Question Video: Describing the Motion of Free Electrons in a Silicon Lattice | Nagwa Question Video: Describing the Motion of Free Electrons in a Silicon Lattice | Nagwa

# Question Video: Describing the Motion of Free Electrons in a Silicon Lattice Physics • Third Year of Secondary School

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Which of the following most correctly describes the net direction of motion of free electrons within a silicon lattice? [A] The net motion of free electrons is across the longest side of the lattice. [B] The net motion of free electrons is across the shortest side of the lattice. [C] There is no net motion of free electrons. [D] The net motion of free electrons is periodic.

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### Video Transcript

Which of the following most correctly describes the net direction of motion of free electrons within a silicon lattice? (A) The net motion of free electrons is across the longest side of the lattice. (B) The net motion of free electrons is across the shortest side of the lattice. (C) There is no net motion of free electrons. Or (D) the net motion of free electrons is periodic.

To begin, we should recall that free electrons are higher-energy electrons that aren’t strictly bound to any one particular atom. Rather, they′re known as free charge carriers, since they’re able to move relatively freely around the silicon lattice, helping to make a substance more conductive. This is why free electrons are useful to a semiconductor. They’re charged particles that are available to easily respond to an applied potential difference. Thus, the semiconductor is able to conduct electricity or facilitate the flow of electric charge.

But note that in this question, there’s no mention of a potential difference being applied to the lattice, which would move the electrons in one direction, and also no mention of a difference in density of electrons in one area of the lattice compared to another. While either of these would influence the overall direction of motion of free electrons in the lattice, that’s not what this question is asking about.

But also, on their own, free electrons don’t just stay still. Rather, they’re constantly moving around due to their thermal energy. And they repeatedly collide with other free electrons and atoms in the lattice. This type of motion, in which a particle is rapidly moving around and experiencing collisions, is characterized by its apparent randomness. A free electron is equally likely to be redirected in any direction at any time, meaning that any given free electron tends to have net zero displacement.

Knowing this, answer option (C) seems like the best choice. It is true that, generally, free electrons experience random motion due to repeated collisions. And such random movement does not constitute the net movement of electrons in any one direction. Thus, we know that answer choice (C) is correct. There is no net motion of free electrons in a silicon lattice.

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