Magnetic Field Given Radius and Energy

[balling12]

Homework Statement

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?

Homework Equations

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

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.

 

[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.

 

[balling12]

ok but don't I still have to find the value for v?

 

[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).

 

[balling12]

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

 

[Delphi51]

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