Question

An electron is released form rest in a region of space where a uniform electric field is present. Joanna claims that its kinetic and potential energies both increase as it moves from its initial position to its final position. Sonya claims that they both decrease. Which one, if either, is correct?

Joanna, because the electron moves opposite to the direction of the field
Sonya, because the electron moves opposite to the direction of the field.
Joanna, because the electron moves in the direction of the field.
Sonya, because the electron moves in the direction of the field.
Neither, because the kinetic energy increases while the electron moves to a point at a higher potential.

Answer 1

The kinetic energy of the electron increases as it moves in the direction of the electric field, while the potential energy of the electron decreases.

Step-by-step explanation:

In this situation, neither Joanna nor Sonya is entirely correct. The kinetic energy of the electron increases as it moves from its initial position to its final position in the direction of the electric field. This is because the electric field does positive work on the electron, increasing its kinetic energy. On the other hand, the potential energy of the electron decreases as it moves in the direction of the electric field. This is because the electric field does negative work on the electron, decreasing its potential energy. Therefore, both the kinetic and potential energies of the electron change, but in opposite directions.

Answer 2

Neither.

Step-by-step explanation:

When an electron is released from rest, in an uniform electric field, it will accelerate moving in a direction opposite to the field (as the field has the direction that it would take a positive test charge, and the electron carries a negative charge).

It will move towards a point with a higher potential, so its kinetic energy will increase, while its potential energy will decrease:

⇒ ΔK + ΔU = 0 ⇒ ΔK = -ΔU = - (-e*ΔV)

As ΔV>0, we conclude that the electric potential energy decreases while the kinetic energy increases in the same proportion, in order to energy be conserved, in absence of non-conservative forces.