Problem 11-55 Potential and kinematics in thundercloud - A

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


Positive charges move with the field and negative charges move against the field.
At its initial position the electron has some electrical potential energy \((U_E)\). Which is correct?

(A)   Since its velocity is initially zero its \((U_E)\) must be zero also.

(B)   The \((U_E)\) can be defined as any value we like. Only changes in potential energies have meaning.

(C)   The \((U_E)\) has a value that must be found by calculation.