Power: resistance, R, in ohms, Ω

The idea

Resistance, measured in ohms (Ω), opposes the flow of electric current. In the context of electrical power, resistance is a key factor in determining the rate at which energy is transferred. The power P dissipated by a resistor is calculated using the equation P = I²R, where I is the current in amperes and R is the resistance in ohms.

For example: If a current of 3 A flows through a 4 Ω resistor, the power dissipated as heat is 3² × 4 = 9 × 4 = 36 W. This equation highlights that power loss is proportional to the square of the current, which is crucial for understanding heating effects in circuits.

What earns the marks

• State that resistance is measured in ohms (Ω) and opposes current flow.
• Recall and apply the equation P = I²R to calculate power, current, or resistance.
• Rearrange the equation correctly, for example, R = P ÷ I² or I = √(P ÷ R).
• Explain that power dissipated by a resistor is transferred as thermal energy to the surroundings.
• Recognise that doubling the current increases the power dissipated by a factor of four due to the I² term.

Common mistakes and how to put them right

• Forgetting to square the current when calculating power.

  Put it right: Calculate I² first before multiplying by R.

• Incorrectly rearranging to find current, such as I = P ÷ R.

  Put it right: To take the square root, so I = √(P ÷ R).

• Using incorrect units for substitution.

  Put it right: Ensure current is in amperes (A) and resistance is in ohms (Ω) before calculating power in watts (W).

Exam tips

• Write down P = I²R before substituting any numbers to ensure you do not forget the squared term.
• When finding current from power and resistance, do not forget the final step of taking the square root.