Question 1

Consider a conducting bar that is free to slide along conducting rails in an upward magnetic field as shown in the diagram below. We push the bar so that it moves to the right at a constant speed. Which way will current flow through the bar, the rails, and the light bulb?

Accepted Answer

The answer of Consider a conducting bar that is free...

Question 2

A commercial jet flies above a region of the earth where the earth's magnetic field points nearly
- straight upward, which part of the airplane becomes negatively charged?

Accepted Answer

The answer of A commercial jet flies above a region...

Question 3

A commercial jet flies above a region of the earth where the earth's magnetic field points nearly
-in the jet's direction of motion, which part of the airplane becomes negatively charged?

Accepted Answer

The airplane is moving parallel to the Earth's magnetic field, so there is no perpendicular component of velocity to induce a charge separation due to the magnetic force.

Question 4

A commercial jet flies above a region of the earth where the earth's magnetic field points nearly
- perpendicular to the jet's path from left to right, which part of the airplane becomes negatively charged?

Accepted Answer

The answer of A commercial jet flies above a region...

Question 5

A squat refrigerator magnet sits on a table with its north pole facing up and its south pole down. We push a ring of wire lying on the table directly toward the magnet. What direction does current flow in the ring when viewed from above? (A wooden table will not affect the magnet's field.)

Accepted Answer

When the ring is pushed toward the magnet, the magnetic field through the ring increases. According to Lenz's law, the induced current will flow in such a direction as to oppose the change causing it. Since the north pole of the magnet is facing up, to oppose the increase in magnetic flux, the current must flow in a direction that creates a north pole on the approaching face of the ring, which, when viewed from above, is counterclockwise.

Question 6

Imagine that we constrain a rectangular loop to move through a square region where the magnetic field $\vec{B}$ is uniform and downward (in the $-z$ direction), as shown below. Assume that $\vec{B} \approx 0$ outside that region.
  
We can divide the loop's motion up into five phases:
For each of the five phases, which of the following is true?
-(a) The loop has not yet entered the magnetic field.

Accepted Answer

Since the loop has not yet entered the magnetic field, there is no change in magnetic flux through the loop, and thus, according to Faraday's law of electromagnetic induction, no current is induced.

Question 7

Imagine that we constrain a rectangular loop to move through a square region where the magnetic field $\vec{B}$ is uniform and downward (in the $-z$ direction), as shown below. Assume that $\vec{B} \approx 0$ outside that region.
  
We can divide the loop's motion up into five phases:
For each of the five phases, which of the following is true?
-(b) The loop is partway into the field (as shown).

Accepted Answer

When a loop is partway into a magnetic field, the changing magnetic flux induces a current. The direction of the induced current depends on the direction of the magnetic field and the motion of the loop. According to Lenz's law, the induced current will flow in a direction that opposes the change in magnetic flux. Without specific details on the direction of the magnetic field and the motion of the loop, the exact direction (clockwise or counterclockwise) cannot be determined from the information given. However, since the question implies a change in magnetic flux through the loop (by stating it is partway into the field), an induced current is expected, ruling out option C. The correct direction (A or B) would depend on further details not provided in the question. Given the choices and the lack of specific detail, this response assumes a scenario where the conditions would result in a counterclockwise current, but it's important to note that the actual direction would depend on the specifics of the magnetic field and loop orientation.

Question 8

Imagine that we constrain a rectangular loop to move through a square region where the magnetic field $\vec{B}$ is uniform and downward (in the $-z$ direction), as shown below. Assume that $\vec{B} \approx 0$ outside that region.
  
We can divide the loop's motion up into five phases:
For each of the five phases, which of the following is true?
-(c) The loop is entirely inside the field.

Accepted Answer

Current does not flow in the loop because there is no change in the magnetic field through the loop. For current to be induced, there must be a change in the magnetic flux through the loop according to Faraday's law of electromagnetic induction.

Question 9

Imagine that we constrain a rectangular loop to move through a square region where the magnetic field $\vec{B}$ is uniform and downward (in the $-z$ direction), as shown below. Assume that $\vec{B} \approx 0$ outside that region.
  
