Year 9 Science · Unit 3 · Lesson 18
Challenge Worksheet
Learning Goals
1. The lesson calls the rule for currents at a junction Kirchhoff’s current law. State the rule, and explain why it is really about conservation of charge.
2. Fill in both columns for each feature.
| Feature | Series circuit | Parallel circuit |
|---|---|---|
| Voltage across each component | ||
| Current through each component | ||
| Effect of one break in the circuit | ||
| How total resistance changes as components are added | ||
| Use in home wiring |
Circuit: two resistors, two ways
Two resistors, 4 Ω and 12 Ω, are wired to a 12 V battery: first end to end in one loop (series), then side by side (parallel).
3. Series: find R total, the current, and how the 12 V is split.
Total resistance Ω
Current A
Voltage across 4 Ω V
Voltage across 12 Ω V
4. Parallel: find the current through each resistor and from the battery, then use R = V ÷ I for the total resistance. How does it compare with the 4 Ω resistor?
Current through 4 Ω A
Current through 12 Ω A
Current from the battery A
Total resistance Ω
Data: what the class measured
Table 1 shows what Liam’s class measured when they built both circuits.
| Circuit | Meter | Reading |
|---|---|---|
| Series | Ammeter in the loop | 0.74 A |
| Series | Voltmeter on the 4 Ω resistor | 2.9 V |
| Series | Voltmeter on the 12 Ω resistor | 8.8 V |
| Parallel | Ammeter on the 4 Ω branch | 2.80 A |
| Parallel | Ammeter on the 12 Ω branch | 0.95 A |
| Parallel | Ammeter next to the battery | 3.75 A |
Source: sample class results.
5. (a) Check the parallel readings against the junction rule from Question 1, and the two series voltmeter readings against the ratio of the resistors. (b) The reading next to the battery is lower than your Question 4 answer. Work out the percentage difference, (calculated − measured) ÷ calculated × 100, and suggest why.
(b) Percentage difference %
Find the mistakes: a student’s answer
“In a parallel circuit, the current is the same in all branches, just like in a series circuit. This is why parallel circuits are used in homes: because series circuits are used for appliances that need to be switched independently. Also, when you add more resistors in series, the total resistance decreases because there are more paths for current to flow.”
6. The answer contains three errors. For each one, say what is wrong, write a correct version, and name the circuit rule that shows the original is wrong.
| Error | What is wrong | Correct version and rule |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
Someone claims…
“Wire the lamps in a camping light in parallel. The current splits between the branches, so each lamp takes less and the battery will last longer than it would with the lamps in series.”
7. Evaluate this claim. Use your answers to Questions 3 and 4, and compare the two ways of wiring the camping light.