Year 9 Science · Unit 3 · Lesson 19
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Learning Goals
1. What is an ohmic conductor? Name one material from the lesson that is close to ohmic when its temperature is kept constant.
Order the steps: solving an Ohm’s law problem
Number the steps from 1 to 5. Step 1 is what you do first.
| Order | Step |
|---|---|
| Calculate, and write the answer with its unit. | |
| Write down the two known values and the quantity you need to find. | |
| Check the answer by putting all three values back into V = I × R. | |
| Write Ohm’s law with the unknown as the subject, such as I = V ÷ R. | |
| Substitute the known values, in volts, amps or ohms. |
Real-world context: a bathroom fan heater
A fan heater is plugged into a bathroom power point on the 230 V mains supply. On its high setting, the heater’s hot element has a resistance of 25 Ω. On its low setting, the resistance is 50 Ω. The bathroom’s power circuit has a 16 A circuit breaker.
Source: sample values for this worksheet.
2. Calculate the current drawn by the heater on its high setting. Show the formula, the substitution and the unit.
Current on high A
3. Is the current on high below the breaker’s limit? Explain what a circuit breaker is for.
4. Calculate the current on the low setting, and use Ohm’s law to explain how it compares with the current on high.
Current on low A
Investigation: a resistor and a lamp
Noah’s group set a variable power supply to five voltages, and read the current through a resistor and then through a small lamp.
| Voltage (V) | Current through the resistor (A) | Current through the lamp (A) |
|---|---|---|
| 1.0 | 0.05 | 0.10 |
| 2.0 | 0.10 | 0.16 |
| 3.0 | 0.15 | 0.20 |
| 4.0 | 0.20 | 0.23 |
| 6.0 | 0.30 | 0.28 |
Source: sample class results.
5. Use R = V ÷ I to calculate the resistance of each component at 1.0 V, 3.0 V and 6.0 V. Give the lamp’s values to one decimal place.
| Voltage (V) | Resistance of the resistor (Ω) | Resistance of the lamp (Ω) |
|---|---|---|
| 1.0 | ||
| 3.0 | ||
| 6.0 |
6. Which of the two is ohmic? Back up your answer with your Question 5 results, and suggest why the lamp behaves differently.
7. A lamp’s filament often breaks at the moment it is switched on, rather than while it is glowing. Use your results and what you know about resistance and temperature to explain why.