# Physics - Electron Drift Velocity

> Source: https://ollybritton.com/notes/a-level/physics/topics/electron-drift-velocity/ · Updated: 2021-01-08 · Tags: physics, year-1, school

### 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??
$$
0
$$

##### 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 $I$??
$$
I = Anev
$$

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

##### ![PHOTO ELECTRON DRIFT VELOCITY WIRE](electron-drift-velocity-wire.png) What equation does this photo represent??
$$
I = Anev
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) What is the formula for the volume of the wire??
$$
V = Al
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) 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??
$$
Aln
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) If you knew the total charge, $Q$, and the individual charge $e$, how could you work out the number of electrons in the wire??
$$
\frac{Q}{e}
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) 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??
$$
Q = Alne
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) 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 $t$.

$$
I = Avne
$$

##### ![PHOTO ELECTRON DRIFT VELOCITY LENGTH](electron-drift-velocity-length.png) What do you get if you do $$\frac{Alne}{t}$$??
$$
Avne
$$

##### 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.

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