What magnetic field is required to constrain an electron with a kinetic energy of 400eV to a circular path of radius .80m? (answer: 8.4x10^-5T) What equation(s) do I use? Any help is appreciated!

Answer 1

V = sqrt (2E/m) relates velocity to energy.

E = 400 eV *1.6*10-19 J/eV = 6.4*10^-17 J

e V B = m V^2/R
That equates the magnetic force to the centripetal force

B = m V/(R e) = [sqrt(2 E m)]/(R e)