All answers have been checked against the answer key, and should be presumed to be correct. You should ask for help in the recitations if you are unable to obtain these results.
10. B = 7.6 Tesla, West
12. (a) South pole, (b) I = 3.5 x 102 A, (c) F' = 5.22 N, regardless of which way the wire tips.
22. I2 = 26 A upward
24. a = 1.3 x 10-3 g
28. (a) Ba = B2 -
B1 = (2.0 x 10-5 T/A)(15A - I) up
(b) Bb = B2 +
B1 = (2.0 x 10-5 T/A)(15A + I) down
32. B = 12 Tesla
36. I = 91.9 A.
56. The permeability is 2.7 x 10-5 Tm/A
4. When the capacitor is charging, B = 5.00 x 10-8 Tesla. After the capacitor is charged, the electric field doesn't change, so the magnetic field is zero.
2. The lines of force encircle the wire, going into the paper above the wire, and come out of the paper below the wire.
3. The magnetic field lines go counterclockwise about the wire.
4. Currents are a source of magnetic fields, and magnetic fields add as vectors, so the total field of the earth and your wires may point in a different direction than the earth's field alone. AC current produces an alternating magnetic field, so it doesn't affect the compass on the average so much, but DC current produces a steady magnetic field that would definitely affect a compass.
5. Both electric and magnetic fields surround a moving charge.
6. There can be a weak attraction of a magnet to non-ferromagnetic materials, because many materials are paramagnetic to some degree, which means that they become magnetized a little in the presence of a magnetic field.
7. No. Both bars cannot be magnets, because magnets would repel when like poles were placed together.
8. The fields depend on distance similarly to an electric dipole. We didn't study these, so you can skip this question. The details are in section 17-6.
9. The two rods that are magnetized will repel when the like poles are put together. The other rod will always be attracted to the magnets, never repelled.
10. You can make an electromagnet by making a coil of wire. This behaves the same way as a compass, with a north and south pole.
11. The magnetic field points from the north pole to the south pole, so it points to the right. the current is carried away from you, so the right hand rule says that the force on the current is downward.
12. A magnetic field does not act on charges at rest, so it cannot start a charge moving. However, an electric field can cause a charge to move.
13. A charged particle encircles the magnetic field lines. An electric field in the same direction will cause the particle to move in a spiral path, with an acceleration parallel to both fields.
14. Using the left hand rule is equivalent to changing the sign of the current, since it causes all of the directions to be reversed. You an try this in a few cases to see how it works.
15. Yes. The particle could move parallel to the magnetic field, so it feels no force. Another possibility is that an electric field could counteract the force of the magnetic field.
16. No, there could be an electric field deflecting the particle.
17. The kinetic energy will not change, because the magnetic field does no work on the particle, since it is perpendicular to the velocity.
18. The magnetic field produced by the wire is out of the paper above the wire, and into the paper below the wire. Therefore, the right hand rule applied to the Lorentz force shows that force on the the upper positive charge will point down toward the wire, the force on the upper negative charge will point to the left, the force on the lower negative charge will point up toward the wire, and the force on the lower positive charge will point to the left. Remember that the right hand rule is reversed for the negative charges.
19. A strong magnet held near a TV would cause the electrons be deflected. They would move in circular paths about the magnetic field direction instead of travelling straight to the TV screen.
20. No. The particle would enter the field in a straight line, and then start to curve in a circular path. When it gets half way around the circular path, it would be at the edge of the region containing the field again, and would leave the region in the opposite direction from which it arrived. It would then travel back in a straight line toward the place from which it came.
21. A magnetic field would never deflect a particle parallel to its velocity. An electric field could deflect it any direction. Also, the magnetic force would depend on how fast the particle moved, but the electric force would not.
22. The magnetic field lines come out of the paper above the wire and enter it below the wire. Therefore, the field lines point downward as the beam of electrons approaches. The electrons are flowing into the paper, so the current flows out of the paper. The right hand rule then shows that the force is to the right.
23. The magnetic field lines due to the first current encircle it. Assume they go counterclockwise. Then they point upward to the right and downward to the left. The magnetic force then points in opposite directions on either side of the second wire, and tends to twist it to align it with the original wire in such a way that the currents would flow in the same direction.
24.
(a) If the force is repulsive, the currents must be in opposite directions.
(b) The equilibrium is not stable, because the upper wire would try to
slide off to one side or the other.
25. We skipped the section on galvanometers, so you can skip this question.
26. We skipped the section on the Hall effect, which is described here, but this question can be analyzed using the Lorentz force on the charge carriers, so you should be able to answer it. As drawn, the current runs from left to right in the semiconductor, because the positive terminal of the battery is on the left. This means positive charges are moving to the right, or negative charges are moving to the left. The magnetic field points down. Either sign of charge carrier would then be pushed toward point a by the Lorentz force. If the charge carriers are positive, a will become positively charged relative to b, and vice versa if the charge carriers are negative. We are told that a is at a higher potential, which means its relative charge is positive. That means the charge carriers are positive.
27. We skipped the discussion of mass spectrometers, so you can skip this question.
28. The iron tends to become magnetized by the magnet in such a way that it is attracted to whichever end is closest.
29. The nail does not become magnetized until it reacts to the magnetic field of the permanent magnet, and the spins align in it, making it a magnet too. Then it can attract the paper clip.
30. Relays are just switches which operate when current in the coil magnetizes iron inside the coil, attracting the metal of the switch to close a circuit. In case (a), a battery or low-voltage power supply would be connected to a switch which would be pressed to ring the door bell, and then to the coil of the relay. The switch inside the relay could be of the normally-closed type, and placed in series with the doorbell switch, so that when the bell is rung, the current flows briefly until the doorbell rings, and then turns off again. The brief current could go to an electromagnet causing the bell to be struck once. In real doorbells, the relay and electromagnet are often combined in one unit. The circuit is opened at the same time the striker moves to hit the bell.
In case (b), a battery or low-voltage power supply would be connected to a switch operated by the user and to the relay coils. Then closing the switch would operate a switch inside the relay, which would be in a circuit of possibly much higher voltage.
There are no questions relevant to displacement currents in chapter 22.
| Physics 222 | Department of Physics | University of Tennessee |