Physics 72: Chapter 27-28 Problem Set Part 1 Summer 2010

Name: Score:

Lecturer: Mr Carlos Baldo III Section: X7-7

Encircle the letter of the correct answer. Solve each problem clearly and completely for full credit.

1. Magnets. Consider various configurations of a bar magnet with its

polarity labeled N for north and S for south, and an unknown magnet (poles labeled X and Y) with its magnetic field lines. Which

configuration is stable?

2. Source. Which of the following cannot be a source of magnetic field?

A. A rotating point charge B. A charging RC circuit connected to an AC source

C. A bouncing charged dragonball

D. A jogging negatively-charged gummy bear E. A uniformly-charged Porsche parked on a sidewalk

3. Solenoids. Consider two very long solenoids S1 and S2. S1 has N number of loops and length L while S2

has 2N number of loops and length 2L. If these solenoids are connected in series to an emf source, compare the relative magnitudes of the magnetic field (BS1 and BS2) inside the two solenoids.

A. BS1 = 4 BS2

B. BS1 = 2 BS2 C. BS1 = BS2

D. BS1 = ½ BS2

E. BS1 = ¼ BS2

4. May the force be with you. Which of the following statements is/are TRUE about magnetic field and

magnetic forces? A. The magnetic force acting on a moving charged object is parallel to the magnetic field

B. The magnetic force acting on a moving charged object is perpendicular to the magnetic field

C. The magnetic field always points away from the North pole towards the South pole

D. A and C

E. B and C

5. Synchrotron. A +5nC charge with an unknown speed is shot into a hole L of a closed chamber, which produces a uniform magnetic field B=1.0mT. The charge comes out

through hole M with speed of 0.5m/s. What is the speed of the charge at hole L?

A. 2.5 m/s

B. 1.0 m/s

C. 0.1 m/s D. 0.5 m/s

E. Cannot be determined

For the next two numbers, consider the figure at the right. The Final Four opens the second round of

competition with a geeky challenge. Each one of them chooses a point charge, places them near point Q and let this point charge move with the same speed but at any particular direction.

6. UAAP. Which among the following universities have their point charge produce a

magnetic field directed into the page at point Q? A. Only UP

B. UP and Ateneo

C. UP and FEU D. UP and UE

E. All universities except Ateneo

Physics 72: Chapter 27-28 Problem Set Part 1 Summer 2010

v I

7. DLSU. If a positively charged green archer is placed to rest at point Q, what will be the direction of the

magnetic force that it will experience? A. Upward

B. Downward C. To the left

D. To the right

E. No magnetic force

8. Magnetic Force. A proton is released from rest in a region of uniform electric field E and magnetic field B as shown. Which line best describes the trajectory

of the proton?

9. Magnetic Force. Three particles of identical speeds and charges, immersed in a

uniform magnetic field is directed out of the page, follow the paths shown. Which

of the following particles is/are negative? A. a & b

B. b only C. c only

D. a & c

E. None of the above

10. Current-carrying wire. A current-carrying wire (I = 1.0 A) is immersed in a uniform magnetic field with magnitude of 2.0 T directed to the positive y-axis.

What is the total force on the wire? A. Zero

B. 6 N, along +z-axis

C. 8 N, along +z-axis

D. 0.06 N, along +z-axis

E. 0.08 N, along +z-axis

11. Two wires. Two parallel wires W1 and W2 lying on the floor carry identical currents in the same direction. If the current in W1 is increased by a factor of two. Which of the following relations about the magnetic

force (FW1 and FW2) acting on each wire is TRUE?

A. 1 2W WF F=

� �

B. 1 2W WF F= −

� �

C. 1 22W WF F=

� �

D. 1 22W WF F= −

� �

E. 1 24W WF F=

� �

12. Bear Hugs. Consider a neutral gummy bear moving at a constant speed

away from a conducting loop with constant current I. It experiences a

magnetic force such that negative charges are propelled towards its feet. What is the direction of the current through the loop?

A. Into the page

B. Out of the page

C. Counter-clockwise D. Clockwise

E. Clockwise then counter-clockwise

Physics 72: Chapter 27-28 Problem Set Part 1 Summer 2010

For the next two numbers, consider a positron (positively-charged electron) moving downward. An electron

at a distance R from the positron is moving to the right with velocity v.

13. Interacting charges. What is the magnitude and direction of the magnetic field at the location of the electron due to the positron?

A. ZERO

B.

o

2

µ ev

4π R , out of the page

C.

o

2

µ ev

4π R , into the page

D.

2 2

o

2

µ e v

4π R , out of the page

E.

2 2

o

2

µ e v

4π R , into the page

14. Magnetic force. What is the magnitude and direction of the magnetic force acting on the electron?

A. ZERO

B.

2 2

o

2

µ e v

4π R , out of the page

C.

2 2

o

2

µ e v

4π R , into the page

D.

2 2

o

2

µ e v

4π R , upward

E.

2 2

o

2

µ e v

4π R , downward

For the next two numbers, consider a system of two infinitely long current-carrying wires with current

direction and magnitude indicated in the figure.

15. Interacting wires. What is the magnitude of the net magnetic field at point P?

A. ZERO

B.

oµ I

πR

C.

oµ I

2πR

D.

o

2

µ I

2πR

E.

o

2

µ I

4πR

16. Force uli. If an electron moving at velocity v downwards is at point P, what is the net magnetic force

exerted on the electron?

A. Zero

B.

oevµ I

πR , left

C.

oevµ I

2πR , right

D.

o

2

evµ I

2πR , upward

E.

o

2

evµ I

4πR , downward

Physics 72: Chapter 27-28 Problem Set Part 1 Summer 2010

17. Magnetic Flux. Consider a cube with side lengths S. The magnetic flux through one of the surfaces is -

10Wb. What is the total magnetic flux through the remaining faces of the square? A. 5 Wb

B. –10 Wb C. –15 Wb

D. 10 Wb

E. 15 Wb

18. Infinite wire. A long, straight wire lies along the y-axis and carries a current in the negative y- direction. In addition to the magnetic field due to the

current in the wire, a uniform magnetic field Bo is in the positive x-direction. At which of the following points will the magnetic field be greatest?

A. a

B. b

C. c

D. all points have equal magnetic field E. cannot be determined

For the next two numbers, consider 2 electrons moving in opposite directions but with the same speed and

two infinitely long wires with same length and current I directed into the page placed at the corners of a

square with side-length L, as shown in the figure.

19. Point and wire. What is the direction of the magnetic field at point P? A. No magnetic field at point P

B. Into the page C. Out of the page

D. Upward

E. Downward

20. FB. If another electron is placed at point P with initially velocity directed out of the page, what is the direction of the magnetic force acting on the electron?

A. The net force is zero. B. Into the page

C. Out of the page D. Upward

E. Downward

For the next two numbers, consider the figure at the right. All four element markers (Fire, Air, Water and Earth)

are equidistant from point P and have wires carrying current as shown. A magnetic sensor placed at P detects that the net magnetic field at P.

21. MD. What should be the magnitude and direction of the current through

a fourth wire to be placed in the Water marker so that the net magnetic field

at P is directed to the right? A. Zero

B. I, into the page C. I, out of the page

D. 2I, into the page E. 2I, out of the page

22. Velocity. If an electron moving with velocity v is to be placed instead of a current-carrying wire, what should be direction of velocity of the electron so

that the net magnetic field at P is directed to the right?

A. Zero

B. Into the page C. Out of the page

D. To the right

E. To the left

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