# Problem 11-55 Potential and kinematics in thundercloud - Part 6 - B

Under a particular thundercloud, there is a uniform electric field of $9.3 \times10^3\; V/m$ upward.

An electron in the field, initially at rest, travels freely for $3.5 \times 10^{-6} \; m$ before undergoing a collision with a gas molecule. Just before it hits the molecule, what is its speed? Use energy methods.

[Ans. $1.1 \times 10^5\; m/s$]

$E_{K1} + U_1 = E_{K2} + U_2 \\ U = qEy$

Correct!

Which values of $E_K$ and $U$ are correct?

Solution $E_{K1}$ $U_1$ $E_{K2}$ $U_2$
(A) 0 0 $½\;mv_2{^2}$ $qEy$
(B) 0 0 $½\;mv_2{^2}$ $- qEy$
(C) 0 0 $- ½\;mv_2{^2}$ $- qEy$
(D) 0 0 $- ½\;mv_2{^2}$ $qEy$