# Help with two quick electric potential and voltage problesm

1. Feb 16, 2012

### jhcollaz

1. The problem statement, all variables and given/known data

1) How much work is needed (done by the field) to move a -6.0 µC charge from ground to a point whose potential is +7.00 V higher?

2)An electron starts from rest 74.5 cm from a fixed point charge with Q = -0.150 µC. How fast will the electron be moving when it is very far away?

It says my answer is not right. I only have two attempts at these problems so im not sure what to do. Do i include the sign of the charge?

2. Relevant equations

For (1)

w=q*v

For (2)

W = (1/2)*m*V^2 -(1/2)*m*V^2
W = K*(qQ/r)

3. The attempt at a solution

(1)
w=q*v
=(6*10^-6) * 7
= 0.000042 J

(2)
W = K*(qQ/r)
=(9*10^9)* ( ((1.6*10^19)*(0.125*10^-6)) / 0.745 )
W = 2.41611*10^-16

W = (1/2)*m*V^2 -(1/2)*m*V^2
2.41611*10^-16 = (1/2)*(9.11*10^-6)*V^2 -0
v = 2.30311e7

2. Feb 16, 2012

### Delphi51

I would include a minus sign on (1) because work is done by the potential as it attracts the negative charge to the positive plate.
In (2), you wrote .125 for the charge when it says .15 in the question, and it has thrown off your answer. No minus in (2) since directions are not involved.