### Fall 2008 Physics 102 Hour Exam 2(27 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 131. The exam period was 90 minutes; the mean score was 96.9; the median was 98. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

• 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 the following situation.

A positively charged particle in a uniform external magnetic field moves in a circular path in the clockwise direction, parallel to the plane of the page. In what direction do the magnetic field lines point?

(a)   out of the page
(b)   into the page
(c)   to the left
(d)   to the right
(e)   in a clockwise pattern parallel to the plane of the page

### QUESTION 2**

What should one do to maximize the magnitude of the magnetic force acting on a charged particle moving in a magnetic field?

I. Maximize the strength of the magnetic field
II. Minimize the particle's velocity
III. Ensure that the particle is moving in the same direction as the magnetic field lines

(a)   I only
(b)   I and II only
(c)   I and III only
(d)   II and III only
(e)   I, II, and III

### QUESTION 3**

Which one of those shown at right is not a possible trajectory of a charged particle in a uniform magnetic field?

(a)
(b)
(c)
(d)
(e)

### QUESTION 4*

A positively charged particle of q = 2.0 C moves upward into an area where both a magnetic field and an electric field are acting. The magnetic field has a magnitude of B = 4.0 × 10-4 T and the electric field has a magnitude of E = 0.1 N/C.

At what velocity v must the particle be moving if it is not deflected when it enters this area?

(a)   v = 4.0 × 10-3 m/s
(b)   v = 125 m/s
(c)   v = 250 m/s
(d)   v = 500 m/s
(e)   The particle will be deflected to the left regardless of its velocity.

### QUESTION 5**

A current-carrying wire in a magnetic field is subject to a magnetic force. If the current in the wire is doubled, what happens to the magnetic force acting on the wire?

(a)   It is quartered.
(b)   It is halved.
(c)   It is unchanged.
(d)   It is doubled.

### QUESTION 6*

Two wires carry current in opposite directions.

Which one of the graphs at right represents the magnetic force acting on each wire?

(a)
(b)
(c)
(d)
(e)

### QUESTION 7***

A current-carrying wire passes through a uniform external magnetic field, as shown above.

At which point is the total magnetic field the strongest?

(a)   A
(b)   B
(c)   C
(d)   D
(e)   The total magnetic field strength is uniform throughout.

### QUESTION 8*

A conducting bar of length 2 cm slides along metal rails at a speed of v = 1 cm/s. The bar and rails are in a magnetic field of 2 T, pointing into of the page.

What is the induced emf ε in the bar and rails?

(a)   ε = 2 × 10-5 V
(b)   ε = 2 × 10-4 V
(c)   ε = 4 × 10-4 V
(d)   ε = 2 × 10-3 V
(e)   ε = 4 × 10-3 V

### QUESTION 9**

Two long parallel wires are 1 cm apart as shown in the picture below. The current I1 in the top wire is 15 A. The magnetic field B at point P (3 cm below the bottom wire) is measured to be 3.5 × 10-5 T and points out of the page.

What is the magnitude and direction of the current I2 in the bottom wire?

(a)   I2 = 7.5 A to the left
(b)   I2 = 7.5 A to the right
(c)   I2 = 0 A
(d)   I2 = 16.5 A to the left
(e)   I2 = 16.5 A to the right

### QUESTION 10**

This and the next question pertain to the the following situation.

A 4 cm × 3 cm rectangular loop is made of a wire with resistance per unit length of 25 Ohm/m. The loop is placed in a region of uniform magnetic field, B = 5 T. The direction of B is perpendicular to the plane of the loop and points into the page as shown.

The magnetic field starts to increase at a uniform rate of 0.1 T/sec. What is the magnitude of the induced current in the loop?

(a)   I = 0 A
(b)   I = 3.4 × 10-5 A
(c)   I = 1.2 × 10-4 A
(d)   I = 0.02 A
(e)   I = 4.8 A

### QUESTION 11**

What is the direction of the induced current?

(a)   counterclockwise
(b)   clockwise
(c)   There is no induced current.

### QUESTION 12**

A loop of wire is moved at a constant speed along the x axis over a fixed permanent magnet as shown.

Which picture best describes the EMF generated in the loop as a function of position of the ring?

(a)
(b)
(c)

### QUESTION 13**

Two identical bar magnets are dropped from the same height. Magnet B passes through a tube wrapped with a coil of wire which is connected to a light bulb. Which one hits the ground first?

