### Summer 2009 Physics 102 Hour Exam 1(25 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 94. The exam period was 75 minutes; the mean score was 63.3 the median was 66. 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:

In lecture a van de Graaff generator was used to build up a large charge. Nearby, there was a grounded metal sphere; and between the grounded metal sphere and the generator, an (initially) uncharged metal sphere hung from a string, allowed to swing back and forth between the generator and the grounded sphere.

What can we say about that hanging metal sphere?

(a)   Because it was initially uncharged, it would initially experience no net force from the van de Graaff generator.

(b)   A charged van de Graaff generator will attract the hanging metal sphere, regardless of the sign of the charge on the van de Graaff generator.

(c)   The negatively-charged van de Graaff generator attracted the neutral hanging sphere but a positively-charged generator would repel the neutral hanging sphere.

### QUESTION 2***

Because it is grounded, the metal sphere on the right side (of the diagram) can never acquire a net electric charge.

(T)   True
(F)   False

### QUESTION 3**

A battery is used to charge the two metal plates of a capacitor. The battery is then removed, leaving the plates electrically isolated. Next, the plates are pulled apart. As they are pulled apart, which one of the following is true?

(a)   The capacitance increases and the voltage decreases.
(b)   The capacitance and the charge both decrease.
(c)   The voltage increases but the charge remains the same.

### QUESTION 4**

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

A pair of charges sits on the x-axis, a charge Q1 = +9 nC at the origin and Q2 = +4 nC at x = 5. Assume no other charges are present. All coordinates are in meters.

At which one of these points is the net electric field equal to zero?

(a)   x = 15
(b)   x = 9
(c)   x = 4
(d)   x = 3
(e)   x = 2

### QUESTION 5*

What is the potential at x = -5 ?

(a)   -12.6 V
(b)   +12.6 V
(c)   +19.8 V

### QUESTION 6**

How much work is required to move the +4 nC charge (Q2) from its original location at x = 5 to x = 3 ?

(a)   -43.2 nJ
(b)   -23 nJ
(c)   23 nJ
(d)   43.2 nJ
(e)   108 nJ

### QUESTION 7**

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

A negative charge Q1 = -8 nC is fixed on the x-axis at (x = -4, y = 0). A positive charge Q2 = +8 nC is fixed on the x-axis at (x = 4, y = 0). For the sake of reference, consider point A, at (x = 0, y = 5). All locations are in meters.

What is the sign of the x-component of the electric field at A?

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

### QUESTION 8*

What is the sign of the y-component of the electric field at A?

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

### QUESTION 9**

What is the magnitude of the electric field at A?

(a)   0 N/C
(b)   1.76 N/C
(c)   2.19 N/C
(d)   2.74 N/C
(e)   3.51 N/C

### QUESTION 10**

A third charge (q = 4 nC) is brought in from very far away and put at point A. What total potential energy is stored in all three charges?

(a)   72 nJ
(b)   9 nJ
(c)   0
(d)   -9 nJ
(e)   -72 nJ

### QUESTION 11*

A current I9 = 2 A flows through the 9 ohm resistor. At what rate does the battery deliver energy to the entire circuit?

(a)   9 W
(b)   27 W
(c)   54 W
(d)   84.6 W
(e)   108 W

### QUESTION 12**

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

Consider this circuit.

What is the voltage across the 6 μF capacitor?

(a)   10 V
(b)   12 V
(c)   18 V
(d)   20 V
(e)   30 V

### QUESTION 13**

The charge stored on the 9 μF capacitor is larger than the charge stored on the 12 μF capacitor.

(T)   True
(F)   False

### QUESTION 14*

What total energy is stored in this circuit?

(a)   2700 μJ
(b)   4050 μJ
(c)   5850 μJ
(d)   12150 μJ
(e)   24300 μJ

### QUESTION 15**

Now, imagine a second circuit just like this one, except that a dielectric will be inserted between the plates of the 9 μF capacitor. What dielectric constant would cause a 30 V battery to deliver a charge of 300 μC to this modified circuit?

(a)   2.0
(b)   2.25
(c)   2.5

### QUESTION 16*

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

Five resistors are connected as shown.

Find the single resistance equivalent to this network.

(a)   21.3 Ω
(b)   25.6 Ω
(c)   32 Ω
(d)   34 Ω
(e)   88 Ω

### QUESTION 17***

Now remove the diagonal 24 Ω resistor, leaving a total of four resistors. What effect does this have on the voltage across the 16 Ω resistor?

(a)   The voltage is greater without that resistor.
(b)   The voltage will be unaffected by the removal of that resistor.
(c)   The voltage is smaller without that resistor.

### QUESTION 18*

Does this network have the same total resistance as the original one (with all five resistors present)?

(a)   Yes, it has the same resistance
(b)   No, it is different

### QUESTION 19*

Which one of the following equations is true?

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

### QUESTION 20*

The equation

6 I1 - 8 I2 - 4 = 0

is valid for this circuit.

(T)   True
(F)   False

### QUESTION 21*

The equation

-9 + 6 I1 – 22 I3 – 14 = 0

is valid for this circuit.

(T)   True
(F)   False

### QUESTION 22**

This and the next three questions are about the circuit shown below:

The capacitor in this circuit is initially uncharged. At time t = 0, switch S1 is closed but switch S2 is left open. After a very long time has elapsed, a charge Q = 3.0 μC will be stored on the capacitor. What must the charge Q on the capacitor have been after a time equal to 2 time constants (t = 2τ) had elapsed?

(a)   0.4 μC
(b)   2.6 μC
(c)   3.0 μC

### QUESTION 23*

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 2400 ohm resistor?

(a)   5.7 mA
(b)   8.5 mA
(c)   14.2 mA

### QUESTION 24*

With switch S1 open and switch S2 closed, what is the time constant τ which governs the rate of discharge of the capacitor?

(a)   0.35 ms
(b)   0.53 ms
(c)   0.88 ms

### QUESTION 25***

Repeat the experiment, with (once again) the capacitor initially uncharged, but at t = 0 close both switches. After a long time has elapsed, what will the charge on the capacitor be?

(a)   1.2 μC
(b)   1.8 μC
(c)   3.0 μC:

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.