# AP Physics 2 : Magnetic Fields

## Example Questions

### Example Question #1 : Magnetic Fields

Suppose that a proton moves perpendicularly through a magnetic field at a speed of . If this proton experiences a magnetic force of , what is the strength of the magnetic field?

.

Explanation:

To solve this question, we need to relate the speed and charge of the particle with the magnetic force it experiences in order to solve for the magnetic field strength. Thus, we'll need to use the following equation:

Also, we are told that the particle is moving perpendicularly to the magnetic field.

Rearrange to solve for the magnetic field, then plug in known values and solve.

### Example Question #2 : Magnetic Fields

Suppose that a positively charged particle with charge  moves in a circular path of radius  in a constant magnetic field of strength . If the magnetic field strength is doubled to , what effect does this have on the radius of the circular path that this charge takes?

Explanation:

To answer this question, we need to realize that the particle is moving in a circular path because of some sort of centripetal force. Since the charge is moving while within a constant magnetic field, we can conclude that it is the magnetic force that is responsible for the centripetal force that keeps this charge moving in a circle. Thus, we need to relate the centripetal force to the magnetic force.

The above equation shows us that the radius of the circular path is directy proportional to the mass and velocity of the particle, and inversely proportional to the charge of the particle and the magnetic field strength. Thus, if the value of the magnetic field is doubled, the above equation predicts that the value of the radius would be cut in half.

### Example Question #3 : Magnetic Fields

loops of current carrying wire form a solenoid of length  that carries  and have radius . Determine the magnetic field at the center of the solenoid.

Explanation:

Using:

Where:

is the magnetic field

is the number of coils

is the current in the solenoid

is the length of the solenoid

is

Plugging in values:

### Example Question #1 : Electricity And Magnetism

There is a loop with a radius of  and a current of . Determine the magnitude of the magnetic field at the center of the loop.

None of these

Explanation:

Using the Biot-Savart law:

Where is the radius of the loop

is the current

is the distance from the center of the loop

Plugging in values:

### Example Question #1 : Magnetism And Electromagnetism

A circular circuit is powered by a  battery. How will the magnetic field change if the battery is removed and placed in the opposite direction?

The magnetic field will have the same magnitude, albeit in the opposite direction

The magnetic field will have the same magnitude and direction

None of these

The magnetic field will double in magnitude and flip directions

The magnetic field will become zero

The magnetic field will have the same magnitude, albeit in the opposite direction

Explanation:

Reversing the battery will reverse the direction of the current. Using the right hand rule, it can be seen that this will also reverse the direction of the magnetic field. Since the magnitude of the current stays the same, the magnitude of the magnetic field will as well.

### Example Question #6 : Magnetic Fields

A circular circuit is powered by a  battery. How will the magnetic field change if a second  battery is added in the same direction as the first?

The magnetic field will stay the same

The magnetic field will become zero

The magnetic field will double in magnitude and have the same direction.

None of these

The magnetic field will double in magnitude and have the same direction.

Explanation:

Based on the Biot-Savart law:

Doubling the voltage will double the current, which will double the magnetic field. The direction will stay the same.

### Example Question #1 : Magnetic Fields

If the north end of a magnetic points towards the geographic north pole, that means that the geographic north pole is a magnetic __________ pole.

Electrical

North

Mono

South

None of these

South

Explanation:

Magnets will align themselves with the surrounding magnetic field. Thus, if the north pole of a magnet is pointing north, the direction of the magnetic field must be pointing north. Magnetic fields point towards magnetic south poles, so the geographic north pole is actually a magnetic south pole.

### Example Question #8 : Magnetic Fields

An infinitely long wire carries a current of  determine the magnitude of the magnetic field  away.

None of these

Explanation:

Magnetic field of an infinitely long wire:

Where

Plugging in values:

### Example Question #9 : Magnetic Fields

A circuit contains a  battery and a  resistor in series. Determine the magnitude of the magnetic field outside of the loop  away from the wire.

None of these

Explanation:

Using

Converting  to  and plugging in values

Determining current:

### Example Question #10 : Magnetic Fields

How strong would a magnetic field need to be in order to make a particle with a mass of  and a charge of  move in a circular path with a speed of  and a radius of ?