OCR AS Physics

Revision Notes

4.2.1 Resistance


  • Resistance is defined as the opposition to current
    • For a given potential difference: The higher the resistance the lower the current
  • Wires are often made from copper because copper has a low electrical resistance
    • Materials with low resistance are known as good conductors
  • The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it

Resistance equation, downloadable AS & A Level Physics revision notes


  • Resistance is measured in Ohms (Ω)
    • Ω is the Greek capital letter ‘Omega’
    • An Ohm is defined as one volt per ampere (1 V A-1)
  • The resistance controls the size of the current in a circuit
    • A higher resistance means a smaller current
    • A lower resistance means a larger current
  • All electrical components, including wires, have some value of resistance

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 resistance equation 

Resistance Equation

Step 3: Rearrange for V

V = IR

Step 4: Substitute in the values

V = 0.3 × 10 = 3 V

Exam Tip

Although all electrical components have resistance, the resistance of wires is taken to be 0 in exam questions.


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