2.1 Conduction of Electricity

Question 1

What is electric current? State its units.

Answer 1

The rate of flow of charge. It is measured in amps A where A = Cs−1.

Question 2

Give the symbol equation relating charge, current and time (include units).

Answer 2

I = ΔQ/Δt
Q = charge (Coulombs)
I = current (Amps)
t = time (seconds) where Δ is the change

Question 3

In electricity what does ‘e’ represent? What are the units of ‘e’?

Answer 3

e is the elementary charge - ie. a proton has charge +e, an electron has charge -e
The units are Coulombs.

Question 4

Give two examples of possible charge carriers.

Answer 4

1. Electrons - in metals.
2. Ions - in electrolytes (aqueous
   solutions).

Question 5

True or false? Current flows from negative to positive.

Answer 5

False.
Conventional current is the ‘flow of positive charge’ - it is in the opposite direction to the movement of the electrons in the circuit.

Question 6

How can you measure the current in a circuit?

Answer 6

You can measure the current in a circuit with an ammeter connected in series with the component.

Question 7

What is meant by ‘mean drift velocity’?

Answer 7

The average velocity of the charge carriers due to the applied electric field.
It has to be an average because they’re often moving randomly in all directions.

Question 8

What equation uses the drift velocity to calculate current?

Answer 8

I = Anev
Where I = current,
A = cross-sectional area of conductor,
n = number density of charge carriers,
e = the elementary charge,
v = mean drift velocity

Question 9

What are the units of ‘number density of charge carriers’ in the equation relating current and drift velocity?

Answer 9

m^(-3)