Electric Current

Electric current is the rate of flow of charge carriers and is measured in units of amperes (A) or amps
Charge can be either positive or negative and is measured in units coulombs (C)
When two oppositely charged conductors are connected together (by a length of wire)
- The charge will flow between the two conductors, giving rise to a current
- The greater the flow of charge, the greater the electric current

Flow of charge, downloadable AS & A Level Physics revision notes

Charge can flow between two conductors. The direction of conventional current in metal is from positive to negative

In electrical wires, the current is a flow of electrons
Electrons are negatively charged so they flow away from the negative terminal of a cell towards the positive terminal
Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
- This is the opposite of the direction of electron flow, as the conventional current was described before the electric current was really understood

Direct current flows from the positive to the negative terminal of the battery in a circuit. Electrons flow in the opposite direction

The potential difference in the circuit causes the current to flow

Current is measured using an ammeter

Direct current (dc) flows through the circuit in one direction
- The direction of conventional current is from the positive terminal to the negative one
- This is opposite to the electrons flow
Direct current is produced when from cells and batteries

2-5-2-dc-current

Direct current is current that only flows in one direction and has one value

The equation for current is:

$I = \frac{∆ q}{∆ t}$

Where:
- I = current (A)
- Δq = change in charge (C)
- Δt = time interval (s)

Worked example

When will 8 mA of current pass through an electrical circuit?

A. When 1 J of energy is used by 1 C of charge

B. When a charge of 4 C passes in 500 s

C. When a charge of 8 C passes in 100 s

D. When a charge of 1 C passes in 8 s

Answer: B

The equation relating current, charge and time is:

$I = \frac{∆ q}{∆ t}$

Consider option B:

$I = \frac{4}{500} = 8 \times 10^{- 3} = 8 mA$

Therefore, the correct answer is B

A is incorrect as it does not contain a value for charge or time, so it can be ruled out
C is incorrect as:

$I = \frac{8}{100} = 80 \times 10^{- 3} = 80 mA$

D is incorrect as:

$I = \frac{1}{8} = 125 \times 10^{- 3} = 125 mA$

Exam Tip

Remember that conventional current flow is always from the positive to the negative terminal. This is important in understanding circuits and whether components such as an LED, which only works if the diode points in the direction of conventional current flow.

Current can be positive or negative. It is still a scalar quantity, but the sign just indicates its direction. Direct current (dc) will only ever be in one direction.

Electric Current (SL IB Physics)

Revision Note