Electric & Magnetic fields, application of Lorenz's Law to -eV electron

[spenghali]

Homework Statement

A velocity selector consists of electric and magnetic fields described by the expressions vector E = E k-hat and vector B = B j-hat, with B = 0.0130 T. Find the value of E such that a 830 -eV electron moving along the negative x-axis is undeflected?

Homework Equations

The Attempt at a Solution

So I know i want to find E such that an electron with velocity = -v i-hat is undeflected. I also know form Lorenz's force that:

F = q[E + (v x B)] = 0

So i really just need to make E equal to the cross product (v x B) but opposite sign. I did this using using the fact that 830 -eV is the kinetic energy, set this equal to 0.5mv^2 and solved for v to be 4.268837e16 m/s.

Next I computed the cross product to get a value of -5.549e14 k-hat. So E should be equal to this but with opposite sign, not working however for my online submission, any suggestions as to where I am going wrong?

 

[Dick]

4.268837e16 m/s is ridiculous. The speed of light is about 3e8 m/s. I'd check that part of the calculation first.

 

[spenghali]

right right, i should have noticed that