9

When a refrigerator/freezer compressor motor initially engages, the motor that is powered by  draws initially  of current.  After a few seconds the back emf has been built up to its value of  at which point the compressor motor now draws only .  What is the resistance of the motor’s armature?

 

A.

B.

C.

 

D.

 

 

 

 

So the Correct Answer is C !

10

If the emf induced in a circular coil exceeds the break down voltage of air, there will be a spark of electricity across the diameter of the coil.  Consider a circular coil with  turns of wire around it and a radius of .  An induced emf of  would result in the sparking across the coil.  What is the change in magnetic field passing through the coil that would create this sparking?

 

A.

C.

 

 

 

 

B.

D.

 

Solving for

 

 

- sign dropped as it indicates polarity not value.

 

 

So Correct answer is B !

11

A long straight wire has a current  going to the right as shown below.  What is the magnitude and direction of the magnetic field created at a point  below the wire?

 

A.

C.

 

 

 

 

B.

D.

 

By the right hand rule, the direction will be into the paper ().

 

 

So Correct Answer is D !

12

As shown above a wire is  long and is carrying a current of .  It is sitting in a constant external magnetic field, which has a field strength of  at an angle of .  What is the magnitude and direction of the force acting on the wire?

 

A.

C.

 

 

 

 

B.

D.

 

 

Using the right hand rule for cross products  points out of the paper .  The magnitude of the force is found by

 

 

So Correct Answer is B !

 

 

 

Please send any comments or questions about this page to ddonovan@nmu.edu

This page last updated on October 16, 2020