Module 8: Magnetic Forces and Fields

Question 1

Answer 1

the positive y-direction.

Question 2

1. into the plane of the figure
2. out of the plane of the figure
3. misleading question—the magnetic field at P is zero
4. to the right
5. to the left

Answer 2

1. into the plane of the figure

Question 3

1. direction 3
2. direction 1
3. the B field is zero
4. direction 2
5. direction 4

Answer 3

1. direction 3

Question 4

1. in the direction of the velocity
2. toward the upper point charge (the force is attractive)
3. none of the above
4. opposite to the direction of the velocity
5. away from the upper point charge (the force is repulsive)

Answer 4

2. toward the upper point charge (the force is attractive)

Question 5

1. the positive x-direction.
2. the negative x-direction.
3. none of the above
4. the negative y-direction.
5. the positive y-direction.

Answer 5

1. the positive x-direction.

Question 6

1. upward (toward AB)
2. to the right
3. to the left
4. misleading question—the net magnetic force is zero
5. downward (away from AB)

Answer 6

1. upward (toward AB)

Question 7

1. paths a, b, c, and d
2. The answer depends on whether the integral goes clockwise or counterclockwise around the path.
3. paths a and c
4. paths b and d
5. path a only

Answer 7

3. paths a and c

Question 8

1. into the page
2. zero
3. left
4. out of the page
5. right

Answer 8

4. out of the page

Question 9

1. arrangement 1
2. arrangement 2
3. arrangement 3
4. same for all

Answer 9

2. arrangement 2

Question 10

1. 1 is positive, 2 is neutral, and 3 is negative.
2. 1 is positive, 2 is negative, and 3 is neutral.
3. 1 is neutral, 2 is positive, and 3 is negative.
4. 1 is neutral, 2 is negative, and 3 is positive.
5. 1 is negative, 2 is neutral, and 3 is positive.

Answer 10

5.1 is negative, 2 is neutral, and 3 is positive.

Question 11

A charged particle is moving with speed v perpendicular to a uniform magnetic field. A second identical charged particle is moving with a speed of 2v perpendicular to the same magnetic field. If the frequency of revolution of the first particle is f, the frequency of revolution of the second particle is

1. 2f
2. f.
3. 4f
4. f/4
5. f/2

Answer 11

2. f.

Question 12

1. A
2. B
3. C
4. D
5. The magnetic field is zero at that point.

Answer 12

2. B

Question 13

1. upward
2. downward
3. into the page
4. out of the page
5. The magnetic force is zero since the velocity is parallel to the current.

Answer 13

1. upward

Question 14

Magnetic field lines point ___ N poles and ___ S poles.

Answer 14

Magnetic field lines point away from N poles and toward S poles.

Question 15

The direction of the magnetic field is ___ to the field line at any point in space. A small compass will point ___ relative to the field line.

Answer 15

The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.

Question 16

The strength of a magnetic field is ___ the closeness of the field lines.

Answer 16

The strength of a magnetic field is proportional to the closeness of the field lines.

Question 17

T or F. Magnetic field lines can never cross.

Answer 17

True.

(This means the field is unique at any point in space)

Question 18

Where do magnetic field lines terminate?

Answer 18

Magnetic field lines are continuous, forming closed loops without a beginning or end. They are directed from the north pole to the south pole.

Question 19

A particle moving in a magnetic field will experience a magnetic force at right angles to both ___ and ___.

Answer 19

A particle moving in a magnetic field will experience a magnetic force at right angles to both the charge’s velocity and the magnetic field.

Question 20

Magnetic Force RHR

Answer 20

Point your fingers toward the velocity, then curl them towards the magnetic field. Your thumb is pointing in the direction of magnetic force.

Question 21

What do magnetic field lines look like around a long, straight current-carrying wire?

Answer 21

Circular loops

Question 22

In what direction is the magnetic force on current flowing in a straight path?

Answer 22

The magnetic force is perpendicular to both the current and the magnetic field.

Question 23

The ___ on a current loop in a uniform magnetic field is zero.

Answer 23

The net force on a current loop in a uniform magnetic field is zero.

(But the net torque is not, in general, equal to zero.)

Question 24

RHR for finding the direction of the magnetic field created by a moving charge at a point near the charge.

Answer 24

Point your fingers toward the charge’s velocity, then curl your finger towards the distance vector through the smallest possible angle. The magnetic field at the point is in the direction of your thumb.

Question 25

RHR for determining the direction of magnetic field lines outside a long straight wire.

Answer 25

Point your thumb in the direction of current, then curl your fingers around the wire. Your fingers are curling in the direction of the circular magnetic field lines.