# Homework Help: Calculate the work needed to move a test charge from point B to D

1. Aug 2, 2012

### shashaeee

Calculate the work needed too move a 1.5μC test charge from point B to D.

Can someone just check if I'm thinking correctly?

Do I just use the formula:

W = qEd ?

For the E, I calculate the magnitude and direction of the electric field at B.
And I use 25cm for the distance and q = 1.5μC

2. Aug 2, 2012

### TSny

W = qEd only works (pun?) if E is uniform. Instead, relate the work to electric potential or electric potential energy.

3. Aug 2, 2012

### shashaeee

Ohhh, I see. So basically, the work needed to bring 1.5μC from a distance (25cm) away to a point 25cm from a charge 75μC?

Would it make sense to use this formula?

W = q(Va - Vb)
W = q[(k*Qa/ra) - (k*Qb/rb)]

q = 1.5μC
Qa = 75μC
Qb = 1.5μC

ra(75μC to D) = 25cm
rb(B to D) = 25cm

4. Aug 2, 2012

### TSny

That's more like it. But, since the charge is being carried from B to D, you'll need to find the potential difference in going from B to D. How would you find the potential at point B due to the two 75 μC charges? Point D?

5. Aug 2, 2012

### shashaeee

ohhh, righttt.

So, rewriting it ..

W = q(VD - VB)

VD = VD1 + VD2

VB = VB1 + VB2

where the distance for VD is 25cm
and the distance for VB is 35cm

6. Aug 2, 2012

Yes, Good!

7. Aug 2, 2012

### shashaeee

oh, thank goodness lol! Thank youu!