### Summer 2008 Physics 102 Hour Exam 1(23 questions)

The grading button and a description of the scoring criteria are at the bottom of this page. Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

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 89. The exam period was 75 minutes; the mean score was 53.0 the median was 52. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• A physics 102 light bulb acts just like a resistor: its resistance is constant, independent of the current flowing through the light bulb. The bulb's brightness increases with increasing current.
• A reminder about prefixes: p (pico) = 10-12; n (nano) = 10-9; μ (micro) = 10-6; m (milli) = 10-3; k (kilo) = 10+3; M or Meg (mega) = 10+6; G or Gig (giga) = 10+9.

### QUESTION 1*

This and the next question pertain to the following situation:

This apparatus consists of two conducting spheres resting on insulated supports, connected by a narrow conducting rod. You should assume that the apparatus is electrically neutral. Bring a negatively-charged rubber rod close to (but not touching) the sphere on the left.

Consider the electrical force, if any, exerted by the rod on the pair of spheres. What can we say about that force? (Ignore all forces other than that electrical force.)?

(a)   The rod repels the apparatus to the right.
(b)   The rod exerts no overall electrical force on the system.
(c)   The rod attracts the apparatus to the left.

### QUESTION 2***

Remove the charged rubber rod (taking it very far away from the pair of spheres) and then remove the conducting connection. What can we say about the spheres?

(a)   The sphere on the left is positively charged; the sphere on the right is negatively charged.
(b)   The spheres are both neutral.
(c)   The sphere on the right is positively charged; the sphere on the left is negatively charged.

### QUESTION 3***

As shown in lecture, a charged capacitor is connected to an electroscope (the device with the pair of hanging gold leaves). As the plates of the capacitor are pushed together, the gold leaves

(a)   collapse (move closer) because the total voltage of the system increases.
(b)   collapse (move closer) because charge moves onto the plates of the capacitor.
(c)   spread farther apart because charge is pushed off the plates of the capacitor.

### QUESTION 4**

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

A pair of charges sits on the x-axis: a charge q = -6 nC at the origin and q = +8 nC at x = 4. Assume no other charges are present.

What is the (electrical) stored energy of this pair of charges?

(a)   - 1.08 × 10-7 J
(b)   - 2.7 × 10-8 J
(c)   0
(d)   + 2.7 × 10-8 J
(e)   + 1.08 × 10-7 J

### QUESTION 5***

At what point between them is the net electric field equal to zero?

(a)   x = 1.71
(b)   x = 1.86
(c)   There is no such point.

### QUESTION 6**

At which of these points on the x-axis is the electric potential zero?

(a)   x = 1.71
(b)   x = 1.86
(c)   The electric potential is not equal to zero at either of the points listed above.

### QUESTION 7*

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

A negative charge q = -7 nC is fixed at the origin. A positive charge q = + 7nC is fixed on the y-axis at y = 6. For the sake of reference, consider point A, at (x = 5, y = 3). All distances quoted are in meters.

In what direction does the electric field at A point?

(a)   There is no electric field at A.

(b)   The field at A points rightward along the page (positive x direction).

(c)   The field at A points leftward along the page (negative x direction).

(d)   The field at A points upward along the page (positive y direction).

(e)   The field at A points downward along the page (negative y direction).

### QUESTION 8***

A third charge (q = 8 nC) is brought in from very far away and put at point A. What is the magnitude of the electric force it experiences?

(a)   0 N
(b)   1.48 × 10-8 N
(c)   1.53 × 10-8 N
(d)   2.96 × 10-8 N
(e)   3.05 × 10-8 N

### QUESTION 9**

What can we say about the work done to bring that 8 nC charge to point A?

(a)   It will depend on the path taken: W < 0 if the particle is brought to A from the upper half of the plane, and positive otherwise.

(b)   It will depend on the path taken: W > 0 if the particle is brought to A from the upper half of the plane, and negative otherwise.

(c)   W = 0

### QUESTION 10**

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

Consider this circuit. (Points A and B in the circuit are labeled for the sake of reference.)

What is the charge on the 6 μF capacitor?

(a)   144 μC
(b)   180 μC
(c)   216 μC
(d)   288 μC
(e)   312 μC

### QUESTION 11*

What energy is stored in the 4 μF capacitor?

(a)   144 μJ
(b)   180 μJ
(c)   216 μJ
(d)   288 μJ
(e)   312 μJ

### QUESTION 12***

Insert a narrow conducting metal slab between the plates of the 4 μF capacitor without touching either plate. Referring to the potentials at points A and B: what effect does this slab have on the difference VA - VB ?

(a)   It has no effect.
(b)   It increases the difference VA - VB.
(c)   It decreases the difference VA - VB.

### QUESTION 13*

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

Six identical resistors are connected as shown. Various batteries may be attached to that resistor network. For the sake of reference, points A, B, and C, and current I, are identified.

Find the single resistance equivalent to the original network of six resistors.

(a)   10.9 Ω
(b)   45 Ω
(c)   55 Ω
(d)   60 Ω
(e)   120 Ω

### QUESTION 14**

If the difference VA - VB between the potentials at A and B is 10 volts, what current I flows through the branch on the right side of the circuit?

(a)   0.167 A
(b)   0.5 A
(c)   .667 A

### QUESTION 15***

Consider connecting points A and C with a wire. What effect would this have on the current I ?

(a)   The current I would be greater with A and C connected than without them connected.
(b)   Connecting A and C with a wire would have no effect on I.
(c)   The current I would be less with A and C connected than without them connected.

### QUESTION 16**

A 10 volt battery is attached to this circuit containing three identical, 20 Ω resistors. What power is dissipated in each resistor?

(a)   5 W
(b)   1.67 W
(c)   0.56 W

### QUESTION 17**

Consider this network of four capacitors. We wish it to have a total capacitance of 14 μF. We will insert a dielectric between the plates of the 4 μF capacitor, filling the space between those plates. What value of dielectric constant must this dielectric possess?

(a)   1
(b)   2
(c)   2.5
(d)   4
(e)   10

### QUESTION 18*

Which one of the following equations is true?

(a)   I3 + I4 = I2
(b)   I1 + I2 - I3 = 0
(c)   I3 = - I4

### QUESTION 19*

Which one of the following equations is true?

(a)   15 I1 + 12 + 16 I3 - 20 = 0
(b)   -23 I1 + 12 - 16 I3 - 20 = 0
(c)   15 I1 + 8 I3 - 6 = 0

### QUESTION 20**

The current through the 10 Ω resistor is 2 A.

(T)   True
(F)   False

### QUESTION 21**

The capacitor in this circuit is initially uncharged. At time t = 0, switch S1 is closed but switch S2 is left open.

What is the charge Q on the capacitor after a time equal to 2 time constants (t = 2τ) has elapsed?

(a)   14.6 μC
(b)   16.9 μC
(c)   108 μC

### QUESTION 22**

After a long time has elapsed, switch S1 is opened and switch S2 is closed. At that very instant, what initial current flows through the 1500 Ω resistor?

(a)   13.3 mA
(b)   24 mA
(c)   30 mA

### QUESTION 23***

Again, assume the capacitor is initially uncharged. At time t = 0, switch S1 (which had been open) is closed. Consider the rate at which the capacitor charges. What effect does switch S2 have on that rate

(a)   If S2 is closed, the capacitor will charge faster.
(b)   If S2 is open, the capacitor will charge faster.
(c)   S2 has no effect on the rate at which the capacitor charges.

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.