# Thread: Work and Electric Potential

1. ## Work and Electric Potential

How much work is needed to move a -7 μC charge from ground to a point whose potential is +6 V higher?

I am confused of this problem.

Do I use ΔU = qΔV or ΔU = -qΔV? Why?

And after I get the result, how do I know if the work was done by the field (electric force) or the agent (external force)?

2. ## Re: Work and Electric Potential

the work done equals the increase in potential regardless of the charge.

3. ## Re: Work and Electric Potential

Which formula do I use?

4. ## Re: Work and Electric Potential

Originally Posted by joshuaa
Which formula do I use?
sigh...

what's the increase in potential here?

5. ## Re: Work and Electric Potential

This one ΔU = qΔV

but why is it also used for the decrease in potential as well?

then when to use ΔU = -qΔV?

6. ## Re: Work and Electric Potential

Originally Posted by joshuaa
This one ΔU = qΔV

but why is it also used for the decrease in potential as well?

then when to use ΔU = -qΔV?
Ok, I see that potential is defined as the work done moving a positive charge

$\Delta U = q \Delta V$

$W = -\Delta U$

so

$W = -q \Delta V$

Here we have

$\Delta U = -6V$

$q=-7 \mu C$

$W = -(-7 )(U_0 - (U_0+6)) \mu J= 7 (-6)\mu J = -42 \mu J$

You have to do work to move a positive charge to a higher potential. Work is done on a positive charge moving to a lower potential

Work is done on a negative charge moving to a higher potential. You have to do work on a negative charge to move it to a lower potential.

7. ## Re: Work and Electric Potential

Just want to clarify one thing -6V you mean ΔV not ΔU, right?

8. ## Re: Work and Electric Potential

Originally Posted by joshuaa
Just want to clarify one thing -6V you mean ΔV not ΔU, right?
yes

9. ## Re: Work and Electric Potential

Delta V is +6 V, why did you make negative?

I did not understand this part Vo - (Vo + 6)

Shoudn't it be V - Vo?

10. ## Re: Work and Electric Potential

Originally Posted by joshuaa
Delta V is +6 V, why did you make negative?

I did not understand this part Vo - (Vo + 6)

Shoudn't it be V - Vo?
you're right.. you should probably just ignore my post.

alright

then thanks