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Last time…

- Combining capacitors
- Begin circuits

Today…

- Resistor circuits
- Start resistor-capacitor circuits

Physics 208 Lecture 13

I2

Iin

I1

I3

I1=I2+I3

I1

I3

Iout

I2

I1+I2=I3

Kirchoff’s junction law- Charge conservation

Iout = Iin

Physics 208 Lecture 13

I

I

I

Quick QuizWhich bulb is brighter?

A

B

Both the same

Current through each must be same

Conservation of current (Kirchoff’s current law)

Charge that goes in must come out

Physics 208 Lecture 13

Work and energy in a circuit

- What happens as charge q moves?
- Uqc=qVc
- Uqd=qVd

- Ohm’s law:
- Charge has lost potential energy
- Energy lost Elost = qIR
- Energy dissipated in resistor as
- Heat (& light in bulb)

- Power dissipated in resistor =

Joules / s = Watts

Physics 208 Lecture 13

Light bulbs and power

- Household voltage is 120V
- Cost
- 24 hours on requires
- MG&E ~ 13¢ / kWatt-hour

19¢ / day

Physics 208 Lecture 13

b

a

I

R1

c

I

e

R2

d

I

Two different bulbs- Current same through each
- Power dissipated different
- Brightness different

Physics 208 Lecture 13

R

2R

=

Resistors in Series- Potentials add
- V = DV1 + DV2 = IR1 + IR2 =
= I (R1+R2)

- V = DV1 + DV2 = IR1 + IR2 =
- The equivalent resistance Req = R1+R2

- I1 = I2 = I

2 resistors in series:

R L

Like summing lengths

Physics 208 Lecture 13

Quick Quiz

What happens to the brightness of the bulb B when the switch is closed?

Gets dimmer

Gets brighter

Stays same

Something else

Battery is constant voltage,Vacross bulbBdoesn’t changeso I through bulb B doesn’t change

Physics 208 Lecture 13

R

Resistors in Parallel- DV = DV1 = DV2
- I = I1 + I2 (lower resistance path has higher current)
- Equivalent Resistance

R/2

Add areas

Physics 208 Lecture 13

Quick Quiz

What happens to the brightness of the bulb A when the switch is closed?

Gets dimmer

Gets brighter

Stays same

Something else

Physics 208 Lecture 13

….

R1

R2

R3

R4

QuestionAs more and more resistors are added to the parallel resistor circuit shown here the total current flowing I…

Increases if each Ri getting bigger

Increases if each Ri getting smaller

Always increases

Always decreases

Stays the same

Each resistor added adds V/Ri to the total current I

Physics 208 Lecture 13

5 A

12 A

Toaster

Coffee Pot

Microwave

Question

You use one power strip to plug in your toaster, coffee pot, microwave.

Everything works great until you plug in your space heater, then you smell smoke. This is because

The resistance of the circuit is too high

The voltage in the circuit is too high

The current in the circuit is too high

Physics 208 Lecture 13

More complicated circuits

- Both series & parallel
- Determine equivalent resistance
- Replace combinations with equivalent resistance

Physics 208 Lecture 13

Quick Quiz

The circuit below contains three 100W light bulbs. The emf e = 110 V. Which light bulb(s) is(are) brightest ?

A. A

B. B

C. C

D. B and C

E. All three are equally bright.

Physics 208 Lecture 13

DV

V

I

R

R

IA

A

e

e

Ammeter

Electrical measurements- A multimeter can measure currents (as an ammeter), potential difference (as a voltmeter)
- Electrical measuring devices must have minimal impact in the circuit

DVV

Voltmeter

IV

IR

DVA

I

- The internal resistance of the
- ammeter must be very small
- I = IA
- = DV+DVA = RI + rAI RI
for rA 0

The internal resistance of the

voltmeter must be very large

I = Iv+IR

DVV = e

Physics 208 Lecture 13

Resistor-capacitor circuit

- Why does QC (charge on capacitor) depend on time after switch is closed?

- Qc decreases: does Qc change faster, slower, same?

- R increases: does Qc change faster, slower, same?

- C increases: does Qc change faster, slower, same?

Physics 208 Lecture 13

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