1. Jan 26, 2004

### KingJaymz

Ok, so I have this problem and I can't figure out where to go because I'm missing the mass. This is the problem:

There is a test charge of +1.0 C. 10 meters away, a negative charge of -0.2 is released. Find the speed of the negative charge when it is 2 meters away from the positive charge.

Is there a way I can get the mass of these charges if I wasn't given them in the first place? Sorry, I'm really rusty with Physics.

2. Jan 26, 2004

### chroot

Staff Emeritus
You are correct. There is no way to solve the problem without knowing the mass of the moving object.

- Warren

3. Jan 26, 2004

### KingJaymz

Hrm. Nice avatar by the way. Venom's awesome. Are you sure there's no way to solve the problem without the mass? I was never given a mass...

4. Jan 26, 2004

### chroot

Staff Emeritus
I am sure.

The easiest way to solve the problem would be the calculate the change in potential energy in moving from 10 meters to 2 meters, and equate that with the change in kinetic energy of the moving charge. That much is easy to find.

Finding the velocity of a particle with a given kinetic energy, however, requires the mass to be given.

- Warren

5. Jan 26, 2004

### KingJaymz

Nuts. That's what I thought. And, in fact, that was the way I was going to tackle the problem. Oh well. I can at least say "Let mass of point charge be x" and such. Thank you so much for your help. Hopefully I'll be able to start helping others with physics soon.

Edit: One last question about this problem. Does anyone know of masses that might be believable for the charges in the problem?

6. Jan 26, 2004

### chroot

Staff Emeritus
The mass could literally be anything at all.

Did someone give this problem to you and expect an answer?

- Warren

7. Jan 26, 2004

### KingJaymz

Yes, but my professor made the problem up himself, so I guess he might have left out the mass by accident.

When I'm finding the difference between the values for the two Electric Potential Energies, am I right in assuming I need to take the absolute value? Or at least stay consistent with whether I take the absolute value with the kinetic energies as well?

8. Jan 26, 2004

### chroot

Staff Emeritus
The potential energy with opposite charges, large r, is small and negative.

The potential energy with opposite charges, small r, is large and negative.

In other words, as the charges come closer, the potential energy is becoming more negative.

Just use a consistent sign convention.

- Warren

9. Jan 26, 2004

### KingJaymz

Thank you for the information. That's sort of what I was figuring. As long as my professor accepts my fabricated mass, I should be fine. Thank you again.