Year 9 Science · Unit 3 · Lesson 17
Challenge Worksheet
Learning Goals
1. The lesson says a current needs three things. Name all three, and for each one describe a fault that would remove it.
Investigation: does the length of a wire affect its resistance?
A class plans to connect a 1.5 V cell, an ammeter and a long, thin nichrome wire in a circuit. By moving a crocodile clip along the wire, they can change the length of wire the current passes through.
2. Plan the investigation.
Hypothesis
Write a hypothesis linking the length of the wire to its resistance, with a reason.
Variables
Method
Safety
Results: what the class measured
The class carried out the investigation, keeping the voltage at 1.5 V throughout. Table 1 shows their results.
| Length of wire (cm) | Current (A) |
|---|---|
| 20 | 1.20 |
| 40 | 0.60 |
| 60 | 0.40 |
| 80 | 0.36 |
| 100 | 0.24 |
Source: sample class results.
3. Draw a line graph of current against length. Leave out any result that does not fit the pattern.
4. (a) Describe the pattern. What happens to the current each time the length doubles, and what does this tell you about the resistance? (b) Identify the anomalous result, estimate what its current should have been, and suggest a cause.
(b) Expected current A
Find the mistakes: a student’s calculation
Question. A 6 V lantern battery lights a lamp. An ammeter shows a current of 250 mA. Calculate the power of the lamp, and the energy it changes in 4 minutes.
Student’s answer. P = V × I = 6 × 250 = 1,500 W. E = P × t = 1,500 × 4 = 6,000 W.
5. The student made three errors. Describe each error, then write a correct solution with working.
Power W
Energy J
6. A student claims: “Adding more batteries to a circuit always makes the bulb brighter.” Evaluate this claim. When is it true, and when might it be false or dangerous?
A model of a circuit
Many teachers explain circuits with a water-pipe model. A pump pushes water around a closed loop of pipes, and a narrow section of pipe slows the flow.
7. Evaluate this model. Say what each part of the model stands for in a circuit, what the model explains well, and where it breaks down.