Resistance

Overview

Students often treat resistance as just another number to substitute into an equation. Keep the physical meaning visible throughout: resistance tells us how difficult it is for charge to move through a component, so it links current, p.d., and circuit design choices.

Key knowledge and explanations

• Introduce resistance as the ratio between p.d. and current for a component and secure the equation V = IR.
• Model rearranging and using the equation in different forms, including unit checks.
• Show the standard experimental method: measure current with an ammeter, measure p.d. with a voltmeter, then calculate resistance from the readings.
• Compare total resistance in series with total resistance in parallel and connect each result to what happens to current in the circuit.
• Use at least one short numerical example for series resistance and one for two resistors in parallel.

Lesson flow

1. Start with a retrieval task on current and p.d., then ask what controls how much current flows for a given p.d.
2. Introduce V = IR and practise direct substitution before moving to rearrangement and unit reasoning.
3. Model or carry out the resistance experiment with correct meter placement, then calculate the resistance from sample readings.
4. Finish with combined-resistance problems in series and parallel, including a comparison of which arrangement gives the smaller total resistance.

Checks for understanding

• Use one quick V = IR question where students must choose the correct rearrangement before substituting values.
• Ask students to sketch or identify the correct circuit for measuring the resistance of a component.
• Give two resistors in series and in parallel and ask which arrangement has the lower total resistance, with a reason.

Common mistakes or misconceptions

• Students often confuse resistance with current or p.d. because all three appear in the same equation. Keep the physical meaning of each quantity visible.
• Some add parallel resistances as if they were in series. Link the lower total resistance in parallel to the idea of extra paths for charge.
• Rearranging V = IR can go wrong when students rely on memory alone. Use formula triangles only if they still explain the meaning of the relationship.

Follow-up

• Set mixed calculation practice so students use resistance fluently before the next practical lesson.
• Carry forward the idea that resistance depends on material and geometry, which becomes the focus of the next lesson.