Confusion about how electric charge is transferred

[intelli78]

I have a question about transfer of electric charge in metals. My understanding is that electrons move in and out of an object at an equal rate when current is flowing (i.e., net change in # of electrons is zero). At the same time, it is possible for an object to become positively or negatively charged by gaining or losing electrons.

These two facts seem contradictory to me; if there is no net change in # of electrons, how can any charge be acquired at all? What am I misunderstanding? Thanks.

 

[Drakkith]

In a closed, isolated circuit, current is the same throughout the circuit, so there is never any net gain or loss of charge. However, you can charge an object because the object is not isolated.

A good example is a capacitor in a circuit. Overall, the circuit is isolated. However, within the circuit the capacitor is not isolated because it is electrically connected to the rest of the circuit, and applying a voltage will cause charges to move off of one plate and onto the other, charging them. The plates are charged, but the net charge of the circuit is still neutral.

Another example is rubbing a balloon to charge it. Current flows to or from the balloon to whatever it is rubbed against, charging both objects. This is okay because neither of the objects are electrically isolated from each other, just like the plates of the capacitor.

I hope that makes sense.

 

[my2cts]

Charge piles up if the rate of current flowing in is NOT equal to the rate of current flowing out.