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First teaching 2023

First exams 2025

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Ohm's Law (CIE A Level Physics)

Revision Note

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Katie M

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Katie M

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Ohm's law

  • Ohm’s law states that:

For a conductor at a constant temperature, the current through it is proportional to the potential difference across it

  • Constant temperature implies constant resistance
  • This is shown the equation below:

V = IR

    • V = potential difference (V)
    • I = current (A)
    • R = resistance (Ω)
  • The relationship between the potential difference across an electrical component (in this case a fixed resistor) and the current can be investigated through a circuit such as the one below

Investigating potential difference and current in a circuit

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

Circuit for plotting graphs of current against voltage

  • By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit
  • Measuring the variation of current with potential difference through the fixed resistor will produce the straight line graph below

Plotting current against voltage

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

Circuit for plotting graphs of current against voltage.

  • Since the gradient is constant, the resistance of the resistor can be calculated by using 1 ÷ gradient of the graph
  • An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin
    • A resistor obeys Ohm’s law
    • A filament lamp does not obey Ohm’s law
  • This applies to any metal wires, provided that the current isn’t large enough to increase their temperature

Worked example

The current flowing through a component varies with the potential difference V across it as shown.

WE - ohms law question image(1), downloadable AS & A Level Physics revision notes

Which graph best represents how the resistance R varies with V?

WE - ohms law question image(2), downloadable AS & A Level Physics revision notes

WE - ohms law answer image, downloadable AS & A Level Physics revision notes

Examiner Tip

  • In maths, the gradient is the slope of the graph
  • The graphs below show a summary of how the slope of the graph represents the gradient 

Different gradients, downloadable AS & A Level Physics revision notes

Graphs showing varying gradients

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.