Question about the electric field

[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??

 

[cepheid]

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.

 

[TSny]

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??

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.