Electricity Electricity

Part 2: Electric CurrentPart 2: Electric Current

A conductor/dielectric isA conductor/dielectric is

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1.1. A material in which A material in which charges are free of charges are free of move about move about

2.2. A material in which A material in which charges are NOT charges are NOT free to move aboutfree to move about

3.3. A material that A material that shields a region shields a region from electric fieldsfrom electric fields

When a charged particle with charge, Q, is When a charged particle with charge, Q, is moved through a distance, d, in a uniform moved through a distance, d, in a uniform electric field E, the work done is given byelectric field E, the work done is given by

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1.1. QE/dQE/d

2.2. Qd/EQd/E

3.3. QEdQEd

The Unit for electric fields is The Unit for electric fields is

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1.1. VoltVolt

2.2. Newton/CoulombNewton/Coulomb

3.3. Volt/meterVolt/meter

4.4. All of the aboveAll of the above

5.5. 1 and 2 1 and 2

6.6. 2 and 32 and 3

7.7. None of the aboveNone of the above

Electric current is the amount of Electric current is the amount of charge moving past a pointcharge moving past a point

Maxwell Demon counting the charges passing a given point

Definition of currentDefinition of currentI = currentI = current

Q = amount of charge that passes point. Q = amount of charge that passes point.

t = time for charge to pass by.t = time for charge to pass by.

IQ

t

Units of CurrentUnits of Current

IQ

t

C ou lom b

sec

1 Ampere (Amp) = 1 Coulomb/second

Electric currents only flow in Electric currents only flow in wires.wires.

1 2

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1.1. TrueTrue

2.2. FalseFalse

Examples of CurrentsExamples of Currents

Solar wind interacting with the earths magnetic field

1) Coronal Mass Ejection

2) Aurora from space

3) Aurora from ground

CME animation

Aurora from space

Aurora effects

3 electron beams in a color TV3 electron beams in a color TV

Tokamak Fusion ExperimentsTokamak Fusion ExperimentsJET discharge

We We usually think of currents inusually think of currents in wires wiresOpening switch

Simple circuitSimple circuit

When a charged When a charged particle passes particle passes through the battery, it through the battery, it gains energy.gains energy.

When the particle When the particle passes through the passes through the light bulb it gives up light bulb it gives up the energy as heat.the energy as heat.

Ohm’s LawOhm’s LawV=IRV=IR

V= Voltage of the V= Voltage of the Battery.Battery.

I=current in circuit.I=current in circuit.

R=Resistance in the R=Resistance in the bulb/resistor. bulb/resistor. (Depends on (Depends on materials and materials and geometry.)geometry.)

Units of ResistanceUnits of Resistance

R=V/I (volts/amps)R=V/I (volts/amps)

By definition, 1Ohm = 1 volt/amp,By definition, 1Ohm = 1 volt/amp,

oror

11=1V/A.=1V/A.

Log Ride Analogy

Water circuit analogyWater circuit analogy

Example problemExample problem

How many amps of current would flow in How many amps of current would flow in a light bulb that has a resistance of 60a light bulb that has a resistance of 60 if if it is connected to a 12 V battery.it is connected to a 12 V battery.

I V R

IV

A

/

.1 2

6 00 2

Power in a circuitPower in a circuit

When Charge When Charge Q Q passes through the passes through the battery it gains an battery it gains an amount of energy amount of energy

E=(E=(Q)VQ)V

(This is the amount of (This is the amount of work the battery does work the battery does on the charge.)on the charge.)

If the charge takes an amount of time If the charge takes an amount of time t to t to pass through the battery, the battery supplies pass through the battery, the battery supplies a power of (does work at a rate of) a power of (does work at a rate of)

PE

t

Q

tV

But

Q

tI

Thus

P IV

The power supplied by the battery must be The power supplied by the battery must be dissipated in the resistor.dissipated in the resistor.

We also know the V=IR.We also know the V=IR.

Power dissipated in resistorPower dissipated in resistor

P IV I IR

P I R

( )2

Example CalculationExample Calculation

What is the resistance What is the resistance and how much and how much current flows through current flows through a 100 W bulb?a 100 W bulb?

Note: The wattage on a Note: The wattage on a bulb is its power bulb is its power output and assumes output and assumes that you will use it in that you will use it in the US where the the US where the voltage in 110 V.voltage in 110 V.

I P V

I WV A

R V I

R VA

/

.

/

.

1 0 011 0 0 9 1

11 00 9 1 1 2 1

Redo the calculations for a 60 W bulbRedo the calculations for a 60 W bulb

I P V

I WV A

R V I

R VA

/

.

/

.

6 011 0 0 5 5

11 00 5 5 2 0 0

If we want to increase the power If we want to increase the power output of a light bulb we shouldoutput of a light bulb we should

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1.1. Increase the Increase the resistance of the resistance of the filamentfilament

2.2. Decrease the Decrease the resistance of the resistance of the filamentfilament

3.3. None of the None of the above.above.

If we were to use a bulb made for the US If we were to use a bulb made for the US (where V=110V) and use it in Europe (where V=110V) and use it in Europe

(V=220V) what would happen to the current (V=220V) what would happen to the current in the bulb relative to the US value.in the bulb relative to the US value.

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68%

4%

28%

1.1. The current would The current would doubledouble

2.2. The current would The current would be cut it halfbe cut it half

3.3. The current would The current would remain the same.remain the same.

What would happen to the What would happen to the power dissipated by the bulb?power dissipated by the bulb?

1 2 3 4 5

44%

15%

4%

33%

4%

1.1. It would quadrupleIt would quadruple

2.2. It would doubleIt would double

3.3. It would not It would not changechange

4.4. It would be cut in It would be cut in halfhalf

5.5. It would be cut by It would be cut by a factor of 4.a factor of 4.