A beam of electrons is moving with constant velocity in a region having simultaneous perpendicular electric and magnetic fields of strength 20 Vm-1 and 0.5 T respectively at right angles to the direction of motion of the electrons. Then the velocity of electrons must be

Calculation:
The motion of the electron is in a region where both electric and magnetic fields are acting on it. For the electron to move with constant velocity, the net force acting on it must be zero. The force due to the electric field (F_e) and the magnetic field (F_m) must balance each other.

The force due to the electric field is given by:

F_e = eE

Where 'e' is the charge of the electron and 'E' is the electric field strength.

The force due to the magnetic field is given by:

F_m = evB

Where 'v' is the velocity of the electron and 'B' is the magnetic field strength.

Since the forces must cancel each other out for constant velocity, we set F_e = F_m:

eE = evB

Simplifying, we get:

v = E / B

Substitute the given values:

v = 20 V/m / 0.5 T

v = 40 m/s

The velocity of the electrons must be 40 m/s.