Year 9 Science · Unit 3 · Lesson 20

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

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Learning Goals

Apply Ohm's Law to calculate current in multiple branches of a parallel household circuit.
Use calculated results to explain which appliance draws the most current and why.
Justify why Australian household wiring uses parallel circuits rather than series.

Warm Up

Odd one out

Circle the item that does not belong in each group. Write the odd one out and explain why it doesn't fit.

#	Group	Odd one out + reason
1	Voltmeter Ammeter Circuit breaker Oscilloscope
2	Current is the same everywhere Voltage is shared One break stops all Each branch has full voltage
3	V = IR I = V ÷ R R = V ÷ I R_total = R₁ + R₂
4	Voltage same across branches Current splits Used in home wiring One component failure stops all

Your Turn

Real-world context

A typical Australian house has three appliances connected in parallel on the same 240 V circuit: a bathroom light globe (resistance 48 Ω), a kitchen toaster (resistance 24 Ω), and a lounge room TV (resistance 120 Ω). Because all three are in parallel, each receives the full 240 V from the mains supply.

(a) Calculate the current drawn by each appliance. Show your working for each. Use I = V ÷ R.

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Bathroom light: I = 240 ÷ _____ = _____ A Kitchen toaster: I = 240 ÷ _____ = _____ A Lounge TV: I = 240 ÷ _____ = _____ A

(b) Which appliance draws the most current? Use the relationship between resistance and current from Ohm's Law to explain why.

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(c) Explain why this household circuit is wired in parallel rather than in series. Give at least two reasons using your knowledge from Lessons 17–19.

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Wrap Up

In one sentence, what was the main idea of this lesson?