Adding a resistor increases total resistance → current decreases
If one component fails → whole circuit breaks
Parallel circuits — three rules
Potential difference is the same across each branch — V₁ = V₂
Current is shared between branches — I = I₁ + I₂
Total resistance is less than the smallest individual resistance
Adding a branch decreases total resistance → total current increases
If one branch fails → other branches keep working
Ammeter & voltmeter placement — always asked
Ammeter — always placed in series with the component being measured
Voltmeter — always placed in parallel with (across) the component being measured
Ammeter has very low resistance (so it doesn't affect the circuit)
Voltmeter has very high resistance (so it draws negligible current)
Exam questions — 7 questions · 12 marks total
1 markSeries rulesAQA 8463 style
In a series circuit, two resistors of 4Ω and 6Ω are connected to a 12V battery. What is the current through the circuit?
A 2 A
B 1.2 A
C 0.5 A
D 3 A
1 markParallel rulesAQA Jun 2022 P1 style
In a parallel circuit, what is true about the potential difference across each branch?
A It is the same throughout the whole circuit
B It is shared between each branch
C It is the same across each branch
D It increases with each additional branch
2 marksV = IR calculationAQA Jun 2023 P1 style
A resistor has a resistance of 15Ω. A current of 0.4A flows through it. Calculate the potential difference across the resistor. Give the unit. (2 marks)
+20 XP
1 markAmmeter placementAQA Jun 2023 P1 Q6.1
A student sets up a circuit to measure the resistance of a filament lamp. How should the ammeter be connected?
A In series with the filament lamp
B In parallel with the filament lamp
C In parallel with the battery
D Between the ammeter and the voltmeter
2 marksParallel currentAQA Nov 2021 P1 style
In a parallel circuit, the total current from the battery is 0.6A. One branch carries 0.2A. What is the current in the other branch? Explain your reasoning. (2 marks)
+20 XP
1 markEffect of adding componentsAQA Jun 2022 P1 Q11 style
A parallel circuit has two branches. A third branch (resistor) is added. What happens to the total resistance of the circuit?
A It increases
B It stays the same
C It decreases
D It depends on the value of the resistor added
4 marksSeries circuit — full calculationAQA 8463 style
A series circuit contains a battery of 9V and two resistors. Resistor 1 has resistance 12Ω. The current in the circuit is 0.3A. (a) Calculate the total resistance of the circuit. (1 mark) (b) Calculate the resistance of Resistor 2. (1 mark) (c) Calculate the potential difference across Resistor 1. (1 mark) (d) State the potential difference across Resistor 2. (1 mark)