We can divide the loop's motion up into five phases:
For each of the five phases, which of the following is true?
-(d) The loop's front end has emerged from the field.

Accepted Answer

The right-hand rule states that if you point your right thumb in the direction of the magnetic field and your fingers in the direction of the current, your palm will point in the direction of the force on the wire. In this case, the magnetic field is pointing into the page and the loop's front end is emerging from the field. This means that the current must be flowing clockwise in the loop in order to produce a force that pushes the loop's front end out of the field.

Question 10

Imagine that we constrain a rectangular loop to move through a square region where the magnetic field $\vec{B}$ is uniform and downward (in the $-z$ direction), as shown below. Assume that $\vec{B} \approx 0$ outside that region.
  
We can divide the loop's motion up into five phases:
For each of the five phases, which of the following is true?
-(e) The loop has now entirely left the field.

Accepted Answer

When the loop has entirely left the magnetic field, there is no change in magnetic flux through the loop, hence according to Faraday's law of electromagnetic induction, no current is induced in the loop.

Question 11

Consider the situation described in problem E9T.7. If current flows in the loop, the magnetic field will exert magnetic forces on the various loop legs, and in some cases there may be a net magnetic force on the loop. If so, we will have to exert a force to keep the loop moving at a constant velocity. In each of the five cases, determine whether the net force we have to exert is
 
-(a) The loop has not yet entered the magnetic field.

Accepted Answer

Since the loop has not yet entered the magnetic field, there is no change in magnetic flux through the loop. According to Faraday's law of electromagnetic induction, an electromotive force (and thus a current) is induced in a loop when there is a change in magnetic flux through the loop. Without a change in magnetic flux, there is no induced current, and thus the force on the loop is zero.

Question 12

Consider the situation described in problem E9T.7. If current flows in the loop, the magnetic field will exert magnetic forces on the various loop legs, and in some cases there may be a net magnetic force on the loop. If so, we will have to exert a force to keep the loop moving at a constant velocity. In each of the five cases, determine whether the net force we have to exert is
 
-The loop is partway into the field (as shown).

Accepted Answer

When a loop is partway into a magnetic field, the induced current creates a force that opposes the change. According to Lenz's Law, the direction of the induced current will be such that it opposes the motion of the loop entering the field, resulting in a force that pushes the loop backward (in the $-x$ direction).

Question 13

Consider the situation described in problem E9T.7. If current flows in the loop, the magnetic field will exert magnetic forces on the various loop legs, and in some cases there may be a net magnetic force on the loop. If so, we will have to exert a force to keep the loop moving at a constant velocity. In each of the five cases, determine whether the net force we have to exert is
 
-(c) The loop is entirely inside the field.

Accepted Answer

When a loop is entirely inside a uniform magnetic field and stationary, there is no change in magnetic flux through the loop. According to Faraday's Law, the induced electromotive force (emf) and thus the induced current is zero when there is no change in flux.

Question 14

Consider the situation described in problem E9T.7. If current flows in the loop, the magnetic field will exert magnetic forces on the various loop legs, and in some cases there may be a net magnetic force on the loop. If so, we will have to exert a force to keep the loop moving at a constant velocity. In each of the five cases, determine whether the net force we have to exert is
 
-The loop's front end has emerged from the field.

Accepted Answer

The answer of Consider the situation described in problem E9T.7....

Question 15

Consider the situation described in problem E9T.7. If current flows in the loop, the magnetic field will exert magnetic forces on the various loop legs, and in some cases there may be a net magnetic force on the loop. If so, we will have to exert a force to keep the loop moving at a constant velocity. In each of the five cases, determine whether the net force we have to exert is
 
-(e) The loop has now entirely left the field.

Accepted Answer

When the loop has entirely left the magnetic field, there is no change in magnetic flux through the loop anymore, so according to Faraday's law of electromagnetic induction, the induced current is zero.

Question 16

The diagram below and to the left shows an electron moving to the right in the plane of the drawing. What is the direction of the electron's magnetic field at points a through $g$ ? Answer using the closest choice given on the right (to indicate a double-letter using the back of the book, point to the letter with two fingers).
 
-(a)

Accepted Answer