2/6/07184 Lecture 171 PHY 184 Spring 2007 Lecture 17 Title: Resistance and Circuits.
2/7/07184 Lecture 181 PHY 184 Spring 2007 Lecture 18 Title: Resistor Circuits.
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Transcript of 2/7/07184 Lecture 181 PHY 184 Spring 2007 Lecture 18 Title: Resistor Circuits.
2/7/07 184 Lecture 18 1
PHY 184PHY 184PHY 184PHY 184
Spring 2007Lecture 18
Title: Resistor Circuits
2/7/07 184 Lecture 18 2
AnnouncementsAnnouncementsAnnouncementsAnnouncements 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
2/7/07 184 Lecture 18 3
Seating Instructions ThursdaySeating Instructions ThursdaySeating Instructions ThursdaySeating Instructions Thursday
Fall Semester 2006Midterm 1Section 1
Alphabetical Seating Order
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.
Section 2
2/7/07 184 Lecture 18 4
Review - Temperature DependenceReview - Temperature DependenceReview - Temperature DependenceReview - Temperature Dependence
The temperature dependence of the resistance of metallic conductors is given by
• R is the resistance at temperature T
• R0 is the resistance at temperature T0
• is the temperature coefficient of electric resistivity for the material under consideration
R R0 R0 T T0 R R0 R0 T T0
0 0 T T0 0 0 T T0
2/7/07 184 Lecture 18 5
Review – Par and Ser ResistorsReview – Par and Ser ResistorsReview – Par and Ser ResistorsReview – 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
1
Req
1
Rii1
n
1
Req
1
Rii1
n
Req Rii1
n
Req Rii1
n
2/7/07 184 Lecture 18 6
Example: Network of ResistorsExample: Network of ResistorsExample: Network of ResistorsExample: Network of Resistors
Consider the network of resistors shown below
Calculate the current flowing in this circuit.
2/7/07 184 Lecture 18 7
Example: Network of Resistors (2)Example: Network of Resistors (2)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
R34 R3 R4
1
R134
1
R1
1
R34
or R134 R1R34
R1 R34
2/7/07 184 Lecture 18 8
Example: Network of Resistors (3)Example: Network of Resistors (3)Example: Network of Resistors (3)Example: Network of Resistors (3)
And now R2, R5, R6, and R134 are in series
R123456 R2 R5 R6 R134
R123456 R2 R5 R6 R1R34
R1 R34
R123456 R2 R5 R6 R1 R3 R4 R1 R3 R4
i VemfR123456
2/7/07 184 Lecture 18 9
Clicker QuestionClicker QuestionClicker QuestionClicker 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
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
2/7/07 184 Lecture 18 10
Clicker QuestionClicker QuestionClicker QuestionClicker Question Consider the circuit on the right. Which statement is correct?
C) R1 and R2 are in parallel
QuickTime™ and a
TIFF (Uncompressed) decompressorare needed to see this picture.
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.
2/7/07 184 Lecture 18 11
More resistors …More resistors …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.
2/7/07 184 Lecture 18 12
More resistors …More resistors …More resistors …More resistors …
R23=12 What is the current through the battery? R1, R23 and R4 are in series.
2/7/07 184 Lecture 18 13
More resistors …More resistors …More resistors …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 currentV23=iR23 =(0.3 A)(12)=3.6 V
2/7/07 184 Lecture 18 14
More resistors …More resistors …More resistors …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.
2/7/07 184 Lecture 18 15
Light Bulbs in Parallel and in SeriesLight Bulbs in Parallel and in SeriesLight Bulbs in Parallel and in SeriesLight 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
2/7/07 184 Lecture 18 16
Clicker QuestionClicker QuestionClicker QuestionClicker 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
2/7/07 184 Lecture 18 17
Clicker QuestionClicker QuestionClicker QuestionClicker 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
2/7/07 184 Lecture 18 18
Light Bulbs in Parallel and SeriesLight Bulbs in Parallel and SeriesLight Bulbs in Parallel and SeriesLight 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
2/7/07 184 Lecture 18 19
Clicker QuestionClicker QuestionClicker QuestionClicker 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
2/7/07 184 Lecture 18 20
Clicker QuestionClicker QuestionClicker QuestionClicker 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
2/7/07 184 Lecture 18 21
Energy and Power in Electric CircuitsEnergy and Power in Electric CircuitsEnergy and Power in Electric CircuitsEnergy 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 energy dU given by
The definition of current is
So we can rewrite the differential electric potential energy as
The definition of power P is
Pitting it together
dU dqV
i dq / dt
dU idtVP dU / dt
P dU
dtidtV
dtiVP
dU
dtidtV
dtiV
2/7/07 184 Lecture 18 22
Energy and PowerEnergy and PowerEnergy and PowerEnergy 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, …
P iV i2R V 2
Rwith
kW h = power times time
1 kW h = 1000 W X 3600 s = 3.6 x 106 joules
2/7/07 184 Lecture 18 23
Temperature Dependence of the Temperature Dependence of the Resistance of a Light BulbResistance of a Light Bulb
Temperature Dependence of the Temperature Dependence of the Resistance of a Light BulbResistance 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
P V 2
R
2/7/07 184 Lecture 18 24
Temperature Dependence of the Temperature Dependence of the Resistance of a Light Bulb (2)Resistance of a Light Bulb (2)
Temperature Dependence of the Temperature Dependence of the Resistance of a Light Bulb (2)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 …
R V 2
P
100 V 2
100 W100
R R0 R0 T T0
R R0 R0 T T0 R0 1 T T0 R0
R
1 T T0
R0 R
1 T T0 100
1+ 4.5 10-3 C-1 2520 C 20 C 8.2
2/7/07 184 Lecture 18 25
Total Energy in a Flashlight BatteryTotal Energy in a Flashlight BatteryTotal Energy in a Flashlight BatteryTotal 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