Q. A beam of well collimated cathode rays travelling with a speed of $5 \times 10^{6}\ ms^{- 1}$ enter a region of mutually perpendicular electric and magnetic fields and emerge undeviated from this region. If $|B| = 0.02\ T$, the magnitude of the electric field is
A) $10^{5}\ Vm^{- 1}$
B) $2.5 \times 10^{8}\ Vm^{- 1}$
C) $1.25 \times 10^{- 10}\ Vm^{- 1}$
D) $2 \times 10^{3}\ Vm^{- 1}$
Explanation
(a) Using $eE = evB\ \Rightarrow E = vB = 5 \times 10^{6} \times 0.02 = 10^{5}Vm^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.