AP Physics 2 : Electromotive Force and Magnetic Flux

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 : Electricity And Magnetism

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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 lower potential than the bottom

 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 higher potential than the bottom

 the top is at a lower 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 : Electricity And Magnetism

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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 potential equals the magnetic field strength.

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.

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 strength created by the separated charge equals the magnetic field strength.

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.

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