What is the direction of the induced current in FIGURE Q30.1?

FIGURE Q30.1

You want to insert a loop of copper wire between the two permanent magnets in FIGURE Q30.2. Is there an attractive magnetic force that tends to pull the loop in, like a magnet pulls on a paper clip? Or do you need to push the loop in against a repulsive force? Explain.

FIGURE Q30.2

A vertical, rectangular loop of copper wire is half in and half out of the horizontal magnetic field in FIGURE Q30.3. (The field is zero beneath the dashed line.) The loop is released and starts to fall. Is there a net magnetic force on the loop? If so, in which direction? Explain.

FIGURE Q30.3

Does the loop of wire in FIGURE Q30.4 have a clockwise current, a counterclockwise current, or no current under the following circumstances? Explain.

1. The magnetic field points out of the figure and is increasing.

2. The magnetic field points out of the figure and is constant.

3. The magnetic field points out of the figure and is decreasing.

FIGURE Q30.4

The two loops of wire in FIGURE Q30.5 are stacked one above the other. Does the upper loop have a clockwise current, a counterclockwise current, or no current at the following times? Explain.

1. Before the switch is closed.

2. Immediately after the switch is closed.

3. Long after the switch is closed.

4. Immediately after the switch is reopened.

FIGURE Q30.5

FIGURE Q30.6 shows a bar magnet being pushed toward a conducting loop from below, along the axis of the loop.

1. What is the current direction in the loop? Explain.

2. Is there a magnetic force on the loop? If so, in which direction? Explain.

   Hint: A current loop is a magnetic dipole.

3. Is there a force on the magnet? If so, in which direction?

FIGURE Q30.6

A bar magnet is pushed toward a loop of wire as shown in FIGURE Q30.7. Is there a current in the loop? If so, in which direction? If not, why not?

FIGURE Q30.7

FIGURE Q30.8 shows a bar magnet, a coil of wire, and a current meter. Is the current through the meter right to left, left to right, or zero for the following circumstances? Explain.

1. The magnet is inserted into the coil.

2. The magnet is held at rest inside the coil.

3. The magnet is withdrawn from the left side of the coil.

FIGURE Q30.8

Is the magnetic field strength in FIGURE Q30.9 increasing, decreasing, or steady? Explain.

FIGURE Q30.9

An inductor with a 2.0 A current stores energy. At what current will the stored energy be twice as large?

1. Can you tell which of the inductors in FIGURE Q30.11 has the larger current through it? If so, which one? Explain.

   FIGURE Q30.11

   

   FIGURE Q30.11 Full Alternative Text

2. Can you tell through which inductor the current is changing more rapidly? If so, which one? Explain.

3. If the current enters the inductor from the bottom, can you tell if the current is increasing, decreasing, or staying the same? If so, which? Explain.

An LC circuit oscillates at a frequency of 2000 Hz. What will the frequency be if the inductance is quadrupled?

Rank in order, from largest to smallest, the three time constants ${\tau }_{\text{A}}$ to ${\tau }_{\text{C}}$ for the three circuits in FIGURE Q30.13. Explain.

FIGURE Q30.13

For the circuit of FIGURE Q30.14:

1. What is the battery current immediately after the switch closes? Explain.

2. What is the battery current after the switch has been closed a long time? Explain.

   FIGURE Q30.14

   

   FIGURE Q30.14 Full Alternative Text