AP Physics C Electricity : Understanding Magnetic Fields and Charges
Study concepts, example questions & explanations for AP Physics C Electricity
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Example Questions
Example Question #1 : Magnetism
A proton traveling 
To calculate the magnetic force of a single charge, we use 
Since this magnetic force causes the proton to travel in a circular path, we set this magnetic force equation equal to the centripetal force equation.
Using the values given in the question, we can solve for the radius.
Example Question #1 : Magnetism
Which of the following best describes the net magnetic flux through a closed sphere, in the presence of a magnet?
Zero regardless of the orientation of the magnet
Negative only if the north pole of the magnet is within the surface
Positive only if the north pole of the magnet is within the surface
Zero only if the magnet is completely enclosed within the surface
More than one of the other options is true
Zero regardless of the orientation of the magnet
The net magnetic flux (or net field flowing in and out) through any closed surface must always be zero. This is because magnetic field lines have no starting or ending points, so any field line going into the surface must also come out. In other words, "there are no magnetic monopoles."
Example Question #1 : Understanding Magnetic Fields And Charges
A particle of charge 
Relevant equations:
Set the magnetic force equal to the centripetal force, since the magnetic force is directed towards the center of the particle's circular path and centripetal force is defined as the net force towards the center of a circular path.
Rearrange to isolate the velocity:
Determine the distance, 
Plug this distance and velocity into 
Example Question #1 : Magnetism
Consider a current-carrying loop with current 
A particle with charge 
The correct answer is zero. To calculate the force of a magnetic field on a moving charged particle, we use the cross product. We know that if the magnetic field is parallel to the velocity vector of the particle, then the force produced is zero.
Because our magnetic field in this case is going in the same direction as the velocity of the particle, we know that the magnetic force on the particle is zero.
Example Question #1 : Understanding Magnetic Fields And Charges
Consider two long, straight, current-carrying wires at distance 
If the two wires described are not held in place, what motion will result from the magnetic fields produced?
The wires will move away from each other
The wires will rotate counterclockwise
The wires will rotate clockwise
The wires will move toward each other
The wires will remain in place
The wires will move away from each other
The answer is that the wires will move from each other. Using our right hand rule, we know that the magnetic fields produced by each wire are in the same direction, as long as their currents oppose. Using the right hand rule again to determine the direction of the force exerted on each wire by the magnetic field with which they are interacting yields a force in the direction away from the other wire for each wire.
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