What Potential Difference Is Needed to Stop an Electron?

[eprparadox]

Homework Statement

What potential difference is needed to stop an electron having an initial speed of 4.2*10^5 m/s?

Homework Equations

W = delta K
W = q*delta V

The Attempt at a Solution

So I'm pretty sure that the potential difference should be negative in order to stop the electron, but I keep getting a positive answer (the correct magnitude, but incorrect sign).

I set delta K = q*delta V.

I know delta K will be less than zero since K(final) < K(initial) and q should also be negative since we're dealing with an electron. But this gives me a positive delta V.

Where am I going wrong?

Thanks in advance.

 

[rock.freak667]

so you have 1/2 mv²=VQ

for an electron Q=-1.60*10^-19C