Work done on a charge as it moves

[reising1]

There's a setup here where a charge q2 travels in a circular motion around a charge q1 at a radius r.

Apparently, the work done on q2 as it moves is equal to the change in its potential energy. I thought work done was E*d*q2?

When do we use W = F*d = Eq*d and when do we use F = delta U

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[shoehornlettuce]

There's a setup here where a charge q2 travels in a circular motion around a charge q1 at a radius r.

Apparently, the work done on q2 as it moves is equal to the change in its potential energy. I thought work done was E*d*q2?

When do we use W = F*d = Eq*d and when do we use F = delta U

They both tell you the same thing. You chose which to use depending on what information you are given. For example, say you are moving something up a ramp and onto a platform that is 5 feet off the ground. On the MCAT it is easier to calculate work done using energy conservation (i.e. delta U = mgh) than to mess with sin, cosine, the angle of the ramp and so on.

As for work done on a charged particle, work is still equal to force * distance. The difference is that you calculate force on a charged particle differently (F=qE). This is simply substituted for force in W=Fd to give W=qEd.

 

[Patassa]

I thought there was no work done on a particle moving in a circle?

N/m, I was thinking about a magnetic field, not an electric one.

 

[reising1]

No, you're right, there is no work done, because the change in potential is 0 since the distance stays the same.