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. \(1.1 \times 10^5\; m/s\)]

Diagram A showing the field direction and the direction of motion of the electron

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

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 \(- ½\;mv_2{^2}\) \(qEy\)