Series and parallel circuits Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
A series circuit is one loop: the current is the same everywhere, and the voltage is shared between the components. A parallel circuit has branches: each branch gets the full voltage, and the current splits between them. House lights are in parallel, so one can be off while the others stay on.
Three things to know
- In a series circuit the current is the same everywhere and the potential differences add up to the supply. Total resistance = R₁ + R₂.
- In a parallel circuit each branch has the full supply p.d. and the branch currents add up to the total. Adding a branch lowers the total resistance.
- A thermistor's resistance falls as it gets hotter; an LDR's falls as light gets brighter. A diode lets current through in one direction only.
Worked example
Resistors of 3 Ω and 5 Ω are in series with a 16 V supply. Find the current.
- Total resistance = 3 + 5 = 8 Ω.
- I = V ÷ R = 16 ÷ 8.
- = 2 A.
Tips and tricks
- Series: same current, shared voltage. Parallel: same voltage, shared current.
- Adding a resistor in parallel makes the total resistance smaller, because the current has another path to take.
It lands in your notebook with its questions as flashcards.
Series and parallel circuits: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Two identical lamps are in series with a 6 V battery. What is the p.d. across each lamp?
The supply p.d. is shared equally between identical components in series.
What is the combined resistance of 5 Ω and 15 Ω in series?
Resistances in series add.
In a parallel circuit, what is the same for every branch?
Each branch is connected directly across the supply.
What happens to the resistance of a thermistor when it is heated?
The usual (NTC) thermistor conducts better when hot.
One lamp in a series circuit breaks. What happens to the others?
There is only one path, and it is now broken.
What does a diode do?
A diode has a very high resistance in the reverse direction.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Series and parallel circuits
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Resistors of 4 Ω and 6 Ω are connected in series to a 5 V supply. Calculate the current.[2]
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0.5 A. Total resistance = 10 Ω; I = 5 ÷ 10.
Two lamps in parallel each take 0.3 A. Calculate the current from the battery.[2]
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0.6 A. The branch currents add.
Give one advantage of wiring the lamps in a house in parallel rather than in series.[2]
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Each lamp gets the full supply voltage and can be switched on and off on its own; if one fails the others stay on.
Describe how the resistance of an LDR changes as the light falling on it gets brighter, and give one use.[2]
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It decreases. Used in circuits that switch lights on automatically when it gets dark.
Answers: Series and parallel circuits
- 1. 0.5 A. Total resistance = 10 Ω; I = 5 ÷ 10.
- 2. 0.6 A. The branch currents add.
- 3. Each lamp gets the full supply voltage and can be switched on and off on its own; if one fails the others stay on.
- 4. It decreases. Used in circuits that switch lights on automatically when it gets dark.