A photoconductor cannot carry an electric current in the dark because all of its valence levels contain two electrons and moving a valence-level electron into one of the empty conduction levels requires too much energy. While the photoconductor would be able to carry current if its electrons could move from one valence level to another, such movement is impossible because
A) the velocity of an electron in the photoconductor is conserved and can't change.
B) no more than two electrons can be in each valence level: one spin-up and one spin-down.
C) the momentum of an electron in the photoconductor is conserved and cannot change.
D) the laws of motion prevent electrons from changing valence levels.

Question

Quizplus · Accepted Answer

B) No more than two electrons can be in each valence level due to the Pauli exclusion principle, which states that no two electrons can have the same set of quantum numbers within an atom. This principle effectively prevents electrons from moving freely between valence levels if those levels are already filled with two electrons (one spin-up and one spin-down), thus hindering the flow of current in the absence of energy input like light.

A Photoconductor Cannot Carry an Electric Current in the Dark