17

An electron has a mass of  and a charge of .  How fast must the electron be going for the magnetic force to cancel the force of gravity if the magnetic field is horizontal and has a strength of ?

 

A.

C.

 

 

 

 

B.

D.

 

 

Solve for

 

 

So Correct Answer is B !

18

A wire is  long and has a current of  running in the wire due North.  The wire is surrounded by an external magnetic field with a field strength of  which is directed due South.  What is the magnitude of the magnetic force the field is exerting on the wire?

 

A.

C.

 

 

 

 

B.

D.

 

 

Since current is north and B field is due south, the cross product is zero!

 

So Correct Answer is D !

19

Two wires are shown above.  Wire 1 carries a current  straight out of the paper.  The wire has a length of .  Wire 2 carries a current  going to the right.  The wire has a length of .  Wire 1 is located above wire 2 by a distance of

.  What is the magnetic force that Wire 1 exerts on Wire 2.

 

A.

B.

C.

 

D.

 

 

The B field for Wire 1 is concentric circles going counter-clockwise as shown above.  The magnetic force of wire 1 on wire 2 is given by

 

 

Since  is parallel to ,

 

So 

 

So the Correct Answer is C !

20

For a magnet, the north pole is the pole where magnetic field lines exit the magnet and the south pole is the pole where the magnetic field lines enter the magnet.  So where is the Earth’s north magnetic pole currently located?

 

A.

In the Northern Hemisphere.

C.

Along the Equator.

 

 

 

 

B.

In the Southern Hemisphere.

D.

The Earth has no magnetic field.

 

The Earth’s north magnetic pole is currently in the Southern Hemisphere.  A compass’s north pole of its needle, points north because the Earth’s south magnetic pole currently lies in the Northern Hemisphere.  The Earth’s magnetic poles do move about. 

 

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