# Homework Help: Electric field decelerating electron

1. Aug 11, 2014

### Daltohn

1. The problem statement, all variables and given/known data
Electron are decelerated by the voltage 1,3V. Initial velocity is 22 Mm/s, what is the final velocity?

2. Relevant equations
W=qU, W=KE_2-KE_1

3. The attempt at a solution
From above equations I get v_2=sqrt(2(W+KE_1)/m). q is the elementary charge -e so the work is negative in the equation. The answer is 5,2 Mm/s though, I get 21,99 Mm/s so barely any work done. It feels simplified, that the voltage could be used like that.

2. Aug 11, 2014

### Staff: Mentor

1.3kV works better.

3. Aug 12, 2014

### Daltohn

Aarghh, this book has so many errors... Thanks. :) While we're at it, our equation for work is charge*voltage, so here the voltage would be positive and the charge negative. Is that how you do it? I'm often confused whether work should be subtracted or added but have negative value. With vectors, when you solve the corresponding scalar equation the magnitude should come out positive, for example with forces even though a force is in the negative direction according to your diagram. But with work you should get a negative value if it is negative, since it is a scalar. Have I understood correctly?