High above the surface of the Earth, charged particles (such as electrons and protons) can become trapped in the Earth's magnetic field in regions known as Van Allen belts. A typical electron in a Van Allen belt has an energy of 55keV and travels in a roughly circular orbit with an average radius of 220m . What is the magnitude of the Earth's magnetic field where such an electron orbits?
I have tried a few different ways, but still having trouble understanding how to solve the problem.

Question

High above the surface of the Earth, charged particles (such as electrons and protons) can become trapped in the Earth's magnetic field in regions known as Van Allen belts. A typical electron in a Van Allen belt has an energy of 55keV and travels in a roughly circular orbit with an average radius of 220m . What is the magnitude of the Earth's magnetic field where such an electron orbits?
I have tried a few different ways, but still having trouble understanding how to solve the problem.

anonymous · Answer

Change 55 keV into J of kinetic energy = (1/2)m v^2
knowing m ,you can get v^2.

use F= m v^2/r to get the force needed to create this circular motion

us F = q v B to get the force due to the magnetic field B

and set F = F to find B.