Current, Potential Difference & Resistance (Edexcel IGCSE Physics): Revision notes

Section 1

What are the key electrical units?

Electricity uses a standard set of units:

Ampere (A) — unit of current
Coulomb (C) — unit of charge
Joule (J) — unit of energy
Ohm (Ω) — unit of resistance
Second (s) — unit of time
Volt (V) — unit of voltage (potential difference)
Watt (W) — unit of power

Knowing these units is essential for checking that equations balance and for giving answers with correct units.

Key termsamperecoulombohmvoltwatt

Section 2

What is current?

Current is the rate of flow of charge. In solid metallic conductors, current is a flow of negatively charged electrons through the material.

The relationship between charge, current and time is:

Q = I × t

where Q is charge (C), I is current (A), and t is time (s).

Key termscurrentelectrons

Example

A current of 2 A flows for 30 s. Charge Q = I × t = 2 × 30 = 60 C.

Section 3

What is voltage and how does it relate to energy?

Voltage (potential difference) is the energy transferred per unit charge passed. The volt is defined as a joule per coulomb (1 V = 1 J/C).

The relationship between energy transferred, charge and voltage is:

E = Q × V

where E is energy transferred (J), Q is charge (C), and V is voltage (V).

Key termsvoltage (potential difference)

Example

A charge of 10 C passes through a component with a voltage of 5 V across it. Energy transferred E = Q × V = 10 × 5 = 50 J.

Section 4

How are voltage, current and resistance related?

Resistance opposes the flow of current. The relationship linking voltage, current and resistance is:

V = I × R

where V is voltage (V), I is current (A), and R is resistance (Ω).

A higher resistance means less current flows for a given voltage.

Key termsresistance

Exam tip

Always rearrange V = IR carefully: I = V/R and R = V/I — examiners award marks for correct rearrangement as well as the final answer.

Section 5

How does current and voltage behave in series and parallel circuits?

Current is conserved at a junction — the total current entering a junction equals the total current leaving it, because charge cannot be created or destroyed
The voltage across two components connected in parallel is the same for each component
In a series circuit, the current is the same through each resistive component; the total resistance is the sum of the individual resistances, so the total voltage is shared between the components

These rules allow the current, voltage and resistance to be calculated for two resistive components connected in series.

Example

Two 4 Ω resistors in series with a 12 V supply: total resistance = 8 Ω, so current I = V/R = 12/8 = 1.5 A, and the voltage across each resistor = I × R = 1.5 × 4 = 6 V.

Must Know

Key units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V), watt (W)
Current is the rate of flow of charge; in metals it is a flow of electrons
Q = I × t
Voltage is energy transferred per unit charge; 1 V = 1 J/C; E = Q × V
V = I × R links voltage, current and resistance
Current is conserved at a junction; voltage across parallel components is the same; series components share the total voltage and have the same current

That's the notes covered.

Carry on to the next subtopic.