Year 9 Science · Unit 3 · Lesson 20

Checkpoint 3, Circuits, Series, Parallel & Ohm's Law

Challenge Worksheet

Name

Date

Class

Learning Goals

Use Ohm’s law and P = V × I to predict the effect of changing a resistance.
Test class results against the rule for currents in a parallel circuit, and correct errors about circuits.
Explain why generators connected to one interconnected grid supply it in parallel.

Warm Up

1. (a) What is the unit of resistance? Use R = V ÷ I to explain what a resistance of 1 ohm means. (b) Why does a torch go dark the moment its switch is opened?

Recall 4 marks

Your Turn

2. A 9.0 V battery drives a current of 0.45 A through a heating coil. (a) Find its resistance. (b) The coil is swapped for one with three times the resistance. Predict the new current. (c) Compare the power in the two cases.

Apply 4 marks

(a) Resistance Ω

(b) New current A

Investigation: two lamps in parallel

Table 1 shows three teams’ ammeter readings for lamps L₁ and L₂, each on its own branch of a 6.0 V circuit.

Table 1. Current readings

Group	Current in L₁ (A)	Current in L₂ (A)	Current from the supply (A)
1	0.30	0.20	0.50
2	0.31	0.19	0.50
3	0.30	0.20	0.25

Source: sample class results.

3. From Group 1’s readings, find the resistance of L₁, of L₂, and of the pair together.

Apply 3 marks

L₁ Ω

L₂ Ω

Total Ω

Go Deeper

4. Which group’s results break a rule for parallel circuits? Name the rule, and suggest what might have gone wrong with that group’s measurement.

Analyse 3 marks

Find the mistakes: a student’s summary

“To measure voltage, you connect a voltmeter in series with the component. In a series circuit, each component gets the same voltage from the battery. Ohmic conductors are different from normal conductors because their resistance increases with temperature. Finally, if one branch of a parallel circuit breaks, the entire circuit stops working because there is no longer a complete path.”

5. Each of the four sentences contains an error. Explain each error, and rewrite the sentence so it is correct.

Analyse 4 marks

Sentence	What is wrong	Correct sentence
1
2
3
4

Show What You Know

6. Why are generators connected to one interconnected grid, such as the National Electricity Market (NEM), wired to that grid in parallel? Describe what would go wrong if generators on that grid were joined in series instead, and what happens in each case when one station shuts down.

Analyse 6 marks