# Homework Help: How can i find the Potential difference?

1. Apr 27, 2012

### yy205001

1. The problem statement, all variables and given/known data
Through what potential difference must an electron be accelerate, starting from rest, to acquire a speed of 0.99c?

vf=0.99c(m/s), vi=0(m/s), m=9.11e-31(kg), q=1.6e-19(C)
2. Relevant equations

E=γmc^2=V*q

3. The attempt at a solution
I plugged in all knowns in the equation and find V(emf).

What i got is 3.63e6 eV but the answer is 3.1e6 V.

What did i do wrong in the progress??

2. Apr 28, 2012

### ehild

The voltage gives the electron extra energy over its rest energy, mc^2. So Vg=mc^2(γ-1).

ehild

3. Apr 28, 2012

### yy205001

But how does (γ-1) come from??

4. Apr 28, 2012

### yy205001

ehild, Never mind, i got it!

But should the unit be eV rather than V?

5. Apr 28, 2012

### ehild

The question is potential difference, that is volts. But the change of energy of the electron is the same number in eV as the acceleration voltage: 1 elementary charge times U potential difference is 1*U =U eV.

ehild

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook