Magnetic field?

Question

An electron with initial kinetic energy of 4.0 keV enters Wall 1 at time t = 0.

Wall 1 contains a uniform magnetic field directed towards it with magnitude 0.010 T. The electron goes through a half-circle then exits Wall 1, headed towards Wall 2 across a gap of 25 cm.

There is an electric potential difference V = 2000 V across the gap, with a polarity such that the electron's speed increases uniformly as it traverses the gap.

Wall 2 contains a uniform magnetic field directed against it with magnitude 0.020 T. The electron goes through a half-circle and the leaves Wall 2.

At what time t does it leave?

I am just stuck with where to start! Please point me in the right direction. Give me the answer so I can check my work. Thanks...

Answer 1

The force on a charge q that moves in a uniform magnetic field B is given by
F=q vxB
Note that x is a vector cross product
or we can write
F=q v B sin(angle between v and B)

If the field is directed towards or away from the direction of the travel there is no force on the charge.

If it is perpendicular however we have
F=q v B
This force is responsible for keeping the electrons in a circular path and does not contribute to velocity.

At the gap it will acquire energy
E=Vq=2000 x 1.602 E-19=3.20 E-16 Joules

However the the magnetic field will have no effect on the electron.