(a)   magnet A
(b)   magnet B
(c)   They hit at the same time.

### QUESTION 14*

This and the next question pertain to the the following situation.

A mechanical crank is used to turn a rectangular loop with area 0.33 m2 at angular frequency ω in the presence of a magnetic field B = 0.8 T as shown. The 200 V peak EMF produced in the loop results in a peak voltage across the primary of the transformer of Vp = 200 V. The secondary windings are connected to an oscilloscope, and the peak voltage is 100 volts.

If the primary coil has 100 windings, how many windings are there in the secondary coil? (Note number of windings on picture is not accurate!)

(a)   Ns = 50
(b)   Ns = 100
(c)   Ns = 200

### QUESTION 15*

What is the period of the oscillating voltage in the secondary circuit?

(a)   T = 2.3 ms
(b)   T = 3.6 ms
(c)   T = 4.1 ms
(d)   T = 8.3 ms
(e)   T = 9.8 ms

### QUESTION 16*

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

The solenoid shown below has length l = 0.15 m and cross-sectional area A = 3.5 × 10-4 m2. Its self-inductance is L = 25 mH. A current I of 2.5 amps flows through its coils in the direction shown in the figure.

Which way does the magnetic field point inside the solenoid?

(a)   to the left
(b)   to the right

### QUESTION 17*

How much magnetic energy, Um, is stored in this solenoid?

(a)   Um = 78 mJ
(b)   Um = 123 mJ
(c)   Um = 175 mJ
(d)   Um = 208 mJ
(e)   Um = 345 mJ

### QUESTION 18*

What is the magnetic field inside the solenoid.

(a)   B = 14 mT
(b)   B = 36 mT
(c)   B = 48 mT
(d)   B = 61 mT
(e)   B = 88 mT

### QUESTION 19***

This and the next question pertain to the the following situation.

A single-turn rectangular wire loop of size 20 cm × 30 cm rotates at a uniform rate of ω = 50 rad/sec around the z-axis as shown. There is a uniform magnetic field B in the +y-direction. The loop has a total resistance of R = 36 Ω.

At the instant shown, an induced current in the loop will

(a)   flow clockwise.
(b)   flow counterclockwise.
(c)   not flow.

### QUESTION 20*

If the induced rms current in the loop is 0.04 A, what is the strength of the magnetic field B?

(a)   B = 0.17 T
(b)   B = 0.29 T
(c)   B = 0.55 T
(d)   B = 0.68 T
(e)   B = 0.79 T

### QUESTION 21*

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

The phasor diagram below yields information on a particular series RLC circuit. It is only partially labeled, but you should know from examining it the proper quantity that is referred to by each number, given in ohms, in the drawing.

What is the total impedance, Z, of the circuit?

(a)   Z = 12 Ω
(b)   Z = 18 Ω
(c)   Z = 36 Ω
(d)   Z = 40 Ω
(e)   Z = 65 Ω

### QUESTION 22*

If the inductance L = 0.02 H, what is the capacitance?

(a)   C = 0.21 μF
(b)   C = 0.59 μF
(c)   C = 3.17 μF
(d)   C = 17.6 μF
(e)   C = 33.3 μF

### QUESTION 23**

To bring the circuit closer to resonance, the frequency of the oscillator should be

(a)   increased.
(b)   decreased.
(c)   left as is, since the circuit is already in resonance.

### QUESTION 24*

This and the next question pertain to the the following situation.

In the following RLC circuit, E(t) is given in volts and time is in seconds.

What is the phase angle Φ by which the voltage leads the current?

(a)   Φ = +73.8°
(b)   Φ = +37.6°
(c)   Φ = 0
(d)   Φ = -37.6°
(e)   Φ = -73.8°

### QUESTION 25**

For which of these is the maximum voltage drop the largest?

(a)   the inductor
(b)   the capacitor
(c)   the resistor

### QUESTION 26*

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

In the drawing below, you are looking at an edge view of a single turn rectangular wire loop, which is being rotated clockwise. There is a uniform magnetic field B which points to the right. The three drawings show the position of the coil at three different times.

For which drawing is the magnitude of the magnetic flux through the loop the largest?

(a)   the left drawing
(b)   the center drawing
(c)   the right drawing

### QUESTION 27*

For which drawing is the magnitude of the induced EMF in the loop the largest?

(a)   the left drawing
(b)   the center drawing
(c)   the right drawing

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.