# Homework Help: Speed of a charge

1. Jun 23, 2006

### renanpo

Hi, I have a question about a problem. Four different charges are placed on the corners of a square, and then a fifth one is placed on the center. I need to find the speed this fifth charge would have at a great distance.

The total energy of the system should be the sum of the work to bring each charge to the square right? So, the total work equals U, and the velocity of the charge can be found from U=Ek, U=mv^2/2. It seems right to me, but I just can't get the right answer. I appreciate the help, thanks.

2. Jun 23, 2006

### renanpo

anyone??

thanks..

3. Jun 23, 2006

### AlphaNumeric

Are all the charges the same? It depends on the system. If the system is symmetric in any kind of way then the central charge won't move.

4. Jun 23, 2006

### renanpo

No.. the charges are different. Q on corner A, 2Q on corner B, -3Q on corner C and 6Q on corner D. The charge that is put in the center of the square is Q..

5. Jun 23, 2006

### Meir Achuz

If the four charges are fixed, you jujst need to find the U of the central charge Q.