4.1 - Charge and Current

Question 1

What is electric current? State its units

Answer 1

The rate of flow of charge. It is measured in Amps.

Question 2

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

Answer 2

Q = charge (Coulombs)
I = current (Amps)
t = time (seconds)

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.
● Units are Coulombs.

Question 4

Give two examples of possible charge carriers.

Answer 4

● Electrons - in metals.
● 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 Kirchhoff’s first law?

Answer 7

All of the current going into a junction is equal to the current leaving the junction. i.e. charge is conserved.

Question 8

What is meant by ‘mean drift velocity’?

Answer 8

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 9

What equation uses the drift velocity to calculate current?

Answer 9

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 10

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

Answer 10

m⁻³

Question 11

Put conductors, semiconductors and insulators in order from highest ‘n’ (number density of charge carriers) to lowest.

Answer 11

● Highest: conductors
● Semiconductors
● Lowest: insulators