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.