Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | Magnetic field lines, like electric field lines, never intersect. Suppose it were possible for two magnetic field lines to intersect at a point in space. Discuss what this would imply about the force(s) that act on a charge moving through such a point, thereby ruling out the possibility of field lines crossing.

| 2 | Suppose you accidentally use your left hand, instead of your right hand, to determine the direction of the magnetic force on a positive charge moving in a magnetic field. Do you get the correct answer? If not, what direction do you get?

| 3 | ssm A charged particle, passing through a certain region of space, has a velocity whose magnitude and direction remain constant, (a) If it is known that the external magnetic field is zero every where in this region, can you conclude that the external electric field is also zero? Explain. (b) If it is known that the external electric field is zero everywhere, can you conclude that the external magnetic field is also zero? Explain.

| 4 | Suppose that the positive charge in Figure 21-10a were launched from the negative plate toward the positive plate, in a direction opposite to the electric field. A sufficiently strong electric field would prevent the charge from striking the positive plate. Suppose the positive charge in Figure 21-10b were launched from the south pole toward the north pole, in a direction opposite to the magnetic field. Would a sufficiently strong magnetic field prevent the charge from reaching the north pole? Account for your answer.

| 5 | Review Conceptual Example 4 and Concept Simulation 21.1 at as background for this question. Three particles move through a constant magnetic field and follow the paths shown in the drawing. Determine whether each particle is positively charged, negatively charged, or neutral. Give a reason for each answer.

Question 5

| 6 | Refer to Figure 21-12. Assume that the particle in the picture is a proton. If an electron is projected at point 1 with the same velocity , it will not follow the same path as the proton, unless the magnetic field is adjusted. Explain how the magnitude and direction of the field must be changed.

| 7 | The drawing shows a top view of four interconnected chambers. A negative charge is fired into chamber 1. By turning on separate magnetic fields in each chamber, the charge can be made to exit from chamber 4, as shown. (a) Describe how the magnetic field in each chamber should be directed. (b) If the speed of the charge is v when it enters chamber 1, what is the speed of the charge when it exits chamber 4? Why?

| 8 | A positive charge moves along a circular path under the influence of a magnetic field. The magnetic field is perpendicular to the plane of the circle, as in Figure 21-12. If the velocity of the particle is reversed at some point along the path, will the particle retrace its path? If not, draw the new path. Explain.

| 9 | When one end of a bar magnet is placed near a TV screen, the picture becomes distorted. Why?

| 10 | ssm The drawing shows a particle carrying a positive charge +q at the coordinate origin, as well as a target located in the third quadrant. A uniform magnetic field is directed perpendicularly into the plane of the paper. The charge can be projected in the plane of the paper only, along the positive or negative x or y axis. Thus, there are four possible directions for the initial velocity of the particle. The particle can be made to hit the target for only two of the four directions. Which two are they? Give your reasoning, and draw the two paths that the particle can follow on its way to the target.

Question 10

| 11 | Refer to Figure 21-17. (a) What happens to the direction of the magnetic force if the current is reversed? (b) What happens to the direction of the force if both the current and the magnetic poles are reversed? Explain your answers.

| 12 | The drawing shows a conducting wire wound into a helical shape. The helix acts like a spring and expands back toward its original shape after its coils are squeezed together and released. The bottom end of the wire just barely touches the mercury (a good electrical conductor) in the cup. After the switch is closed, current in the circuit causes the light bulb to glow. Does the bulb glow continually, glow briefly and then go out, or repeatedly turn on and off like a turn signal on a car? Explain.

| 13 | In Figure 21-28, assume that the current I1 is larger than the current I2. In parts a and b, decide whether there are places where the total magnetic field is zero. State whether they are located to the left of both wires, between the wires, or to the right of both wires. Give your reasoning.

| 14 | The drawing shows an end-on view of three parallel wires that are perpendicular to the plane of the paper. In two of the wires the current is directed into the paper, while in the remaining wire the current is directed out of the paper. The two outermost wires are held rigidly in place. Which way will the middle wire move? Explain.

| 15 | For each electromagnet at the left of the drawing, explain whether it will be attracted to or repelled from the permanent magnet at the right.

Question 15

| 16 | For each electromagnet at the left of the drawing, explain whether it will be attracted to or repelled from the adjacent electromagnet at the right.

Question 16

| 17 | Refer to Figure 21-5. If the earth’s magnetism is assumed to originate from a large circular loop of current within the earth, how is the plane of this current loop oriented relative to the magnetic axis, and what is the direction of the current around the loop?

| 18 | ssm There are four wires viewed end-on in the drawing. They are long, straight, and perpendicular to the plane of the paper. Their cross sections lie at the corners of a square. Currents of the same magnitude are in each of these wires. Choose the direction of the current for each wire, so that when any single current is turned off, the total magnetic field at point P (the center of the square) is directed toward a corner of the square. Account for your answer.

| 19 | Suppose you have two bars, one of which is a permanent magnet and the other of which is not a magnet, but is made from a ferro magnetic material like iron. The two bars look exactly alike. (a) Using a third bar, which is known to be a magnet, how can you determine which of the look-alike bars is the permanent magnet and which is not? (b) Can you determine the identities of the look-alike bars with the aid of a third bar that is not a magnet, but is made from a ferromagnetic material? Give reasons for your answers.

| 20 | In a TV commercial that advertises a soda pop, a strong electro magnet picks up a delivery truck carrying cans of the soft drink. The picture switches to the interior of the truck, where cans are seen to fly upward and stick to the roof just beneath the electromagnet. Are these cans made entirely of aluminum? Justify your answer.

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