1. The problem statement, all variables and given/known data What potential difference is needed to stop an electron having an initial speed of 4.2*10^5 m/s? 2. Relevant equations W = delta K W = q*delta V 3. 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.