PHY 2049: Physics II
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Tutoring Center is open in room NPB 1215,
M-F 12:00AM -4:00PM. It is free.
HITT 1
You buy a “75W” light bulb. The label means that:
A. no matter how you use the bulb, the power will be
75W
B. the bulb was filled with 75W at the factory
C. the actual power dissipated will be much higher than
75W since most of the power appears as heat
D. the bulb is expected to burn out after you use up its
75W
E. none of the above
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Currents
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Sample problem
26-2.
A. 1.9 A
B. 7.1 A
Resistances
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R = ρl/A
Rank them by current, lowest first.
Superconductors
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Resistance is zero –transmission losses,
switches
Magnetic field is expelled - levitation
Small thermal conductivity - thermal
switches
SQUID - low fields and low voltages
Circuits, Junctions
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Charge can not be
created, nor
destroyed
Total Current in =
Total current out
i = 8A -
Resistances n circuit
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in series R = R1 + R2
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In parallel 1/R = 1/R1 + 1/R2
Work and emf
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Pumping of charges
Potential difference around a loop adds
to zero
Resistances
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Each of the resistors in the diagram has
a resistance of 12 Ω. The resistance of
the entire circuit is
More Resistance in parallel
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A coaxial cable consists of a wire of
material A, resistivity ρA radius R1,
surrounded by a shell of another material B
with resistivity ρB . The total radius is R2.
What is its resistance (per unit length)?
 1

1  R2

R
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1
2 
2
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R1   A  B  R1

l
2
1
HITT2
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A total resistance of 3.0 Ω is to be produced
by combining an unknown resistor R with a
12 Ω resistor. What is the value of R and how
is it to be connected to the 12 Ω resistor?
A. 4.0 Ω, parallel
B. 4.0 Ω, series
C. 2.4 Ω, parallel
D. 2.4 Ω, series
E. 9.0 Ω, series