The shape of the lesson
By the end of the lesson, learners can
- Sweep the voltage, trace the curve: Trace the graph for a mystery component and decide what it is from the shape.
- Three shapes to recognise: Resistor, lamp and diode each give a distinctive line.
- Fixed resistor: straight line through the origin; R = V ÷ I constant.
- Filament lamp: curve that flattens; R rises as it heats.
- Diode: conducts forward only; blocks the reverse.
One picture every idea returns to
For a metal resistor at constant temperature the current is proportional to the p.d., a straight line through the origin. A filament lamp gives a curve that flattens as it heats and its resistance rises. A diode allows current in one direction only.
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, The shape tells the story.. Plot current against voltage and each component signs its name in the shape of the line. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Sweep the voltage, trace the curve | Trace the graph for a mystery component and decide what it is from the shape. Project tab 7 of the lab (I-V graphs) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Three shapes to recognise | Resistor, lamp and diode each give a distinctive line. | Frayer Model |
| 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, 15 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 Three shapes to recognise, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| Only a fixed resistor at constant temperature gives a straight line through the origin. | The filament lamp curves because heating it raises its resistance, and the diode passes current in only one direction. |
Current-voltage graphs: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the shape of the I-V graph for each of the following: a fixed resistor at constant temperature, a filament lamp, and a diode.
From a resistor's I-V graph, a current of 0.50 A flows at a potential difference of 2.0 V. Calculate the resistance, and state what its constant value tells you about the graph.
Explain why the I-V graph of a filament lamp is a curve and not a straight line.
For a filament lamp, the current is 1.0 A at 2.0 V and 2.0 A at 8.0 V. Calculate the resistance at each point and state whether the resistance has increased or decreased.
Describe how the current through a diode depends on the direction and size of the potential difference applied to it.
Two I-V graphs are drawn. Graph A is a straight line through the origin; graph B is a curve through the origin that becomes less steep. State which is the fixed resistor and which is the lamp, and give the reason.
Answer key and marking notes
- Fixed resistor: a straight line through the origin. ✓
- Filament lamp: a curve through the origin that flattens as the voltage increases. ✓
- Diode: zero (or near zero) current in reverse, then a steeply rising current once forward voltage passes about 0.6 V. ✓
- R = V ÷ I = 2.0 ÷ 0.50
R = 4.0 Ω
A constant R means the graph is a straight line through the origin. ✓
- As the voltage increases, the current increases, heating the filament. ✓
- The higher temperature increases the resistance of the filament. ✓
- So the current rises less and less steeply, and the graph curves (flattens). ✓
- At 2.0 V: R = 2.0 ÷ 1.0 = 2.0 Ω
At 8.0 V: R = 8.0 ÷ 2.0 = 4.0 Ω
The resistance has increased (from 2.0 Ω to 4.0 Ω). ✓
- In the forward direction, almost no current flows until about 0.6 V, after which the current rises steeply. ✓
- In the reverse direction, effectively no current flows (its resistance is very high). ✓
- Graph A is the fixed resistor: a straight line means constant resistance. ✓
- Graph B is the lamp: the curve becoming less steep shows the resistance increasing as it heats. ✓
4 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 component obeys Ohm's law (its resistance is constant). What does its current-voltage (I-V) graph look like?
- Aa straight line through the origin
- Ba curve that gets steeper
- Ca curve that flattens off
- Da horizontal straight line
The I-V graph for a filament lamp is a curve that becomes less steep as the voltage rises. What does this show about the lamp's resistance?
- AIts resistance increases as the filament gets hotter.
- BIts resistance decreases as it gets hotter.
- CIts resistance stays constant.
- DIts resistance becomes zero.
How does a diode behave in a circuit, as shown by its I-V characteristic?
- AIt allows current to flow in one direction only.
- BIt allows the same current in both directions.
- CIts resistance is always zero.
- DIt blocks current in both directions.
At one point on a component's I-V graph, the voltage is 4.0 V and the current is 0.20 A. What is the resistance at that point?
- A20 Ω
- B0.80 Ω
- C0.05 Ω
- D4.2 Ω
Quiz key
If resistance is constant, current is proportional to voltage, giving a straight line through the origin (an ohmic conductor). A curve would show a changing resistance.
As the voltage rises the filament heats up and its resistance increases, so the current rises less and less steeply, bending the line over. A constant resistance would give a straight line.
A diode conducts only in the forward direction; in reverse it has a very high resistance and almost no current flows. This is why diodes are used to convert a.c. to d.c.
R = V ÷ I = 4.0 ÷ 0.20 = 20 Ω. Option B multiplies the values; C inverts the formula.
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.