# Calculating electric field given velocity and magnetic field

1. Jul 3, 2011

### RKOwens4

1. The problem statement, all variables and given/known data

In the figure below, an electron moves at speed v = 93 m/s along an x axis through uniform electric and magnetic fields. The magnetic field B is directed into the page and has magnitude 4.00 T. In unit vector notation, what is the electric field?

Figure: http://imageshack.us/photo/my-images/233/webassign2.jpg/

2. Relevant equations

v=E/B

3. The attempt at a solution

I know that the answer is 0 for the i and k direction. I thought for the j direction, I just had to use the formula v=E/B, which would give me 372. But that's incorrect. I don't know what else to try.

2. Jul 3, 2011

### rock.freak667

Have you tried -372 ?

3. Jul 3, 2011

### RKOwens4

That did it! Thanks.

4. Jul 3, 2011

### rock.freak667

Good. Usually when they don't specify a direction for the z-axis, you will need to try both + and - signs.

5. Jul 4, 2011

### vela

Staff Emeritus
Note that getting the sign right isn't a matter of guessing. From the right-hand rule, you should be able to see that the magnetic force will point in the +y direction. To cancel it, the electric force must point in the -y direction.