Question

An electron $($mass $=9.1\times {10}^{-31}$; charge $=-1.6\times {10}^{-19}C)$ experiences no deflection if subjected to an electric field of $3.2\times {10}^{5}V/m$ and a magnetic field of $2.0\times {10}^{-3}Wb/m^2$. Both the fields are normal to the path of electron and to each other. Ifthe electric field is removed, then the electron will revolve in an orbit of radius :

• A. $45m$
• B. $4.5m$
• C. $0.45m$
• D. $0.045m$

Answer 1

The correct option is C $0.45m$

$V=\frac{E}{B}$ For no deflection to occur

$V=\frac{3.2\times {10}^5}{2\times {10}^{-3}}=1.6\times {10}^{8}m/s$

$R=\frac{mv}{qB}$

$R=\frac{9.1\times {10}^{-31}\times 1.6\times {10}^8}{1.6\times {10}^{-19}\times 2\times {10}^{-3}}$

$R=0.45m$