# Conceptual Test CH 33: Electromagnetic Induction

## Unlock all answers in this set

question
What is the direction of the induced current in FIGURE Q33.1?
- Magnetic field is going out of the page --> Current is ccw - Magnetic flux is decreasing (because as the bar moves upward, the area is decreasing), therefore the induced current will combat it by moving ccw and magnetic field will be going out of the page aka increase the decreasing field. Book solution: The induced current will be counterclockwise (ccw). As the bar moves upward through the constant-magnetic- field region, the area of the loop decreases, so the flux through the loop decreases. By Lenz's Law, any induced current will tend to oppose this decrease. A ccw current will create a stronger magnetic field inside the loop, tending to increase the flux.
question
You want to insert a loop of copper wire between the two permanent magnets in FIGURE Q33.2. 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. HELP
You need to push against a repulsive force. In its initial position, the loop encircles essentially no magnetic flux, whereas once the loop is positioned between the poles of the magnet, magnetic flux will pass through the loop. Lenz's Law tells us that the current flows in the direction required to oppose a change in flux, so the current will flow in the direction required to create a repulsive force on the copper wire. The current induced in the loop is therefore in the direction shown (into the page on the left side of the copper loop). The magnetic poles of the induced current loop are also shown in the diagram. The resulting magnetic force on the loop is to the right. Note that if the magnetic poles were reversed, the induced current would reverse as well so that the resulting force would still be to the right.
question
A vertical, rectangular loop of copper wire is half in and half out of the horizontal magnetic field in FIGURE Q33.3. (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.
Yes, there is a net upward force. Lenz's Law tells us that a current will be induced in the loop to oppose the change in magnetic flux. The net force will be upward because the upper loop segment traverses the field B whereas the lower loop segment does not (the two side segments produce forces of equal magnitude but in opposite directions). We know the force must be upward because, by Lenz's Law, it must oppose the force due to gravity that is causing the change in flux.
question
Does the loop of wire in FIGURE Q33.4 have a clockwise current, a counterclockwise current, or no current under the following circumstances? Explain. a. The magnetic field points out of the page and is increasing. b. The magnetic field points out of the page and is constant. c. The magnetic field points out of the page and is decreasing.
a. The loop of wire will have a cw current to combat the increasing magnetic field out of the page. b. There will be no current because a changing magnetic flux is necessary to induce a current. c. The loop of wire will have a ccw direction to help the decreasing magnetic field.
question
The two loops of wire in FIGURE Q33.5 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. a. Before the switch is closed. b. Immediately after the switch is closed. c. Long after the switch is closed. d. Immediately after the switch is reopened.
a. There is no current. No magnetic flux is changing, so no magnetic field is induced. b. Cw. An increasing counterclockwise current in the lower loop creates an increasing upward- pointing magnetic field in the upper loop. Thus, a downward magnetic field is induced, resulting from a cw current in the upper loop. c. There will be no current because the current through the lower loop is constant. No changing in magnetic flux in the upper loop. d. Ccw. Immediately after the switch is closed the magnetic field will be decreasing in the lower loop. Thus, a upward magnetic field is induced, resulting from a ccw current in the upper loop.
question
FIGURE Q33.6 shows a bar magnet being pushed toward a conducting loop from below, along the axis of the loop. a. What is the current direction in the loop? Explain. b. Is there a magnetic force on the loop? If so, in which direction? Explain. HELP Hint: A current loop is a magnetic dipole. c. Is there a force on the magnet? If so, in which direction? HELP
a. Clockwise from above. The increasing flux through the loop is due to the upward-pointing field of the north pole of the magnet, so the induced magnetic field is downward. b. Yes, upward. The induced downward field is like a magnet with its north pole on the bottom, which is repulsed by the bar magnet. c. Yes, downward. By Newton's third law, the bar magnet is pushed down.
question
A bar magnet is pushed toward a loop of wire as shown in FIGURE Q33.7. Is there a current in the loop? If so, in which direction? If not, why not?
There is no current induced in the loop. As the bar magnet is pushed upward, the net magnetic flux through the loop is zero. In other words, all flux lines that enter the loop will, by symmetry, also exit the loop. Note that the magnet must be under the center of the loop for this symmetry argument to hold.
question
FIGURE Q33.8 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. a. The magnet is inserted into the coil. b. The magnet is held at rest inside the coil. c. The magnet is withdrawn from the left side of the coil.
a. Left to right. The bar magnet's field inside the coil points from right to left. The flux through the coil is increasing. B induced will point left to right, and is caused by a current flowing left to right through the meter. b. No current. The flux through the coil is not changing. c. Right to left. Flux through the loop decreases as the magnet is withdrawn. Since B original is right to left, B induced is left to right. It is caused by a current flowing right to left through the meter by the RHR.
question
Is the magnetic field strength in FIGURE Q33.9 increasing, decreasing, or steady? Explain.
The magnetic field is out of the page and therefore has a current moving ccw. The given induced current is cw, suggesting that the magnetic field strength is increasing.
question
An inductor with a 2.0 A current stores energy. At what current will the stored energy be twice as large? HELP