Spring 2004 Physics 102 Hour Exam 1
(29 questions)

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This exam consists of 29 questions; true-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 120. When the exam was given, the mean was 75.1; the median was 75. Click here to see the formula sheet that came with the exam.


This and the following three questions pertain to the same situation.

A negatively charged rod is brought close to an isolated conducting sphere. The net charge on the sphere is zero. The net force between the rod and the sphere is

(a)   zero.
(b)   attractive.
(c)   repulsive.


The conducting sphere is now connected to ground. The net charge of the sphere is

(a)   zero
(b)   positive
(c)   negative


The net force between the rod and the conducting sphere is now

(a)   zero.
(b)   attractive.
(c)   repulsive.


The electric field inside the conducting sphere is now zero.

(T)   True
(F)   False


Which one of the circuit diagrams below (a, b, or c) corresponds to the circuit sketch?



This and the following question are related.

Which one of the following describes the function of the Electromotive Force (emf)?

(a)   It produces charge.
(b)   It pushes charge.
(c)   It provides a constant current.


Which of the following also describes the function of the Electromotive Force (emf)?

(a)   It provides a constant power.
(b)   It maintains a constant potential difference.


The sketches below show some patterns for equipotential lines in the plane of the paper. Which of these is the one most closely corresponding to the equipotentials due to two equal positive charges?



Two metal rods have exactly the same resistance. Rod A has a length LA and diameter DA. The length LB and diameter DB of rod B are related to LA and DA by LB = 2 LA and DB = 2 DA. It follows that rod A has a resistivity ρA related to that of rod B, ρB, by:

(a)   ρA = ρB
(b)   ρA = ρB / 2
(c)   ρA = ρB / 4
(d)   ρA = 2 ρB
(e)   ρA = 4 ρB


This and the next question pertain to the following situation.

A uniform electric field is set up between the two parallel plates of a capacitor at a potential difference of 200 V. The distance between the two plates is 0.5 cm.

What is the magnitude of the electric field between the two plates?

(a)   4 × 104 N/C
(b)   2 × 104 N/C
(c)   8 × 103 N/C
(d)   4 × 103 N/C
(e)   1.6 × 103 N/C


A positive charge Q = 4 mC is moved from the plate at positive voltage to the center of the capacitor (at a distance of 0.25 cm from the positive plate). What is the change in the magnitude of the potential energy of the charge Q for this displacement?

(a)   1.6 joules
(b)   0.4 joules
(c)   0.25 joules
(d)   0.03 joules
(e)   0.01 joules


Two parallel plate capacitors are constructed of the same materials and have the geometry as illustrated. The first, with capacitance C1, has plates with dimensions W and L separated by a distance d. The second, with capacitance C2, has plates with dimensions W and 3L separated by a distance 3d. Which one of the following is correct?

(a)   C1 > C2
(b)   C1 = C2
(c)   C1 < C2


This and the next four questions pertain to the following situation:

Eight small conducting spheres (open circles) are fixed at regular intervals on a circle with radius R = 1.2 m in the xy-plane. Each one is charged with 2 nC of positive charge.

What is the electric field strength |E| at the center of the circle?

(a)   100 N/C
(b)   0 N/C
(c)   -100 N/C


Which way does the electric field vector E point at the center of the circle?

(a)   to the right, along the x axis
(b)   out of the paper, along the z axis
(c)   does not point anywhere, because the field strength is zero


Assuming the potential of the charge configuration to be zero an infinite distance away, what is the potential at the center of the circle?

(a)   120 V
(b)   100 V
(c)   0 V
(d)   -100 V
(e)   -144 V


An electron is placed on to the z-axis (which passes out of the page, perpendicular to the plane of the paper, and through the middle of the circle) a long distance away (more than a few hundred meters) and then released from rest. Neglecting gravity, estimate the electronís speed once it reaches the center of the circle.

(a)   65 cm/s
(b)   59 m/s
(c)   6.5 × 102 m/s
(d)   5.9 × 104 m/s
(e)   6.5 × 106 m/s


Now imagine the circle with the 8 charges in the figure lying flat on the ground. How much charge would you have to put on each sphere (the same charge on all 8 spheres) to levitate a small object of mass 1 kg, charged with +1 nC, at a distance 1 m above the center of the circle? (Hint: consider the force equilibrium between gravitational and electric forces).

(a)   120 μC
(b)   34 × 10-3 C
(c)   0.52 C
(d)   13 C
(e)   4.2 × 102 C


This and the next four questions pertain to the following situation:

The resistors R1 and R2 each have resistance 10 Ω. The capacitors C1, C2 and C3 all have capacitance 1 μF. The battery supplies a potential difference of 10 V. A perfect ammeter (without resistance) is connected in the lower left of the circuit, as shown in the figure. The switch on the top right is initially in the open position, and the capacitors are fully charged.

What is the current I through the ammeter with the switch open?

(a)   0.0 A
(b)   1.0 A


What is the effective capacitance between points X and Y with the switch open?

(a)   0.33 μF
(b)   0.5 μF
(c)   0.67 μF
(d)   2 μF
(e)   3 μF


What is the charge on capacitor C2 with the switch open?

(a)   1.11 μC
(b)   2.22 μC
(c)   3.33 μC
(d)   4.44 μC
(e)   6.67 μC


You close the switch and wait until the ammeter settles down. What current I is shown on the ammeter?

(a)   0.5 A
(b)   1.0 A
(c)   2.0 A


What is the charge on capacitor C2 now that the switch is closed?

(a)   0 μC
(b)   2.5 μC
(c)   5.0 μC


This and the next three questions pertain to the following situation:

The circuit below represents a circuit that contains a battery that supplies a potential difference V0 = 10 V, a resistor R1 = 100 kΩ and a capacitor C. Initially, no charge is stored on C and the switch is in the open position (as shown in the figure).

The switch is now closed. What is the current through R1 at the moment just after the switch connects?

(a)   0 mA
(b)   1 × 10-4A
(c)   1 × 106A


Which graph best displays the time evolution of the voltage across C starting at the moment the switch makes the connection, assuming that it stays closed for a long time? The y-axis (vertical) represents voltage, whereas the x-axis (horizontal) represents time.



You open the switch again after 2 seconds and measure the voltage across the capacitor to be 6.32 V. What is the charge now stored on C?

(a)   5.32 μC
(b)   63.6 μC
(c)   126 μC
(d)   3.24 × 10-4 C
(e)   5.32 × 10-2 C


If a dielectric material with dielectric constant, κ > 1, had been placed between the plates of the capacitor (as in the above circuit) before the switch were closed, how would the voltage, Vnew, measured across the capacitor after 2 seconds compare with the value, Vold, measured without the dielectric in place?

(a)   Vnew > Vold
(b)   Vnew = Vold
(c)   Vnew < Vold


This and the next two questions are about the circuit shown here:

What single, equivalent resistor could replace all of the resistors in the circuit?

(a)   20 ohms
(b)   12 ohms
(c)   6 ohms
(d)   4 ohms
(e)   3 ohms


What is the magnitude of the current supplied by the battery?

(a)   0.5 A
(b)   1.0 A
(c)   2.0 A
(d)   4.0 A
(e)   6.0 A


What is the magnitude of the current through the 8 ohm resistor in the diagram?

(a)   0.5 A
(b)   1.0 A
(c)   2.0 A
(d)   4.0 A
(e)   6.0 A