I–V characteristics GCSE physics lesson: resistor, filament lamp, diode, thermistor, LDR

I–V characteristics: the virtual lab

This is a virtual experiment: it follows the model written under it and is not the required practical, or a substitute for it. Change the cell's voltage, then record each reading. Give resistances in ohms (Ω), to the significant figures asked.

1. A resistor's I–V. Change the cell's voltage and record the current (ammeter A1) and the p.d. across the resistor (voltmeter V1): at least 5 readings at different voltages. Look at the graph of your readings: what shape is it?They set up a virtual circuit and record its meters' readings, each checked.
2. Use any one of your resistor readings: what is the resistor's resistance, in ohms?
3. A filament lamp's I–V. Change the cell's voltage from 0 to 12 V and record the current through the lamp (A1) and the p.d. across it (V1): at least 6 readings at different voltages, including 2 V and 12 V. Why does the graph curve?They set up a virtual circuit and record its meters' readings, each checked.
4. From your reading at 2 V: what is the lamp's resistance, in ohms, to 2 significant figures?
5. From your reading at 12 V: what is the lamp's resistance now, in ohms, to 2 significant figures?
6. Why is the lamp's resistance bigger at 12 V than at 2 V?
   1. The current gets smaller at higher voltages
   2. The lamp is brighter, so its resistance is lower
   3. The battery's own resistance gets bigger
   4. A bigger current heats the filament more, and a hotter metal filament has more resistance

I–V characteristics: practice

Show your working on paper. Give resistances in ohms (Ω) and currents in amps (A); turn milliamps into amps first. The stretch is optional.

Diagnostic question: choose and explain

1. On a filament lamp's I–V graph (current up, p.d. across), the point at 4.0 V shows 0.25 A, and the curve's gradient there is 0.025 A per volt. What is the lamp's resistance at 4.0 V?
   1. 40 Ω
   2. 0.0625 Ω
   3. 16 Ω
   4. 0.025 Ω

Practice questions

1. A resistor at a constant temperature has 6.0 V across it and 0.30 A through it. What is its resistance, in ohms?
2. Put each component with what is true of it.They sort 4 items into Resistance rises as it heats up and Resistance falls as it heats up and Resistance falls in brighter light and Lets current through one way only.
3. Which describes a diode's I–V graph?
   1. A straight line through the origin, both ways
   2. A curve that flattens as the p.d. rises, the same shape both ways
   3. Almost no current in reverse; forwards, almost none until about 0.6 V, then a steep rise
   4. The current falls as the p.d. rises
4. A component has 3.0 V across it and a current of 150 mA through it. What is its resistance, in ohms?
5. A component's readings: 2.0 V, 0.40 A; 4.0 V, 0.80 A; 6.0 V, 1.20 A. Which is it most likely to be?
   1. A filament lamp
   2. A resistor at a constant temperature
   3. A diode in reverse
6. Ali says: "A filament lamp's I–V graph curves over because the current goes down at high voltages." What is wrong with that?
   1. Nothing: the current does go down
   2. The current still goes up, by less for each volt, because the resistance rises
   3. The current goes up because the resistance falls
7. A street lamp uses an LDR to switch itself on at dusk. As the sky gets darker, what happens to the LDR's resistance?
   1. It falls
   2. It rises
   3. It stays the same
8. A diode, a lamp and a 6 V battery are in series, and the lamp lights. The battery is turned round. What happens?
   1. The lamp goes out: the diode now blocks the current
   2. The lamp lights the same: the lamp works both ways
   3. The lamp gets brighter
9. A lamp takes 0.20 A at 3.0 V and 0.36 A at 9.0 V. By how many ohms does its resistance rise between the two? (Whole number.)
10. Stretch, a temperature sensor. T1 is a resistor standing in for a thermistor (100 Ω when hot to 2000 Ω when cold); R1 is a fixed 500 Ω resistor and V1 reads the p.d. across it. Set T1 as the thermistor warms up, from 2000 Ω down, and record at least 4 readings.They set up a virtual circuit and record its meters' readings, each checked.
11. Stretch: a fan switches on when V1 reaches 3.0 V. At what resistance of the thermistor, in ohms, does that happen?

Support: For the thermistor and the LDR, say it as a sentence first: hotter, lower resistance; brighter, lower resistance. For the sensing circuit: a smaller resistance in series takes a smaller share of the battery's p.d.

I–V characteristics: exit

Work out V ÷ I for each reading first.

1. A component's readings: 2.0 V, 0.10 A; 4.0 V, 0.20 A; 6.0 V, 0.25 A. What is its resistance at 6.0 V, in ohms?
2. Which component could it be?
   1. A resistor at a constant temperature
   2. A filament lamp, warming up at the highest reading
   3. A thermistor, warming up at the highest reading

Exit check

Three readings: a resistance at a point, and which component behaves that way.

Readings: 2.0 V, 0.10 A; 4.0 V, 0.20 A; 6.0 V, 0.25 A.

I–V characteristics: graphs and resistance

Show your working, with units. For each resistance, write V and I first, then V ÷ I.