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PHY 184. Spring 2007 Lecture 18. Title: Resistor Circuits. Announcements. Midterm 1 will take place in class tomorrow Chapters 16 - 19 Homework Sets 1 - 4 You may bring one 8.5 x 11 inch sheet of equations, front and back, prepared any way you prefer. Bring a calculator

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phy 184

PHY 184

Spring 2007

Lecture 18

Title: Resistor Circuits

184 Lecture 18

announcements
Announcements
  • Midterm 1 will take place in class tomorrow
  • Chapters 16 - 19
    • Homework Sets 1 - 4
    • You may bring one 8.5 x 11 inch sheet of equations, front and back, prepared any way you prefer.
    • Bring a calculator
    • Bring a No. 2 pencil
    • Bring your MSU student ID card
  • We will post Midterm 1 as Corrections Set 1 after the exam
    • You can re-do all the problems in the Exam
    • You will receive 30% credit for the problems you missed
      • To get credit, you must do all the problems in Corrections Set 1, not just the ones you missed

184 Lecture 18

seating instructions thursday

Section 2

Seating Instructions Thursday

  • Please seat yourselves alphabetically.
  • Sit in the row (C, D,…) corresponding to your last name alphabetically.
  • For example, Bauer would sit in row C, Westfall in row O.
  • We will pass out the exam by rows.

Fall Semester 2006

Midterm 1

Section 1

Alphabetical Seating Order

184 Lecture 18

review temperature dependence
Review - Temperature Dependence
  • The temperature dependence of the resistance of metallic conductors is given by
    • Ris the resistance at temperature T
    • R0is the resistance at temperature T0
    • is the temperature coefficient of electric resistivity for the material under consideration

184 Lecture 18

review par and ser resistors
Review – Par and Ser Resistors
  • We can replace n parallel resistors with one equivalent resistor given by
  • We can replace n series resistors with one equivalent resistor given by

184 Lecture 18

example network of resistors
Example: Network of Resistors
  • Consider the network of resistors shown below
  • Calculate the current flowing in this circuit.

184 Lecture 18

example network of resistors 2
Example: Network of Resistors (2)
  • Ok, let’s look at it. R3 and R4 are in series
  • Now note that R34 and R1 are in parallel

184 Lecture 18

example network of resistors 3
Example: Network of Resistors (3)
  • And now R2, R5, R6, and R134 are in series

184 Lecture 18

clicker question
Clicker Question
  • Consider the circuit on the right.
  • Which statement is correct?

A) R2 and R3 are in parallel

B) R1 and R3 are in series

C) R1 and R2 are in parallel

D) Several statements above are correct

184 Lecture 18

clicker question1
Clicker Question
  • Consider the circuit on the right.
  • Which statement is correct?

C) R1 and R2 are in parallel

R1 and R2 have the same voltage across them. R2 and R3 do not have the same voltage drop, so they cannot be in parallel. R1 and R3 do not have the same current flowing through them, so they cannot be in series.

184 Lecture 18

more resistors
More resistors …
  • The figure shows a circuit containing one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 .
  • What is the current through the battery?

Idea: Find the equivalent resistance and

use Ohm’s Law.

R2 and R3 are in parallel.

184 Lecture 18

more resistors1
More resistors …
  • R23=12 W
  • What is the current through the battery?

R1, R23 andR4 are in series.

184 Lecture 18

more resistors2
More resistors …
  • The circuit contains one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 .
  • What is the current i2 through R2?

Key Idea 1: R2 and R3 are in parallel, so they have the same voltage

drop V2=V3=V23

Key Idea 2: R1, R23 and R4 are in series so they have the same current

V23=iR23 =(0.3 A)(12)=3.6 V

184 Lecture 18

more resistors3
More resistors …
  • The figure on the right shows a circuit containing one ideal 12 V battery (no internal resistance) and 4 resistors with R1=20, R2=20 , R3=30, and R4=8 .
  • What is the current i3 through R3?

Key Idea: Conservation of charge

tells us that the current i going through R23 must be equal to the sum of the currents through R2 and R3.

184 Lecture 18

light bulbs in parallel and in series
Light Bulbs in Parallel and in Series

+12 V

- 12V

In parallel:

Observation: Take out one bulb, nothing happens to the others

Assume: the bulbs are almost identical and have the same resistance

184 Lecture 18

clicker question2
Clicker Question

+12 V

- 12V

In parallel:

What voltage drop will be measured across one light bulb?

A) 12 V

B) 24 V

C) 36 V

184 Lecture 18

clicker question3
Clicker Question

+12 V

- 12V

In parallel:

What voltage drop will be measured across one light bulb?

B) 24 V

Since the bulbs are wired in parallel: the voltage drop is the same for all and equal to the voltage supplied by the emf device

184 Lecture 18

light bulbs in parallel and series
Light Bulbs in Parallel and Series

+12 V

- 12V

In series:

Observation: Taking one bulb out breaks the circuit. The more bulbs we put in series, the dimmer they get!

Assume: the bulbs are almost identical and have the same resistance

184 Lecture 18

clicker question4
Clicker Question

+12 V

- 12V

In series:

What voltage drop will be measured across one light bulb?

A) 8 V

B) 12 V

C) 24 V

184 Lecture 18

clicker question5
Clicker Question

+12 V

- 12V

In series:

What voltage drop will be measured across one light bulb?

A) 8 V

In series: Vemf=V1+V2+V3, all resistances are the same.

We measure Vemf/3=24/3=8 V across each bulb

184 Lecture 18

energy and power in electric circuits
Energy and Power in Electric Circuits
  • Consider a simple circuit in which a source of emf with voltage V causes a current i to flow in a circuit.
  • The work required to move a differential amount of charge dq around the circuit is equal to the differential electric potential energydU given by
  • The definition of current is
  • So we can rewrite the differential electric potential energy as
  • The definition of powerP is
  • Pitting it together

184 Lecture 18

energy and power
Energy and Power
  • The power dissipated in a circuit or circuit element is given by the product of the current times the voltage.
  • Using Ohm’s Law we can write equivalent formulations of the power
  • The unit of power is the watt (W).
  • Electrical devices are rated by the amount of power they consume in watts.
  • Electricity bill is based on how many kilowatt-hours of electrical energy you consume.
  • The energy is converted to heat, motion, light, …

with

kW h = power times time

1 kW h = 1000 W X 3600 s = 3.6 x 106 joules

184 Lecture 18

temperature dependence of the resistance of a light bulb
Temperature Dependence of the Resistance of a Light Bulb
  • A 100 W light bulb is connected to a source of emf with Vemf = 100 V.
  • When the light bulb is operating, the temperature of its tungsten filament is 2520 °C.
  • Question:
  • What is the resistance of the light bulb at room temperature (20 °C)?
  • Answer:
  • Power when lighted

184 Lecture 18

temperature dependence of the resistance of a light bulb 2
Temperature Dependence of the Resistance of a Light Bulb (2)
  • … so
  • The temperature dependence of the resistance
  • … solve for the resistance at room temperature, R0
  • Look up the temperature coefficient for tungsten …

184 Lecture 18

total energy in a flashlight battery
Total Energy in a Flashlight Battery
  • A standard flashlight battery can deliver about 2.0 Wh of energy before it runs down.
  • If a battery costs US$ 0.80, what is the cost of operating a 100 W lamp for 8.0 hours using standard batteries?

Answer: $320

184 Lecture 18