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Exam III review.
RII-1
A copper wire and an aluminum wire both have the same
dimensions (same length, same cross-sectional area).
Which one statement is true?
B) Both wires have the same resistance and the same resistivity.
C) The wires have different resistances and different resistivities.
D) The wires have the same resistance but different resistivities.
E) The wires have different resistances but the same resistivity.
RII-2.
Which has higher resistance ?
B) 100 W bulb C) 60 W bulb
Hint: P = IV = I2R = V2/R
RII-3
Consider the following circuit containing three ordinary household
light bulbs, a 100W light bulb, a 60W bulb, and a 40W bulb.
100W
60W
40W
Which one statement is true?
B) The 100W bulb will be brightest.
C) The 60W bulb will be brightest.
D) The 40W bulb will be brightest.
E) All the light bulbs glow with the same brightness.
Answer: the 40W bulb is brightest! (Since it has the largest R).
In comparing the power (or brightness) of two light bulbs, you must first be clear whether
the current or the voltage is the same for the two. It makes all the difference. If the two
bulbs are in series, then the current is the same, and according to P = I2 R , the larger
resistance bulb has more power, and is brighter. But if the bulbs are in parallel, the
voltage is the same, and then according to P = V2 /R, the larger resistance bulb has less
power, and is dimmer.
RII-4.
A 600W hairdryer is attached to 120VAC circuit. What is the peak
current through the hairdryer (to within 5%)?
B) 10A
C) 7A
D) 5A
HINT: P = IV = I2R = V2/R
Answer: 7A. Use P=IV, (works OK with rms values and Pavg). Irms=P/V = 600/120 = 5A,
Ipeak =(2)1/2 Irms = 7A.
RII-5
The voltage between points a and b in the circuit shown is
measured with an ideal voltmeter.
R
R
What does the voltmeter read?
Hint: Recall that an ideal voltmeter
V
R
has infinite internal resistance.
a
b
B) V/2
D) V/4
C) V/3
E) zero
RII-6
Consider the circuit below. What is the total resistance which the
battery sees?
I=
1V ? R
1
=2 2
1
R2
=2
B) 1 
C) 2 
D) 0.5 
E) 0.25 
In the circuit above, the resistor R2 is increased to 5. What
happens to the current through from the battery?
B) increases
C) decreases
D) remains constant
The battery current decreases, since the total resistance seen by the battery increases.
RII-7
R
C
R
The capacitor C is charged up to a voltage V. The switch is closed
at time t=0 connecting the capacitor to two identical resistors as
shown.
Immediately after the switch is closed, how does the current
through the second resistor (on far right) compare with the current
through the capacitor?
B) IR = Ic ,
C) IR = (1/2) Ic ,
D) IR = 2 Ic ,
E) IR = 0
Immediately after the switch is closed, how does the voltage across
the second resistor (on far right) compare with the voltage across
the capacitor?
B) VR = Vc ,
C) VR = (1/2) Vc ,
D) VR = 2 Vc
E) VR = 0 /don't know
Answer: IR = Ic The circuit elements are in series.
Answer: VR = (1/2) Vc By Kirchhoff's voltage law, we have Vc = VR1 + VR2 = 2VR.
In the circuit below which resistors must have the same
current flowing through them? Do not assume that the resistors are
identical.
RII-8
R2
1V
R1
R3
R4
R5
B) R2 / R5 only
C) R3/R4 only
D) R2 / R5 and R3/R4 only
E) R1/ R2 / R5 only
Which resistors have the same voltage drop?
B) R2 / R5 only
C) R3/R4 only
D) R2 / R5 and R3/R4 only
E) R1/ R2 / R5 only
Answer: Only R3 and R4 are guaranteed to have the same voltage.Answer: Only R2 and
R5 areguaranteed to have the same current.
RII-9
A charged particle is orbiting in a uniform B-field in the sense
shown.
Is the particle positive or negative?
B(in)
B) positive
C) negative
D) impossible to tell/don't know
Two particles with the same charge but different masses are
moving in circular orbits in a magnetic field. They have the same
speed. Which one will have the larger radius orbit?
B) Neither, the orbits have the same size.
C) The larger mass particle will have a larger orbit.
D) The smaller mass particle will have a larger orbit.
Answer: The direction of acceleration and net force must be toward the center of the
circle. For a negative particle, the force on particle is toward the center of the circle.
RII-10
A coil of wire carrying current I can rotate freely about an axis in a
magnetic field. If released from rest in the position shown, which
way does it rotate?
B) right side will move out of
page
B
I
C) left side will move out of
page.
D) loop will not rotate at all
axis
Answer: Right side will move out of page. Use Fwire = I L B
Two bar magnets are brought near each other as shown.
The magnets...
RII-11.
B) attract
C) repel
D) exert no net force on each other.
S
N
S
N
Answer:
RII-12
A coil of wire attached to a resistor is in an
oscillating magnetic field, as shown. The Bfield vs. time is shown. At what point in
time is the current through the resistor zero?
B-field (T)
(B)

(C)
0
B
(oscilating)
(D)
(E)
time
RII-13
A circuit with a battery and a variable resistor is near a loop of
wire as shown. When the resistance R is decreased, the induced
current in the loop is ..
B) CW
R
C) CCW
D) ZERO
Answer: CW. When R decreases, the current I = V/R in the circuit increases, causing an
increasing B-field out of the page at the position of the loop. A CW current in the loop
will fight the increase in B.
RII-14
A uniform solid sphere of copper rotates about a stationary axis in
a uniform magnetic field B. Are there
eddy currents?
B
B) Yes, there are eddy currents.
C) No, no eddy currents.
ANSWER: NO EDDY CURRENTS. The flux through any loop rotating with the sphere
does not change. Therefore no emf, no eddy currents.