AP Physics 2 : Electricity and Magnetism

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Electricity And Magnetism

There is a particle with a charge of  moving  perpendicular through a magnetic field with a strength of . What is the force on the particle?

Possible Answers:

Correct answer:

Explanation:

The equation for force on a moving charged particle in a magnetic field is

.

Because the charge is moving perpendicularly through the magnetic field, we don't have to worry about the cross product, and the equation becomes simple multiplication. 

Therefore, the force experienced by the particle is 3619N.

Example Question #2 : Magnetism And Electromagnetism

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A conductive rod is moving through a region of magnetic field, directed out of the page as diagrammed above. As a result of its motion, the mobile charge in the rod separates, creating an electric potential across the length of the rod. The length of the rod is 0.12m and the magnitude of the magnetic field is 0.022T. If the rod is moving with velocity , what is the magnitude and direction of the potential from one end of the rod to the other?

Possible Answers:

 the top is at a higher potential than the bottom

 the top is at a higher potential than the bottom

 the top is at a lower potential than the bottom

 the top is at a lower potential than the bottom

 the top is at a higher potential than the bottom

Correct answer:

 the top is at a higher potential than the bottom

Explanation:

For a conductor moving in a magnetic field, cutting straight across the field lines, a potential is generated  where  is the potential or EMF,  is the magnetic field strength, and  is the conductor's velocity relative to the field.

As the rod moves, the positive charge feels an upward-directed force by the right-hand rule, and the negative a downward force resulting in the top being at a higher potential than the bottom of the rod.

Example Question #3 : Magnetism And Electromagnetism

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A conductive rod is moving through a region of magnetic field, as diagrammed above. As a result of its motion, mobile charge carriers in the conductor separate, creating an electric potential across the rod. When, if ever, do the charge carriers cease this motion?

Possible Answers:

The motion stops when the magnetic field created by the separated charges equals the external magnetic field.

The motion stops when the electric field strength created by the separated charge equals the magnetic field strength.

The motion stops when the electric potential equals the magnetic field strength.

The motion does not stop. Mobile charge carriers continue to separate as long as the rod remains in motion.

The motion stops when the electric field created by the separated charges creates an equal and opposite force to the magnetic force created by the rod's motion.

Correct answer:

The motion stops when the electric field created by the separated charges creates an equal and opposite force to the magnetic force created by the rod's motion.

Explanation:

The separated charge creates a potential . This potential results in an electric field  When this induced electric field creates a force  equal to the magnetic force  on the mobile charge carriers, motion stops. Of course, if an electric circuit is created drawing current from the rod, motion will resume to rebuild the field.

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?

.

Possible Answers:

Correct answer:

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?

Possible Answers:

Correct answer:

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 #1 : 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.

Possible Answers:

Correct answer:

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.

Possible Answers:

None of these

Correct answer:

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?

Possible Answers:

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

The magnetic field will become zero

The magnetic field will have the same magnitude and direction

None of these

The magnetic field will double in magnitude and flip directions

Correct answer:

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 #1 : Magnetism And Electromagnetism

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?

Possible Answers:

The magnetic field will quadruple

The magnetic field will stay the same

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

The magnetic field will become zero

None of these

Correct answer:

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.

Possible Answers:

Electrical

None of these

North

South

Mono

Correct answer:

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.

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