# Homework Help: Question about charge

1. Sep 23, 2007

### FeynmanMH42

2.1 Two objects, A and B, are initially uncharged. A negative charge of 4 nC flows from A to B. What is then the charge on each object?

My problem with this is that
a) the two objects are uncharged; to me "flow" of charge indicates current - I can understand the two objects becoming charged by static, but why on earth would a current flow between two uncharged objects?

b) A negative charge of 4 nC... I know A will be left with a positive charge of 4 nC, and B will be left with a negative charge of 4 nC. So my instinct would be to say that A has a charge of 4 nC and B has -4 nC... however, it says a negative charge of 4 nC flows! This means A is left with -4 nC (because 4 nC has left it... but it was actually a negative charge that left it, so -4 nC is actually positive! I hope you're following this.)

So, what will the charges be?

2. Sep 23, 2007

### Proggle

a) Don't understand your point here. How does "being charged by static" not involve a current? Charge movement implies current, even if equilibrium is quickly reached.

b) If A loses 4 nC of negative charge (let's say electrons), it will have a net charge of +4 nC. B will pick up the electrons and have a -4 nC net charge.

3. Sep 23, 2007

### Astronuc

Staff Emeritus
a. Just assume a voltage is applied temporarily between A and B.

Conservation of charge means a - charge removed from A and transferred to B must leave behind a corresponding + charge at B.

4 nC is the magnitude. Instead the problem could have stated a charge of -4 nC is transferred from A to B, instead of saying a negative charge of 4 nC.

4. Sep 23, 2007

### FeynmanMH42

Ahh, right. So charge is a scalar quantity, then?

5. Sep 23, 2007

### Astronuc

Staff Emeritus
Yes. Charge is a scalar. The electric field and electric force are vector quantities.