# Homework Help: Calculate charge on a body(in Coulombs)

1. Sep 19, 2004

### HurricaneJesus

All right i have a simple problem but i just want to verify my results, so any help would be greatly appreciated. The problem is as follows:

7.1* 10^12 electrons are added to a body that already has a negative charge of 0.2uC(the u is micro 10^-6). Calculate the final charge on this body.

Now i have
(7.1* 10^12)(1.602* 10^-19) + (0.2* 10^-6)

which gave me 1.337 uC

is this correct?

2. Sep 19, 2004

### HurricaneJesus

whoops, i think it should have been:

(7.1* 10^12)(- 1.602* 10^-19) + (0.2* 10^-6)

which gave me 0.937 uC

3. Sep 19, 2004

### HurricaneJesus

new info, 1C is equal to 6.24* 10^18 electrons

4. Sep 19, 2004

### HurricaneJesus

ok, formula should be:

(7.1* 10^12)/(6.24* 10^18) + (0.2*10^-6)

= 1.337 uC which is the same as the first result i got...... 0_o
i am going to go with this one.

is this correct?

5. Sep 19, 2004

### enigma

Staff Emeritus
Check all of your negative signs. The problem states that the body already has a negative charge,

Your magnitude looks right, but the sign is wrong.

[mod hat]
Moving the thread to homework help.
[/mod hat]

6. Sep 19, 2004

### HurricaneJesus

Thanks you were right about the sign, it was supposed to be -1.337 uC

Also, sorry about the location of my topic i only put it there because the course i am taking is electrical engineering and there was a section that matched exactly. I just joined the forum and i already know i will be here every day to read up on the info i need. Thanks again for the help, i will keep all homework related problems here.