Ohm's Law (Cambridge (CIE) A Level Physics): Revision Note

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

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

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