Electricity and magnetism · GCSE Physics
Series and parallel circuits
Teacher-written GCSE Physics revision on series and parallel circuits: current the same in series, potential difference the same in parallel, adding resistances, and why household lamps are in parallel.
Series shares the current; parallel shares the potential difference. Series: I the same, V adds, R adds. Parallel: V the same, I adds, total R falls.
The important bits
What you need to know
- 1
A series circuit has one loop. Current is the same at every point because charge has nowhere to split. I_total = I_1 = I_2.
- 2
In series, potential differences add to the supply: V_total = V_1 + V_2. The larger resistor takes the larger share of the potential difference.
- 3
Series resistances add: R_total = R_1 + R_2 + … Adding a resistor in series increases total resistance and decreases the current from the cell.
- 4
A parallel circuit has junctions. Potential difference is the same across each branch: V_total = V_1 = V_2.
- 5
In parallel, currents in the branches add to the current in the main loop: I_total = I_1 + I_2. Charge is still conserved.
- 6
Total resistance in parallel is less than the smallest branch. Adding another identical branch decreases total R and increases the current from the cell.
- 7
Household lamps are in parallel so each gets the full mains potential difference and can be switched separately. Fairy lights in series all go out if one bulb fails.
- 8
A cell or battery in a circuit supplies a potential difference. Two identical cells in series add their voltages; in parallel they share the current but the voltage stays that of one cell.
Quotations worth analysing
Short evidence. Real method.
“In a series circuit the current is the same through each component.”
One path, no junctions. If an ammeter reading changes along a series loop, the circuit is broken or you have miswired the meter.
“In a parallel circuit the potential difference across each component is the same.”
Each branch is connected across the same two points. That is why each mains lamp in a house can be rated 230 V.
Go deeper
Series versus parallel is the whole circuit topic
If there is only one loop, it is series: the current has nowhere to split, so an ammeter reads the same in every gap. The cell’s potential difference is shared, and a bigger resistor takes a bigger share. Add a resistor in series and the total resistance rises, so the current from the cell falls. If there are junctions, it is parallel: each branch sees the full cell potential difference, and the currents in the branches add to the current in the main loop. Add another identical branch and the total resistance falls, so the current from the cell rises. That is why fairy lights in series all go out when one bulb fails, and why house lights are in parallel — each lamp can be switched alone, and each gets 230 V.
Go deeper
Find the total resistance before you find anything else
Most calculation questions are two steps: combine resistances, then use V = IR on the result. Series is addition. Parallel of two resistors can be found from 1/R_total = 1/R_1 + 1/R_2 on Higher papers, or by reasoning that two equal resistors in parallel give half the resistance of one. Mixed circuits need a sketch: combine the parallel pair first, then add the series remainder. Students add 4 Ω and 6 Ω in parallel and get 10 Ω, which is the series answer, and then cannot understand why the battery current came out too small. If the total resistance fell when you added a branch, you are in parallel. If it rose when you added a component in the same loop, you are in series. Let that check catch you before the arithmetic hardens.
See the idea in action
A 12 V battery supplies a 6.0 Ω resistor in series with a 3.0 Ω resistor. Total R = 9.0 Ω. Current in the loop I = V/R = 12 / 9.0 = 1.33 A (same through both). p.d. across 6.0 Ω = IR = 1.33 × 6.0 ≈ 8.0 V. Across 3.0 Ω ≈ 4.0 V. They add to 12 V. If the 3.0 Ω resistor is instead connected in parallel with the 6.0 Ω across 12 V, each branch sees 12 V. Currents are 12/6.0 = 2.0 A and 12/3.0 = 4.0 A. Total current from the battery = 6.0 A, much larger, because total resistance has fallen below 3.0 Ω.
Exam technique
Turn knowledge into marks
Sketch the circuit and mark where current splits before you calculate. Series: I the same, V adds, R adds. Parallel: V the same, I adds. Then use V = IR on one component at a time.
Common mistakes
Do not give these marks away
- 01
Adding resistances in parallel as if they were in series, or saying current is used up around a loop.
- 02
Sharing potential difference in a parallel pair, or sharing current in a series pair.
- 03
Wiring household lamps in series in an exam diagram, so they would all go out together and each would get less than 230 V.
In a series circuit, which quantity is the same through every component?
APotential difference
BResistance
CCurrent
DPower
Show the answer
Current. There is only one path, so charge cannot pile up. Current is the same at every point. Potential difference is shared and power need not be equal.
Quick questions
If this is the bit you searched
What is the difference between series and parallel circuits?
Series: one loop, same current, p.d. shared, resistances add. Parallel: branches, same p.d. across each branch, currents add, total resistance falls as branches are added.
Why are lights in a house connected in parallel?
Each lamp gets the full 230 V, can be switched on its own, and the others stay on if one lamp fails.
What happens to total resistance when you add a resistor in series?
It increases, so the current from the supply decreases if the supply voltage is unchanged.
What happens to total resistance when you add another parallel branch?
It decreases, because there is an extra path for charge. The current from the supply increases.