Chapter 24: Magnetism

78. Your friend says that, when a compass is taken across the equator, it turns around and points in the opposite direction. Your other friend says that this is not true, that people in the Southern Hemisphere use the south magnetic pole of the compass to point toward the nearest pole. You're on; what do you say?

First friend: the Magnetic field of the Earth is continuous from pole to pole, and doesn't make a turnaround at the Earth's equator; so a compass needle that is aligned with the Earth's field likewise does not turn around at the equator. Second friend: The same pole points southward in the Northern Hemisphere. A compass does not turnaround when crossing the equator.

19. In what direction relative to a magnetic field does a charged particle move in order to experience maximum deflecting force? Minimum deflecting force?

Force is maximum when motion is perpendicular to the field; minimum when parallel to the field.

44. Why aren't permanent magnets really permanent?

The magnet is "permanent" as long as nothing knocks its electrons out of alignment (ex. other magnets, heat, vibration). But since it can be demagnetized and remagnetized it is not permanent.

59. Can an electron at rest in a stationary magnetic field be set into motion by the magnetic field? What about the electron placed at rest in an electric field?

An electron must be moving across magnetic field lines in order to feel a magnetic force. So an electron at rest in a stationary magnetic field will feel no force to set it in motion. In an electric field, however, an electron can be accelerated whether or not it is already moving. (A combination of magnetic and electric fields is used in particle accelerators such as cyclotrons. The electric field accelerates the charged particle in its direction, and the magnetic field accelerates it perpendicular to its direction.)

10. At the micro level, what is the difference between an unmagnetized iron nail and a magnetized iron nail?

In an unmagnetized nail, the magnetic domains have a random orientation so that the net magnetism adds to zero. In a magnetized nail, many of the magnetic domains are aligned.

5. In what way are magnetic poles very different from electric charges?

Magnetic poles cannot be isolated; electric charges can.

54. A straight current-carrying wire is horizontal and oriented south to north. When a compass needle is placed below or above it, in what direction does the compass needle point?

The compass needle points perpendicular to the wire.

26. Why are there probably no permanently aligned magnetic domains in Earth's core?

The core is too hot to allow atoms to maintain constant magnetic orientation.

62. A magnetic field can deflect a beam of electrons, but it cannot do work on the electrons to change their speed. Why?

The direction of a magnetic force is always perpendicular to the velocity of an electron, so it cannot do work on the electron.

16. Why does a piece of iron in a current-carrying loop increase the magnetic field strength?

The electromagnetic field is amplified because it aligns the domains in the iron.