Year 9 Science · Unit 3 · Lesson 20

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

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Learning Goals

Identify the odd one out among circuit units, rules and materials.
Apply Ohm’s law to the branches of a parallel kitchen circuit.
Explain why kitchen power points and house lights are put on circuits with different breakers.

Warm Up

1. Find the odd one out in each group, and say what the other three have in common.

Recall 4 marks

#	Group	Odd one out, and why
1	Volt · Kilogram · Ampere · Ohm
2	Each branch has the full voltage · Current is the same everywhere · The voltage is shared · One break stops everything
3	V = I × R · I = V ÷ R · Total R = R₁ + R₂ · R = V ÷ I
4	Battery · Solar cell · Generator · Lamp

2. Copper is used for the wires in a power cord, and plastic for their covering. Explain why, using the words conductor, insulator and resistance.

Recall 2 marks

Your Turn

Real-world context: a kitchen circuit

A kettle, a sandwich press and a heat lamp run from the power points on one kitchen circuit. The circuit runs on the 230 V mains, its power points are wired in parallel, and it has a 20 A circuit breaker.

Table 1. Appliances on the kitchen circuit

Appliance	Resistance when running (Ω)	Current (A)
Kettle	23
Sandwich press	46
Heat lamp	230

Source: sample values for this worksheet.

3. Use I = V ÷ R to complete Table 1. Show your working for each appliance.

Apply 3 marks

4. Which appliance draws the most current, and what property of it causes this?

Apply 2 marks

Go Deeper

5. With all three switched on, find (a) the current in the main supply wire, (b) the resistance of the whole circuit, using R = V ÷ I, and (c) the kettle’s power, using P = V × I.

Apply 3 marks

(a) Total current A

(b) Total resistance Ω

6. A second kettle like the first is added. Would the 20 A breaker cut the power? Then explain why the whole circuit’s resistance is lower than any one appliance’s.

Analyse 3 marks

New total current A

Show What You Know

Scenario: planning the circuits in a new house

An electrician puts the kitchen power points on their own circuit with a 20 A circuit breaker. All eight lights in the bedrooms and hallway share one lighting circuit with a 10 A circuit breaker. Each light uses 23 W, and every circuit runs on the 230 V mains supply.

7. Explain the electrician’s plan. Use P = V × I to find the current drawn by the lights, compare it with your kitchen results, and explain why each circuit has the breaker it has.

Analyse 5 marks

One light A

All eight lights A