Edexcel International A Level Physics

Revision Notes

2.34 Ohm's Law

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Defining Resistance


  • Resistance is defined as the opposition to current
  • It is further defined by Ohm's Law, which says that the resistance of a conductor is given by the ratio of potential difference across it to the current flowing in it

R space equals space V over I

  • For a given potential difference, then, the higher the resistance the lower the current

Resistance equation, downloadable AS & A Level Physics revision notes

Resistance of a component is the ratio of the potential difference and current

  • Resistance is measured in Ohms (Ω)
  • An Ohm is defined as one volt per ampere
  • 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

Measuring Resistance

  • To find the resistance of a component, a simple circuit can be used, containing:
    • A power supply
    • A component (such as a lamp or resistor)
    • An ammeter in series with the component
    • A voltmeter in parallel with the component

 Resistance circuit, downloadable AS & A Level Physics revision notes

A circuit to determine the resistance of a component

  • The power supply should be set to a low voltage to avoid heating the component, typically 1-2 V
  • Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively
  • Finally, these readings should be substituted into the resistance equation

Worked example

A charge of 5.0 C passes through a resistor of resistance R Ω at a constant rate in 30 s.

If the potential difference across the resistor is 2.0 V, calculate the value of R.

Ohm's Law

  • Ohm’s law is defined as:

 The current through a component is directly proportional to the potential difference across it, providing the temperature is constant

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


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

Ohm’s law

  • By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit.
  • The variation of current with potential difference through the fixed resistor can be plotted on a pair of axes
    • This will produce a straight-line graph

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 R 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 does obey Ohm’s law
    • A filament lamp does not obey Ohm’s law
  • Any metal wires will follow Ohm's Law, 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

Exam Tip

Resistance is used to control current. Increasing the resistance in a circuit will reduce the current. Don't get the cause and effect mixed up here. Reducing current does not increase resistance - it's the other way round!

Using graphs;

  • 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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