Year 9 Science · Unit 3 · Lesson 19
Challenge Worksheet
Learning Goals
1. For an ohmic conductor, what does the gradient of its V–I graph tell you? What does a curved V–I graph tell you?
Real-world context: a boat’s spotlight
A boat has a 12 V spotlight with a halogen lamp. When the lamp is glowing at full brightness on 12 V, it uses 36 W.
Source: sample values for this worksheet.
2. (a) Use P = V × I to find the current at full brightness. (b) Find the filament’s resistance at full brightness. (c) The battery runs down to 11 V. Predict the current, assuming the resistance does not change.
(a) Current A
(b) Resistance Ω
(c) Current at 11 V A
Investigation: a thermistor
A thermistor is a component whose resistance changes with temperature. A class placed one in a beaker of water, connected it to a 6.0 V supply, and measured the current as they heated the water.
| Water temperature (°C) | Current (A) | Resistance (Ω) |
|---|---|---|
| 20 | 0.012 | 500 |
| 40 | 0.024 | |
| 60 | 0.048 | |
| 80 | 0.080 |
Source: sample class results.
3. Use R = V ÷ I to complete Table 1. The first row is done for you.
4. What happens to the thermistor’s resistance between 20 °C and 80 °C? How is this the opposite of a lamp filament? Suggest one device in which a thermistor could be useful, and how it would work.
Find the mistakes: a student’s homework
Question. A resistor has 4.5 V across it, and the current through it is 150 mA. (a) Calculate its resistance. (b) What voltage is needed for a current of 0.40 A?
Student’s answer. (a) R = I ÷ V = 150 ÷ 4.5 = 33.3 Ω. (b) V = I ÷ R = 0.40 ÷ 33.3 = 0.012 V.
5. Explain what went wrong in each part of the homework, and give the correct answers with working.
(a) Resistance Ω
(b) Voltage needed V
A student claims…
“A light bulb must follow Ohm’s law, because you can always measure the voltage across it and the current through it, and then use R = V ÷ I to calculate its resistance. Since you can calculate R for any values of V and I, Ohm’s law must apply.”
6. Evaluate the student’s reasoning. Say which part is correct, explain the flaw, and describe the V–I graph you would expect for an ohmic conductor and for a lamp filament. Use evidence from this worksheet.