Current, Resistance & Potential Difference (AQA GCSE Physics: Combined Science)

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Current, Resistance & Potential Difference

  • Resistance is defined as the opposition to current:
    • The higher the resistance of a circuit, the lower the current
    • This means that good conductors have a low resistance and insulators have a high resistance

  • The symbol for resistance is R
  • It is measured in Ohms (Ω)
    • Ω is the Greek capital letter ‘Omega’
    • An Ohm is defined as one volt per ampere (1 V / A)

  • The resistance of a circuit can be increased by adding resistors (or variable resistors) to it
  • Every electrical component has a resistance, even wires
    • In exam questions, the resistance of the wires and batteries are assumed to be negligible

Resistance & Current, downloadable IGCSE & GCSE Physics revision notes

High resistance means there is lower current and vice versa

  • The current I through a component depends on both the resistance R of the component and the potential difference V across the component
    • The greater the resistance R of the component, the lower the current I for a given potential difference V across the component
    • The lower the resistance R of the component, the greater the current I for a given potential difference V across the component

Examiner Tip

Resistance can be tricky to grasp conceptually because the definition 'opposition to current' is a bit difficult to pin down. 

From the point of view of a tiny electron flowing through a wire, the wire is this huge structure of metal ions arranged in a lattice. There are millions of other electrons flowing through the wire too. Every time an electron hits the lattice, the electron transfers some of its energy to the lattice. The electron loses energy and slows down a bit, and the lattice gains energy and vibrates more and more with each collision. The more it shakes, the more the electrons collide with it.

The wire heats up due to the collisions of electrons with the lattice of the metal ions in the wire. The hotter it gets, the more it vibrates, the more collisions there are. The more collisions there are, the hotter it gets... and on and on and on.

Calculating Current, Resistance & Potential Difference

  • The current, resistance and potential difference of a component in a circuit are calculated using the equation:

Ohm's law, downloadable AS & A Level Physics revision notes

  • This equation can be rearranged with the help of the following formula triangle:

VIR triangle (3), IGCSE & GCSE Physics revision notes

Voltage, current, resistance formula triangle

Worked example

Calculate the potential difference through a resistor of resistance 10 Ω if there is a current of 0.3 A through it.

Step 1: List the known quantities

    • Resistance, R = 10 Ω
    • Current, I = 0.3 A

Step 2: Write the equation relating resistance, potential difference and current

V = IR

Step 3: Substitute in the values

V = 0.3 × 10 = 3 V

Examiner Tip

Remember that 'voltage' and 'potential difference' are the same, either wording will be accepted in your exam answers

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.