Question

A beam of light travelling along $X$-axis is described by the electric field $E _{ y }=900 \sin \omega( t - x / c )$. The ratio of electric force to magnetic force on a charge $q$ moving along $Y$-axis with a speed of $3 \times 10^7 \,ms ^{-1}$ will be : [Given speed of light $=3 \times 10^8 \,ms ^{-1}$ ]

• A. $1: 1$
• B. $1: 10$
• C. $10: 1$
• D. $1: 2$

Answer 1

Answer: (C)

$ E _{ y }=900 \sin \left(\omega t -\frac{\omega x }{ c }\right)$
$ E _0=900$

$F _{ E }= qE _0 $
$ F _{ B }= qvB _0$
$ \frac{ F _{ E }}{ F _{ B }}=\frac{ E _0}{ vB _0}=\frac{ c }{ v }=\frac{3 \times 10^8}{3 \times 10^7}=10: 1$