What does conservation of charge say about a junction?

The current entering the junction equals the current leaving it.

What does conservation of energy say about p.d.s in a series circuit?

The p.d.s across the components add up to the supply p.d.

What is the p.d. across each branch of a parallel circuit?

The same as the supply p.d. (the p.d. across the whole parallel combination).

What is the equation for resistors in parallel?

1 / R = 1 / R₁ + 1 / R₂ + ...

Why is the combined resistance in parallel less than the smallest resistance?

Each extra path allows more current for the same p.d., so R = V / I decreases.

How can you derive the series resistance equation?

Same I through each, V = V₁ + V₂, and V = IR, so R = R₁ + R₂.

How can you derive the parallel resistance equation?

I = I₁ + I₂ with the same V across each, and I = V / R, so 1 / R = 1 / R₁ + 1 / R₂.

What happens to the other lamps in a series circuit when one lamp filament breaks?

All of them go out, because the circuit is broken and the current is zero.

In a series-parallel circuit, how do you find the p.d. across a parallel group?

Subtract the p.d.s across the series components from the supply p.d.

Series and parallel circuits and conservation lawsEdexcel A-Level Physics: Flashcards

Card 1 of 120 of 12 known

Question

What does conservation of charge say about current in a series circuit?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

What does conservation of charge say about current in a series circuit?: The current is the same at every point, because charge cannot build up or be lost.
What does conservation of charge say about a junction?: The current entering the junction equals the current leaving it.
What does conservation of energy say about p.d.s in a series circuit?: The p.d.s across the components add up to the supply p.d.
What is the p.d. across each branch of a parallel circuit?: The same as the supply p.d. (the p.d. across the whole parallel combination).
What is the equation for resistors in series?: R = R₁ + R₂ + ...
What is the equation for resistors in parallel?: 1 / R = 1 / R₁ + 1 / R₂ + ...
Why is the combined resistance in parallel less than the smallest resistance?: Each extra path allows more current for the same p.d., so R = V / I decreases.
What is the combined resistance of N identical resistors R in parallel?: R / N.
How can you derive the series resistance equation?: Same I through each, V = V₁ + V₂, and V = IR, so R = R₁ + R₂.
How can you derive the parallel resistance equation?: I = I₁ + I₂ with the same V across each, and I = V / R, so 1 / R = 1 / R₁ + 1 / R₂.
What happens to the other lamps in a series circuit when one lamp filament breaks?: All of them go out, because the circuit is broken and the current is zero.
In a series-parallel circuit, how do you find the p.d. across a parallel group?: Subtract the p.d.s across the series components from the supply p.d.

Exam questions on Series and parallel circuits and conservation laws

A string of decorative lights consists of 20 identical lamps connected in series to a 230 V supply.
Explain, using conservation of energy, why the potential differences across the 20 lamps add up to the p.d. of the supply.2 marks
Two resistors, of resistance 6.0 Ω and 12 Ω, are connected in parallel across a 12 V battery that has negligible internal resistance.
Explain why the combined resistance is less than the resistance of the 6.0 Ω resistor alone.2 marks
A circuit contains a 9.0 V battery of negligible internal resistance. A 15 Ω resistor is connected in series with a parallel combination of a 30 Ω resistor and a 60 Ω resistor.
Calculate the total resistance of the circuit.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).