The shape of the lesson
By the end of the lesson, learners can
- Set the voltage, read the current: Change the p.d. across a resistor and watch the current and the resistance ratio.
- What changes a wire’s resistance: Resistance depends on the dimensions of the wire as well as the material.
- Match the pair: use the voltage across a component with the current through that same component.
- Three forms: R = V ÷ I, V = IR, I = V ÷ R.
- Units: volts, amperes and ohms; one ohm is one volt per ampere.
- Hold conditions steady: the simple proportion holds only while the temperature is constant.
One picture every idea returns to
Resistance is the ratio of the potential difference across a component to the current through it: R = V divided by I, measured in ohms. A longer wire has more resistance; a thicker wire has less.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Holding back the current.. Push the same voltage through a thin wire and a thick wire and very different currents flow. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Set the voltage, read the current | Change the p.d. across a resistor and watch the current and the resistance ratio. Project tab 6 of the lab (Resistance) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: What changes a wire’s resistance | Resistance depends on the dimensions of the wire as well as the material. Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in What changes a wire’s resistance, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Resistance is voltage divided by current, R = V divided by I, not the other way round. | A longer wire has more resistance, and a thicker (wider) wire has less. |
Resistance (R = V / I): worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Write the equation that defines resistance, name each quantity, and state the unit of each.
A current of 0.40 A flows through a component when the potential difference across it is 6.0 V. Calculate its resistance.
A 24 Ω resistor is connected across a 12 V supply. Calculate the current through it.
A heating element has a resistance of 30 Ω and carries a current of 8.0 A. Calculate the potential difference across it.
A fixed resistor is kept at a constant temperature. When the potential difference across it is 4.0 V, the current is 0.25 A.
(a) Calculate its resistance.
(b) The potential difference is increased to 8.0 V. Calculate the new current.
A student measures the current through a filament lamp at several voltages and finds that the value of V ÷ I is larger at high voltages than at low voltages.
(a) State what happens to the resistance of the lamp as the voltage increases.
(b) Explain why, in terms of the filament.
Answer key and marking notes
- R = V ÷ I, where V is the potential difference (volts), I is the current (amperes) and R is the resistance (ohms). ✓
- A resistance of 1 Ω passes a current of 1 A when 1 V is across it. ✓
- R = V ÷ I = 6.0 ÷ 0.40
R = 15 Ω
- I = V ÷ R = 12 ÷ 24
I = 0.50 A
- V = IR = 8.0 × 30
V = 240 V
- (a) R = V ÷ I = 4.0 ÷ 0.25
R = 16 Ω
(b) I = V ÷ R = 8.0 ÷ 16I = 0.50 A
double the voltage, double the current, since R is unchanged
- (a) The resistance increases as the voltage increases. ✓
- (b) A larger voltage drives a larger current, which heats the filament so its temperature rises. ✓
- The hotter metal resists the current more, so R = V ÷ I gives a larger value. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A potential difference of 6.0 V drives a current of 0.50 A through a resistor. What is its resistance?
- A3.0 Ω
- B12 Ω
- C0.083 Ω
- D6.5 Ω
Two wires are made of the same material and have the same thickness, but wire X is longer than wire Y. How do their resistances compare?
- AWire X has the greater resistance.
- BWire Y has the greater resistance.
- CThey have equal resistance.
- DLength has no effect on resistance.
Quiz key
R = V ÷ I = 6.0 ÷ 0.50 = 12 Ω. Option A multiplies the values; C inverts the formula.
Resistance increases with length, so the longer wire X has the greater resistance. A thicker wire would reduce resistance, but here the thickness is the same.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.