# Physics - Electron Drift Velocity

### Questions for Teacher

If there’s no potential difference in a wire, what happens to the electrons?

They will drift randomly.

What happens to the randomly drifting electrons in a wire when a potential difference is applied?

They start to drift in the same direction.

What is meant by electron drift velocity?

The average displacement of the electrons along the a wire per second.

Why do electrons move slowly through a metal?

Because the electrons are colliding with the lattice of atoms.

What is the advantage of using metal to conduct electricty?

They have lots of free electrons.

What is the disadvantage of using metal to conduct electricity?

There’s lots of atoms so the electrons can’t move far without colliding with something.

What is the advantage of using semiconductors to conduct electricity?

The electrons can move very quickly.

What is the disadvantage of using semiconductors to conduct electricity?

There’s not many free electrons.

Which has a higher number density: metals or semiconducts?

Metals.

What is roughly the number density of a good insulator?

In relation to electron drift velocity, what is number density?

The number of charge carriers per metre cubed.

What 4 things does the electron drift velocity in a wire depend on?

- The current
- The number of electrons
- The area of the wire
- The charge of the charage carrier

Does a larger number of electrons increase or decrease the electron drift velocity?

Decrease.

Does a larger wire area increase or decrease the electron drift velocity?

Decrease.

Does a larger charge of the charge carries in a wire increase or decreate the electron drift velocity?

Decrease.

What is electron drift velocity formula with the subject $I$?

Instead of $e$ representing charge, what is sometimes used?

What equation does this photo represent?

What is the formula for the volume of the wire?

If you knew the volume of the wire, $Al$, and the number density, $n$, how could you work out the number of electrons in the wire?

If you knew the total charge, $Q$, and the individual charge $e$, how could you work out the number of electrons in the wire?

The two formulas for the number of electrons in the wire are $Aln$ and $\frac{Q}{e}$, what is an expression for the charge in the wire?

The formula for the total charge in the wire is $Q = Alne$. How could you turn this into a formula for current, $I$?

Divide both sides by time $t$.

\[I = Avne\] What do you get if you do

\[\frac{Alne}{t}\]
?

Why does the current decrease when you increase the temperature of the wire?

- Electrons have more kinetic energy
- More collisions

### 2022-03-09

How would you explain “mean drift velocity” in an exam?

The average displacement per second due to a potential difference in the conductor.