Question

If an electron moves from rest from a point at which the potential is $50V$ to another point at which the potential is $70V$, then its kinetic energy in the final state will be

• A. $1J$
• B. $3.2\times {10}^{-18}J$
• C. $3.2\times {10}^{-10}J$
• D. $10J$

Answer 1

The correct option is B $3.2\times {10}^{-18}J$

We know that potential energy of a charge is given by $U=qV$ where q is the charge and V is the potential of the point where charge is present.

Initial kinetic energy $K_i=0$ since particle is at rest.

From conservation of mechanical energy-

$K_f+U_f=K_i+U_i$

$⟹K_f+(-e)\times 70=0+(-e)\times 50$

$⟹K_f=20e$

$⟹K_f=1.6\times {10}^{-19}\times 20$

$⟹K_f=3.2\times {10}^{-18}J$

Answer-(B)