Conductivity and Resistivity Resistance and Ohm’s Law ...phy21bio/Presentations/Phy1022Ch3132.pdf · 1 PHYS 1022: Chap. 31-32, Pg 2 Topics: • Conductivity and Resistivity •
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PHYS 1022: Chap. 31-32, Pg 2
Topics: • Conductivity and Resistivity • Resistance and Ohm’s Law
• Capacitors in series and parallel • Resistors in series and parallel • Kirchhoff’s Rules
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PHYS 1022: Chap. 31-32, Pg 3
PHYS 1022: Chap. 31-32, Pg 4
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PHYS 1022: Chap. 31-32, Pg 5 New Topic
PHYS 1022: Chap. 31-32, Pg 6
The electric field is directly proportional to the current density in the wire.
ρ is called resistivity, R is the resistance a measure of how hard a charge can move in a material
SI unit: V.m / A = Ω. m
A perfect conductor has zero resistivity. A perfect insulator has infinite resistivity The reciprocal of resistivity (1/ρ) is called conductivity σ.
€
V = RI
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PHYS 1022: Chap. 31-32, Pg 7
The resistivity of a material depends on temperature. Metal near room temperature:
PHYS 1022: Chap. 31-32, Pg 8
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PHYS 1022: Chap. 31-32, Pg 9
PHYS 1022: Chap. 31-32, Pg 10
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PHYS 1022: Chap. 31-32, Pg 11
PHYS 1022: Chap. 31-32, Pg 12
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PHYS 1022: Chap. 31-32, Pg 13
Topics: • Conductivity and Resistivity • Resistance and Ohm’s Law
• Capacitors in series and parallel • Resistors in series and parallel • Kirchhoff’s Rules
PHYS 1022: Chap. 31-32, Pg 14 New Topic
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PHYS 1022: Chap. 31-32, Pg 15
C is called an equivalent capacitor.
Since Q = C V, we have C1V+ C2V + C3V = CV
PHYS 1022: Chap. 31-32, Pg 16
1 1
1 C C 1
2 C = + 1
3 C +
Since V = Q/C, we have Q/C1+ Q/C2 + Q/C3 = Q/C
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PHYS 1022: Chap. 31-32, Pg 17
What is the equivalent capacitance of the combination
shown ?
o
o
C C C
Ceq
1) 3C/2 2) 2C/3 3) 3C 4) 2C 5) C/2
PHYS 1022: Chap. 31-32, Pg 18
How does the voltage V1 across the C1 compare to the voltage
V2 across C2?
1) V1 = V2 2) V1 > V2 3) V1 < V2 4) all voltages are
zero
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PHYS 1022: Chap. 31-32, Pg 19
How does the charge Q1 across C1 compare to the charge Q2
across C2?
1) Q1 = Q2
2) Q1 < Q2
3) Q1 > Q2
4) all charges are zero
PHYS 1022: Chap. 31-32, Pg 20
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PHYS 1022: Chap. 31-32, Pg 21
Find the charge, potential difference, and energy stored in each capacitor.
PHYS 1022: Chap. 31-32, Pg 22 New Topic
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PHYS 1022: Chap. 31-32, Pg 23
PHYS 1022: Chap. 31-32, Pg 24 Req is the reciprocal of the sum of the reciprocals.
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PHYS 1022: Chap. 31-32, Pg 25 New Topic
PHYS 1022: Chap. 31-32, Pg 26
(or: what goes in has to come out!)
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PHYS 1022: Chap. 31-32, Pg 27
“The sum of voltage drops and gains around any closed circuit loop must be zero”
Sign convention: – sign for voltage drop and +
sign voltage gain.
ΔV1 + ΔV2 + ΔV3 + ΔV4 + … = 0
It depends on several factors: 1. emf or resistor
2. direction of current 3. direction of loop travel
PHYS 1022: Chap. 31-32, Pg 28
What is the current at point P?
5 A
8 A
2 A
P
1) 2 A 2) 5 A 3) 6 A 4) 10 A
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PHYS 1022: Chap. 31-32, Pg 29
PHYS 1022: Chap. 31-32, Pg 30
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PHYS 1022: Chap. 31-32, Pg 31
PHYS 1022: Chap. 31-32, Pg 32
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PHYS 1022: Chap. 31-32, Pg 33
PHYS 1022: Chap. 31-32, Pg 34
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PHYS 1022: Chap. 31-32, Pg 35
2 Ω 4 Ω
–IR = –8 V –IR = – 4 V
+ε = 12 V
Start
+ 12 V – 8 V – 4 V = 0 ΔV =
Current in each resistor?
Total resistance = 6 Ω
Ohm’s Law: I = V / R = 12 V / 6 Ω
= 2 A
Voltage drop across resistor?
V = I R
12 V I
PHYS 1022: Chap. 31-32, Pg 36
–I2R1 –I2R2
+ε
Start + ε – I2R1 – I2R2 ΔV = = 0
+ I3R3
–ε
Start – ε + I3 R3 ΔV = = 0
The first thing we have to do is? Define currents!
I1
I2
I3
Now apply Loop rule to each loop R1 R2
R3
Next: Define travel direction: top loop (CW)
bottom loop (CW)
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PHYS 1022: Chap. 31-32, Pg 37
PHYS 1022: Chap. 31-32, Pg 38
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PHYS 1022: Chap. 31-32, Pg 39
PHYS 1022: Chap. 31-32, Pg 40
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