wires so as to minimize the electrical resistance.The cables are usually copper wire.

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Ohm’s LawThe relation between the current flowing through a

device, the voltage across the device, and resistance of the device is Ohm’s law,

(Current) =

I = V/R

Ampere is unit of current; symbol is AVolt is unit of voltage; symbol is VOhm is unit of resistance; symbol is Ω

(Voltage)(Resistance)

When you double the voltage in a simple electric circuit, you double the

A. current.B. resistance.C. Both A and B.D. Neither A nor B.

Ohm’s LawCHECK YOURSELF

Current = resistancevoltage

When you double the voltage in a simple electric circuit, you double the

A. current.B. resistance.C. Both A and B.D. Neither A nor B.

Explanation:

This is a straightforward application of Ohm’s law.

Ohm’s LawCHECK YOUR ANSWER

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Example: Car HeadlampA 12V car battery is attached to a headlamp and 2

A of current flows. What is the resistance of the headlamp? .

12V

CU

RR

EN

T

Headlamp

Ammeter

+-

24-Oct-10

Example: Car HeadlampA 12V car battery is attached to a headlamp and 2

A of current flows. What is the resistance of the headlamp? .

12V

CU

RR

EN

T

Headlamp

Ammeter

+-

Ohm’s Law: I = V/R

R = V/I

= (12V)/(2 A) = 6 Ω

How much current flow would there be if two batteries were put together to get a voltage of 24V ?

Current in Nerves & MusclesNervous systems in animals

use electrical currents to signal the contraction and relaxation of muscles.

Frog leg jumps when electrical current passes through it.

Conduction in Human Heart

The most important electrical signal in our body is the periodic signal that contracts and relaxes our heart muscle to pump blood.

SA

AV

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Current and Electric ShockThe damaging effects of shock are the result

of current passing through the body.

Current (A) Effect

0.001 Can be felt

0.005 Is painful

0.010 Causes involuntary muscle contractions (spasms)

0.015 Causes loss of muscle control

0.070 If through the heart, serious disruption; probably fatal if current lasts for more than 1 s

Effects of Electric Shock on Human BodyFrom Ohm's law, current depends on voltage and on electrical resistance.

When dry, skin’s resistance around 100,000 Ω.

Resistance drops as low as 100 Ω when wet and salty.

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Check Yourself

If your resistance is 100,000 Ω, what is the current passing through you when you touch an electric socket (120 volts)?

What if your resistance is only 100 Ω?

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Check Yourself

If your resistance is 100,000 Ω, what is the current passing through you when you touch an electric socket (120 volts)?I = V/R = (120 V)/(105 Ω) = 0.0012 A

What if your resistance is only 100 Ω?

Current will be 1000 times larger; 1.2 A

Third prong is “safety” ground; connects case of appliance to “ground”

Shock Prevention: Grounding Appliances

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Ohmic HeatingAs current flows through a

resistance, heat is produced. Flowing electrons strike atoms in a conductor, heating the material.

Toaster

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Rear Window Defogger

Ohmic heating evaporates fog on car window.

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Demo: Ohmic CookingAn electric current running through a

hot dog generates enough heat to cook it.

Electric Energy and PowerWhen a charge q moves across a voltage difference V, its potential energy changes:

Therefore, the electric power involved is

∆PE = qV

IVVt

qt

qVt

PEP =∆

=∆

=∆

=

I is current in Amperes; V is voltage difference in Volts

24-Oct-10 Physics 1 (Garcia) SJSU

Electric PowerPower is rate at which energy is delivered.

Power = (Voltage) x (Current)For example,

(100 Watts) = (120 Volts) x ( 5/6 Ampere)

Commercial power voltage in the US is 120 Volts.In Europe voltage is 240 Volts.

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Check YourselfA light bulb draws 60 W of power when connected

to a 120 V electric power outlet. What is the current flowing through the bulb?

What is the electrical resistance of the bulb?

How much energy does the bulb use in one second?

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Check YourselfA light bulb draws 60 W of power when connected

to a 120 V electric power outlet. What is the current flowing through the bulb?

P = IV so I = P/V = (60 W)/(120 V) = 0.5 A

What is the electrical resistance of the bulb?

I = V/R so R = V/I = (120 V)/(0.5 A) = 240 ΩHow much energy does the bulb use in one

second?P = ∆E/(∆t) so ∆E = P•(∆t) = (60 W)•(1 s) = 60J

Electric Circuit• Make electric circuit by connecting voltage

source and resistive object(s) together in a loop with metal wires that are (nearly) perfect conductors

• This is a closed circuit -- there is a continuous path through conducting material from the + terminal to the - terminal of the voltage source

9V +-

Battery (voltage source)

Resistance(light bulb)

Wires(assumed to be perfect conductors)

Electric CircuitCircuit• Any closed path around which current (electrons) can flow. • A complete circuit has a voltage source and one or more

resistances (such as light bulbs), connected by “ideal”wires. There may be a switch to open and close the circuit.

• “Ideal” wires have no resistance.

Electric circuit.Water circuit.

Circuit Diagram“Ideal” wires “Ideal” wiresCould be light bulb,

heater, etc.

+ -

Key Points of Lecture 25Key Points of Lecture 25

Before Wednesday, read Hewitt Chap. 23 (second half).

Homework #17 due by 11:00 PM Tuesday Oct. 26.

Homework #18 due by 11:00 PM Friday Oct. 29.

• Electric Current• DC & AC• Voltage Sources• Resistance• Ohm’s Law• Ohmic Heating• Electric Power• Circuits