An electron accelerated from rest through a voltage of 780 v enters a region of constant magnetic field. part a part complete if the electron follows a circular path with a radius of 25 cm , what is the magnitude of the magnetic field? express your answer using two significant figures.

The electron is accelerated through a potential difference of , so the kinetic energy gained by the electron is equal to its variation of electrical potential energy:

where
m is the electron mass
v is the final speed of the electron
e is the electron charge
is the potential difference

Re-arranging this equation, we can find the speed of the electron before entering the magnetic field:


Now the electron enters the magnetic field. The Lorentz force provides the centripetal force that keeps the electron in circular orbit:

where B is the intensity of the magnetic field and r is the orbital radius. Since the radius is r=25 cm=0.25 m, we can re-arrange this equation to find B:

Pretty sure the answer is b : )