Download Exam1, 1402, Summer II, 2008

Review for the 2nd exam, 2426, Chpt. 25 – 28. Lianxi Ma
First part: multiple choice.
1.
(b) One ampere of current means:
(a)
(b)
(c)
(d)
(e)
1 W of power
1 C of charges flows through a certain cross-section area per second
1 V of voltage
1 C of charges flows through a 1 m2 of cross-section area per second
1  of resistance
2.
(c) One Ohm of resistance of a conductor means
(a)
(b)
(c)
(d)
(e)
1 V of voltage
1 A of current
If 1 V of voltage is applied, the current is 1 A
Its resistivity is 1  m
Its length is 1 m and its cross-section area is 1 m2.
3.
(d) In lab 3: Ohm’s law, the light bulb’s behavior is non-ohmic. The conclusion is based on
the fact that
(a)
(b)
(c)
(d)
(e)
V – I curve is a straight line – R of the light bulb is a constant
R of the light bulb can’t be measured
The brightness of the light bulb changes when the rheostat is rotated
V – I curve is not a straight line – R changes with V
Lab manual suggests it
4.
(a) In the circuit shown in (a), the two bulbs A and B are identical. Bulb B is removed and
the circuit is completed as shown in (b). Compared to the brightness of bulb A in (a), bulb A
in (b) is
(a)
(b)
(c)
(d)
Brighter
Less bright
Just as bright
Any of the above, depending on the rated wattage of the bulb.
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5.
(b) For an ideal ammeter and an ideal voltmeter
(a)
(b)
(c)
(d)
(e)
6.
Ammeter has infinite resistance and should be connected in parallel with the circuit
element being measured.
Ammeter has zero resistance and should be connected in series with the circuit
element being measured
Voltmeter has zero resistance and should be connected in parallel with the circuit
element being measured
Voltmeter has infinite resistance and should be connected in series with the circuit
element being measured
Both ammeter and voltmeter have zero resistance.
(b) Which of the two arrangements shown has the smaller equivalent resistance between
points a and b?
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(a)
(b)
(c)
(d)
(e)
7.
The series arrangement
The parallel arrangement
They are the same
The answer depends on the values of the individual resistances R1, R2, and R3.
The answer depends on the values of the total current I.
(c) A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are
connected in parallel as shown below. The voltage between points a and b is 120 V. Which
bulb glows the brightest (consumes the most power)?
(a)
(b)
(c)
(d)
The 120-V, 60-W light bulb
The 120-V, 120-W light bulb
The 120-V, 240-W light bulb
All three light bulbs glow with equal brightness
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8. (a) A 120-V, 60-W light bulb, a 120-V, 120-W light bulb, and a 120-V, 240-W light bulb are
connected in series as shown above. The voltage between points a and b is 120 V. Through
which bulb is there the greatest voltage drop?
(a)
(b)
(c)
(d)
the 120-V, 60-W light bulb
the 120-V, 120-W light bulb
the 120-V, 240-W light bulb
All three light bulbs have the same voltage drop.
9. (a) Three identical light bulbs are connected to a source of emf as shown. Which bulb is
brightest?
(a)
(b)
(c)
(d)
(e)
10.
Light bulb A
Light bulb B
Light bulb C
Both light bulbs B and C (Both are equally bright and are brighter than light bulb A.)
All bulbs are equally bright
(d) A battery, a capacitor, and a resistor are connected in series. Which of the following
affect(s) the maximum charge stored on the capacitor?
(a) the emf  of the battery
(b) the capacitance C of the capacitor
(c) the resistance R of the resistor
(d) both  and C
(e) all three of , C, and R
11. (d) A particle with a positive charge moves in the xz-plane as shown. The magnetic field is
in the positive z-direction. The magnetic force on the particle is in
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(a)
(b)
(c)
(d)
(e)
the positive x-direction.
the negative x-direction.
the positive y-direction.
the negative y-direction.
none of these
12. (d) A particle with charge q = –1 C is moving in the positive z-direction at 5 m/s. The
magnetic field at its position is B = 3iˆ – 4 ˆj T . What is the magnetic force on the particle?





