Resistance (Edexcel IGCSE Physics (Modular))

Calculating current, resistance & potential difference

• Resistance is defined as

The opposition of a component to the flow of electric current through it

• Resistance is measured in units of ohms (Ω)

  • A resistance of 1 Ω is equivalent to a voltage across a component of 1 V which produces a current of 1 A through it

• The resistance of a component controls the size of the current in a circuit

• For a given voltage across a component:

  • The higher the resistance, the lower the current that can flow

  • The lower the resistance, the higher the current that can flow

• All electrical components, including wires, have some value of resistance

• Wires are often made from copper because it has a low electrical resistance

  • This is why it is known as a good conductor

Comparing current and resistance

A greater resistance means there is a lower current and vice versa

• The current, resistance and potential difference of a component in a circuit are calculated using the equation:

voltage = current × resistance

• Where:

  • V = voltage, measured in volts (V)

  • I = current, measured in amps (A)

  • R = resistance, measured in ohms (Ω)

• This equation can be rearranged with the help of the following formula triangle:

Voltage current resistance formula triangle

Formula triangle for the voltage, current and resistance equation

• Check out this revision note on speed, distance and time if you need a reminder on how to use formula triangles