# Question about the electric field

1. Feb 22, 2013

### fishingspree2

Hello, please take a look at the first 2 minutes of this video:

He says that we compute the E field, so when we put a charge anywhere, we can know what will happen to the charge because we know the E field due to the other charges.

But wouldn't the added charge disrupt the force equilibrium between the charges, thus forcing them to move to reach a new equilibrium state, thus changing the E field??

Last edited by a moderator: Sep 25, 2014
2. Feb 22, 2013

### cepheid

Staff Emeritus
we are envisioning a scenario in which the source E-field is static. Whatever distribution of source charges is generating it is unaffected by the presence of the test charge, because we've arranged for it to be so.

3. Feb 22, 2013

### TSny

Yes, that's a very good point. Generally, placing a charged object Q at some place P will cause charges in neighboring objects (conductors or insulators) to shift their positions. As you noted, the electric field experienced by object Q will not be the same as the field at P before Q was brought in. This is certainly a complication! See http://web.mit.edu/jbelcher/www/att.html for a visualization of a point charge moving near a conducting sphere. As the point charge moves, the surface charge on the sphere moves around. The electric field produced by the charged sphere changes as the point charge moves.

Last edited: Feb 22, 2013