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Last time…. Combining capacitors Begin circuits. Today…. Resistor circuits Start resistor-capacitor circuits. Resistors. Circuits. Physical layout. Schematic layout. I 2. I in. I 1. I 3. I 1 =I 2 +I 3. I 1. I 3. I out. I 2. I 1 +I 2 =I 3. Kirchoff’s junction law.

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Last time
Last time…

  • Combining capacitors

  • Begin circuits

Today…

  • Resistor circuits

  • Start resistor-capacitor circuits

Physics 208 Lecture 13


Resistors
Resistors

Circuits

Physical layout

Schematic layout

Physics 208 Lecture 13


Kirchoff s junction law

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


Quick quiz

I

I

I

I

Quick Quiz

Which 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
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

60 Watt

Light bulbs and power

  • Household voltage is 120V

  • Cost

    • 24 hours on requires

    • MG&E ~ 13¢ / kWatt-hour

19¢ / day

Physics 208 Lecture 13


Two different bulbs

I

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


Resistors in series

R

R

2R

=

Resistors in Series

  • Potentials add

    • V = DV1 + DV2 = IR1 + IR2 =

      = I (R1+R2)

  • The equivalent resistance Req = R1+R2

  • I1 = I2 = I

2 resistors in series:

R  L

Like summing lengths

Physics 208 Lecture 13


Quick quiz1
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


Resistors in parallel

R

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 quiz2
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


Question

I

….

R1

R2

R3

R4

Question

As 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


10 A

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
More complicated circuits

  • Both series & parallel

    • Determine equivalent resistance

    • Replace combinations with equivalent resistance

Physics 208 Lecture 13


Quick quiz3
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


Electrical measurements

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


Kirchoff s loop law
Kirchoff’s loop law

R1

I1

e

R2

R3

  • Conservation of energy

I2

I3

Physics 208 Lecture 13


Resistor capacitor circuit
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


Rc discharge
RC discharge

  • RC time constant

time t

Physics 208 Lecture 13


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