Resistance (OCR A Level Physics)

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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

Examiner Tip

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

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Ashika

Author: Ashika

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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.