# Electron travelling in Magnetic and Electric Field.

1. Mar 14, 2006

### vg19

I am having trouble figuring out the answer to one MCQ question.

An electron with velocity = 2.0 x 10^6 i m/s enters a region with electric field E = 200 j V/m. Because there is also a magnetic field present, the electron follows a straight line path at a constant velocty. The magnetic field could be:

a) B = (-1 x 10^4 k) T
b) B = (4 x 10^8 k) T
c) B = (1 x 10^-4 j) T
d) B = (1 x 10^4 k) T

I am really really confused with this question. Wont the electric field cause the electron to change speeds and direction? Also, the magnetic field can only alter the direction of the electron correct? So, how can the electron follow a straight line path at all?? AHHH I think something is horribly wrong with my logic and any help would be appreciated.

Thanks

2. Mar 14, 2006

### eep

What if the magnetic field was set up so that the force felt due to the magnetic field counteracts the force of the electric field?

3. Mar 14, 2006

### vg19

Would the magnetic feild not have to be in the -j direction then?

4. Mar 14, 2006

### topsquark

The force from the magnetic field is at right angles to the velocity of the electron:
$$F=qv\, x\, B$$

-Dan