Question

If an electron entering at a magnetic field of 2 × 10^-2 T has a velocity of 3 × 10^7 ms^-1 and describes a circle of radius 8 × 10^-3 m, then find the value of \(\frac {e}{m}\) of the electron.

(a) 1.567 × 10^11 C/Kg

(b) 190 × 10^11 C/Kg

(c) 18 × 10^11 C/Kg

(d) 1.875 × 10^11 C/Kg

I had been asked this question in semester exam.

This interesting question is from Motion in Combined Electric and Magnetic Field in portion Moving Charges and Magnetism of Physics – Class 12

Answer 1

Correct choice is (d) 1.875 × 10^11 C/Kg

The best explanation: Radius, r=\(\frac {mv}{eB}\).

\(\frac {e}{m} = \frac {v}{rB}\)

\(\frac {e}{m} = \frac {3 \, \times \, 10^7}{(8 \, \times 10^{-3} \, \times \, 2 \, \times \, 10^{-2})}\)

\(\frac {e}{m}\) = 1.875 × 10^11 C/Kg.