Magnetic Field Given Radius and Energy

1. Sep 27, 2009

balling12

1. The problem statement, all variables and given/known data

A cosmic-ray proton in interstellar space has an energy of 21.5 MeV and executes a circular orbit having a radius equal to that of Earth's orbit around the Sun (1.50 X 10^11 m). What is the magnetic field in that region of space?

2. Relevant equations
B=(Mo)(I)/ (2pie)(r)
Mo= 4pie X 10^-7

3. The attempt at a solution
My unknown is I which is the current. I am unsure how to use the energy given to find the current.

2. Sep 27, 2009

Delphi51

I think you have the wrong formula!
This is a an orbital motion problem. Ask yourself what provides the centripetal force that makes the proton go around a circle. Of course it is Fm, the magnetic force, so you will then write
Fc = Fm
Fill in the detailed formulas and solve for the B.

3. Sep 27, 2009

balling12

ok but dont I still have to find the value for v?

4. Sep 27, 2009

Delphi51

Yes. You know the kinetic energy, so you should be able to find v using the kinetic energy formula. If you find the v is more than half the speed of light using the regular KE formula, you should switch to the relativistic KE formula (if you have taken it in your course).

5. Sep 27, 2009

balling12

I am totally confused. I am not even sure what MeV means.

6. Sep 27, 2009

Delphi51

MeV means "million electron-volts". If you look up "electron volt" you will find it is 1.6 x 10^-19 Joules.