Table of Contents
#8
#14
#32
#34
#36
#42
#52
#54
#58
#60
8. Why is it not possible for two magnetic field lines not to cross?
There can be only one value of the magnetic field, a vector quantity, at a single point. To say that more than one magnetic field value existed at a single point would be like saying that there was more than one value of a net force.
14. A horizontal wire is aligned along the direction to magnetic north and carries a current northward. Which way will a compass needle point if the compass is placed just above the wire?
The compass needle would point along a northeasterly direction. Prior to the current-carrying wire's appearance, the needle would point to the north. The current carrying-wire has a magnetic field above it that points to the east. Since magnetic fields point to a "south pole," and a compass also points to a magnetic "south pole," the current-carrying wire causes the compass needle to swing toward the east. Combining the wire's magnetic field with the earth's causes the compass needle to point in a northeasterly direction.
32. In what direction must the electric current in the earth's core circulate to produce the earth's magnetic field?
As pointed out on page 497 of your text, the earth's magnetic pole located in northern Canada is a SOUTH magnetic pole (since it attracts the north pole on the compass needle). Using the right-hand rule, the current flows to the west.
34. Under what conditions does the velocity of a charged particle moving in a magnetic field remain constant?
The magnetic force is defined as the product of the particle's charge, the velocity perpendicular to the magnetic field, and the magnetic field (with the direction of the force given by the right-hand rule). Therefore, a particle whose velocity is entirely parallel, or anti-parallel, to the magnetic field experiences no force - and has a constant velocity.
36. An electron beam is directed horizontally toward the west at a laboratory near Chicago. What is the approximate line of the force on these electrons due to the magnetic field of the earth?
Recall that the magnetic force is the product of the charge (here it's negative since these are electrons), the velocity, and the magnetic field, with the direction given by the right-hand rule. The direction of the earth's magnetic field in the 48 states is given on page 497 as to the north and down. Thus, the right-hand rule seems to give the force direction as down and to the south... but here we have a negative charge, so the force direction will be the negative of down and south. That is, the force will be upward and to the north.
42. If a horizontal loop of wire is raised vertically upward from the earth's north magnetic pole, will there be a current in the loop?
Yes. The loop is moving away from the earth, so the magnetic field around the loop is decreasing. Since the magnetic field is changing, Faraday's law tells us that a current will be induced in the wire.
52. Each of the following statements is correct as written. Which one is NOT true if you interchange the capitalized words in each sentence?
a. A changing MAGNETIC field can produce a changing ELECTRIC field.
b. A steady CURRENT produces a changing MAGNETIC FIELD.
c. A changing CURRENT can produce a changing MAGNETIC FIELD.
d. A changing ELECTRIC field can produce a steady MAGNETIC field.
b. A steady magnetic field does not produce a current.
54. How are electromagnetic waves generated?
Electromagnetic waves are produced whenever an electric charge is accelerated (see the section on the electromagnetic spectrum in your text for details).
58. What is the difference between X-rays and gamma rays?
They have different wavelengths and frequencies.
60. How is sound transmitted by an FM radio station?
FM means frequency modulated. Sound is transmitted by modulating the frequency of the electromagnetic waves about the carrier frequency.
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