# Work needed to move charge from infinity to center of sphere

Hey, I recenty took an exam and the professor asked us to turn in a corrected version of the exam based on the comments he made to us (I guess people didn't do as well as he expected). On one of the problems it asks:

How much work does it take for you to move a charge Q from infinity to the center of the sphere (all charge is located at the surface)?

I already calculated the potential from infinity to the surface of the sphere as well as the work required to create the sphere. When I originally did the problem, I did this:

W=(1/2)*Q*(Work to create sphere)

He made a comment circling that and then: Q*delta-V(due to q=4 pi R^2) from infinity to R (R=surface of sphere).

I don't really know what the professor means by that and I can't really ask him right now either. Does anyone have an idea what that comment is supposed to mean or how I should be correcting this problem?

ZapperZ
Staff Emeritus
GTdan said:
Hey, I recenty took an exam and the professor asked us to turn in a corrected version of the exam based on the comments he made to us (I guess people didn't do as well as he expected). On one of the problems it asks:
How much work does it take for you to move a charge Q from infinity to the center of the sphere (all charge is located at the surface)?
I already calculated the potential from infinity to the surface of the sphere as well as the work required to create the sphere. When I originally did the problem, I did this:
W=(1/2)*Q*(Work to create sphere)
He made a comment circling that and then: Q*delta-V(due to q=4 pi R^2) from infinity to R (R=surface of sphere).
I don't really know what the professor means by that and I can't really ask him right now either. Does anyone have an idea what that comment is supposed to mean or how I should be correcting this problem?

The potential from infinity to the surface is Delta(V). All you needed to do is multiply that by Q, and you get the work done to move the charge from infinity to the surface. This is what your Prof. was trying to tell you.

Zz.

oh ok. So the potential that I originally calculated from infinity to the surface (R) time Q was what I needed. I guess I made it harder than it should have been. 