# Homework Help: Magnetic Fields and electrons

1. Mar 2, 2006

### jakeowens

What magnitude and direction would an electric field, E, need to be to cancel out the force on an electron traveling with speed, v, in magnetic field B?

I know the formula for the force on an electron in a magnetic field is F=qvB. The formula for the force on an electron in a electric field is F=qE. can i just set the two equal to eachother? giving me qE=qvB, divide by q, giving me E=vB? is that right?

2. Mar 2, 2006

### Staff: Mentor

Your answer is supposed to have magnitude and direction, so you need to use the vector forms of the equations that you list. Other than that, you are on the right track. BTW, what is the shape of the path of an electron moving with velocity v in a magnetic field? When you get your E(t) field correct, the electron will go back to just a straight path with velocity v.

3. Mar 2, 2006

### jakeowens

I dont know any information other then what was given, thats why I'm slightly confused, im just getting messed up in the directions

4. Mar 2, 2006

### jakeowens

The electric field has to flow in a direction (from positive to negative) in the same direction as the force from the magnetic field correct? that way the electron will be attracted back, canceling out the force from the magnetic field, allowing it to travel in a straight path. Am i right?

5. Mar 2, 2006

### topsquark

Actually because the electron has a negative charge, the electric field will point in the direction opposite to the direction of the velocity. I think what the question is after is the relative directions of the vectors involved, so sketch a quick diagram and make an assumption of what direction the E field is, then figure out the required direction of B.

Note: The magnetic force is NOT F=qvB. Use the vector version of this equation!

-Dan