Q: An electron beam moving with a speed of 2.5 × 107 m/s enters into magnetic field 4 × 10-3 Wb/m2 directed perpendicular to its direction of motion. Find the intensity of the electric field required so that the electron moves undeflected .
Sol: When electron goes undeflected , force due to electric field is equal an opposite to force due to magnetic
field i.e.,
E e = B e v
B = 4 × 10-3 Wb/m2 and v = 2.5 × 107 m/s
E = B v = 4 ×10-3 × 2.5 × 107 = 105 V /m.