# Magnetic fields and Circular Orbits Question

1. Jul 27, 2004

### Sam_The_Great

Hi, does anybody know what approach to take with the following problem.

An electron is accelerated from rest through a potential difference of 500V, then injected into a uniform magnetic field. Once in the magnetic field, it completes half a revolution in 2 ns. What is the radius of the orbit? And what is the magnetic field?

Thanks.

2. Jul 28, 2004

### Staff: Mentor

You'll need to know several things. First, how fast is that electron going when it enters the magnetic field? (Think potential energy changing to kinetic: $q\Delta V = 1/2 mv^2$.) Then you'll need to combine your knowledge of the magnetic force on a moving charge (F = qvB, assuming the field is perpendicular to the velocity) and centripetal force ($F = mv^2/r = m\omega^2 r$). Good luck!

3. Jul 28, 2004

### Sam_The_Great

Thanks you, I think I got the right answer, my magnetic field is just a little huge, like 16.4 T which just doesn't seem right, but I took the approach you put and also since T = 2pir/v and we know how long it took to complete half a revolution, I solved it. Thanks again Doc.

4. Jul 28, 2004

### maverick280857

The question may have been designed to test your ability to integrate circular motion, work and energy and electrodynamics. So the large answer may not be a major factor...

Cheers
Vivek

5. Jul 28, 2004

### Sam_The_Great

Thanks for the assurance vivek. Did I need to integrate? I found the radius from the amount of time it took to complete half a circle. I found the velocity from change in voltage(charge) = 1/2 mv^2. and then I found the mag field from r =vm/qB. That's the correct approach to take right?

Thanks.

6. Jul 28, 2004

### Gza

Integration wasn't involved in the problem. I think he meant it in a literal rather than mathematical sense.

7. Jul 29, 2004

### maverick280857

Yeah,

I meant integration of various topics/ideas of physics...as Gza understood correctly

Cheers
Vivek

8. Jul 29, 2004

### Gokul43201

Staff Emeritus
16T magnets are quite common...why we have a couple in my lab.