C)  20iˆ + 15 ˆj  N; D)  20iˆ  15 ˆj  N;

A) 20iˆ + 15 ˆj N; B) 20iˆ  15 ˆj N


E) 10iˆ + 25 ˆj N
13. (c) A charged particle moves through a region of space that has both a uniform electric field
and a uniform magnetic field. In order for the particle to move through this region at a
constant velocity,
(a)
(b)
(c)
(d)
the electric and magnetic fields must point in the same direction.
the electric and magnetic fields must point in opposite directions.
the electric and magnetic fields must point in perpendicular directions.
The answer depends on the sign of the particle’s electric charge.
14. (a) A circular loop of wire carries a constant current. If the loop is placed in a region of
uniform magnetic field, the net magnetic torque on the loop
(a) Tends to orient the loop so that its plane is perpendicular to the direction of the
magnetic field.
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(b) Tends to orient the loop so that its plane is edge-on to the direction of the magnetic
field.
(c) Tends to make the loop rotate around its axis.
(d) Is zero.
(e) The answer depends on the magnitude and direction of the current and on the
magnitude and direction of the magnetic field.
15. (a) Two positive point charges move side by side in the same direction with the same
velocity. What is the direction of the magnetic force that the upper point charge exerts on the
lower one?
(a)
(b)
(c)
(d)
(e)
toward the upper point charge (the force is attractive)
away from the upper point charge (the force is repulsive)
in the direction of the velocity
opposite to the direction of the velocity
none of the above
16. (c) A long straight wire lies along the y–axis and carries current in the positive y–direction.
A positive point charge moves along the x–axis in the positive x–direction. The magnetic
force that the wire exerts on the point charge is in
(a) the positive x–direction.
(b) the negative x–direction.
(c) the positive y–direction.
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(d) the negative y–direction.
(e) none of the above
17. (c) The long, straight wire AB carries a 14.0-A current as shown. The rectangular loop has
long edges parallel to AB and carries a clockwise 5.00-A current. What is the direction of the
net magnetic force that the straight wire AB exerts on the loop?
(a) to the right
(b) to the left
(c) upward (toward AB)
(d) downward (away from AB)
(e) misleading question — the net magnetic force is zero
18. (b) The figure shows, in cross section, three conductors that carry currents perpendicular to
the plane of the figure. If the currents I1, I2, I3, all have the same magnitude, for which
path(s) is the line integral of the magnetic field equal to zero?
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(a) path a only
(b) paths a and c
(c) paths b and d
(d) paths a, b, c, and d
(e) The answer depends on whether the integral goes clockwise or counterclockwise
around the path.
Part II, Free response problems.
1. When switch S in the figure is open, the voltmeter V of the battery reads 3.08 V. When the switch is
closed, the voltmeter reading drops to 2.97 V, and the ammeter A reads 1.65 A. Find the emf,
internal resistance of the battery, and the circuit resistance R. Assume the two meters are ideal, so
they don’t affect the circuit. (3.08 V, 0.067 , 1.8 )
Figure for Problem 1
2. The region between two concentric conducting spheres with radii a and b is filled with a conducting
material with resistivity . (a) Show that the resistance between the spheres is given by
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 1 1
  
4  a b 
(b) Derive an expression for the current density as a function of radius, in terms of the potential
difference Vab between the sphere. Show that the result in part (a) reduces to R = L/A when the
separation L = b – a between the spheres is small.
R
3. For the circuit shown in the figure find the reading of the idealized ammeter if the battery has an
internal resistance of 3.26 . (0.769 A)
Figure for Problem 3
4. In the circuit shown in the figure find (a) the current in resistor R; (b) the resistance R; (c) the unknown emf E.
(d) If the circuit is broken at point x, what is the current in resistor R? (a. 2.00 A; b. 5.00 ; c. 42.0 V; d. 3.50
A)
Figure for Problem 4
5. You try to reproduce Thomson’s e/m experiment with accelerating V = 150 V and deflecting E = 6.0
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 10 N/C. (a) At what fraction of speed of light do the electrons move? (b) What B do you need? (c)
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With this B, what will happen to the electron beam if you decrease the accelerating V below 150 V?
(a. 0.024c; b. 0.83 T; c. bent upward).
Figure for Problem 5
6. A circular coil 0.0500 m in radius, with 30 turns of wire, lies in a horizontal plane. It carries a
current of 5.00 A in a counterclockwise sense when viewed from above. The coil is in a uniform
magnetic field directed toward the right, with 1.20 T. (a) Find the magnetic moment and the torque
on the coil. (b) If the coil from this position to a position where its magnetic moment is parallel to B,
what is the change in potential energy? (a. 1.18 Am2, 1.41 Nm; b. -1.41 J)
Figure for Problem 6
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7. A wire carrying current I =20 A with length L =10 m is arranged in y direction. “o” is origin of the
coordinates and is in the middle of the wire. The distance between o and P is d = 5 m. (a) Find the
magnitude and direction of the magnetic field at point P. (equation first then numerical calculation)
(b) Write an equation for B when d approaches zero.
y
(a) B 
0 I
L
 5.66 107 T
2
4 d d 2   L / 2 
(b) B 
0 I
2 d
o
I
P
x
8. A solenoid is designed to produce a magnetic field of 0.0270 T at its center. It has radius 1.40 cm
and length 40.0 cm, and the wire can carry a maximum current of 12.0 A. (a) What minimum
number of turns per unit length must the solenoid have? (b) What total length of wire is required?
(1790 turns/m, 63 m)
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