Series and Parallel Circuits: KS3 Physics

In a series circuit, components share one loop and carry the same current. In a parallel circuit, components sit on separate branches and each receives the full supply voltage. These two arrangements behave very differently — and understanding why explains home wiring, fuse boards, and why one blown bulb can kill a whole string of lights.

What is a series circuit?

In a series circuit, components are connected end-to-end in a single, unbroken loop. There is only one path for current to follow.

Key rules for a series circuit:

Current is the same everywhere in the circuit (it has nowhere else to go).

Voltage is shared between the components — the voltages across each component add up to the supply voltage.

Resistance adds up — the total resistance is the sum of all individual resistances: R_total = R₁ + R₂ + R₃ + …

Consequence: If one component breaks (for example, a bulb blows), the circuit is broken and all components stop working. This is what happened with old-fashioned Christmas fairy lights — one faulty bulb put out the whole string.

Example: A 6 V battery connected to two identical bulbs in series. Each bulb gets 3 V (half the supply voltage). The current is the same through both bulbs.

What is a parallel circuit?

In a parallel circuit, components are connected in separate branches between the same two points. There are multiple paths for the current to flow.

Key rules for a parallel circuit:

Voltage is the same across each branch (and equal to the supply voltage).

Current is split between the branches — the total current from the supply equals the sum of the currents through each branch: I_total = I₁ + I₂ + I₃ + …

Resistance decreases — adding more parallel branches gives the current more paths, so the total resistance is less than the smallest individual resistance.

Consequence: If one branch breaks, the other branches are unaffected. Mains wiring in homes is parallel — you can switch off one light without the rest going off.

How do current, voltage and resistance compare?

Property | Series circuit | Parallel circuit
Current | Same through every component | Splits between branches; branch currents add to give total
Voltage | Shared; add up to supply voltage | Same across every branch (equals supply voltage)
Total resistance | Adds up (R_total = R₁ + R₂ + …) | Decreases (less than the smallest branch resistance)
Effect of removing one component | All components stop working | Other components continue working
Used in | Simple switching circuits, old fairy lights | Home wiring, most practical electrical systems

How do you draw circuit diagrams?

At KS3, you use standard circuit symbols to represent components. The most important symbols are:

Component | Symbol
Cell | Long and short parallel lines (+ on the long side)
Battery (multiple cells) | Several pairs of long/short lines
Bulb (lamp) | Circle with a cross inside
Resistor | Rectangle
Switch (open) | A gap in the wire with two dots
Ammeter | Circle labelled A
Voltmeter | Circle labelled V

An ammeter measures current (in amperes, A) and is always connected in series with the component being measured.

A voltmeter measures voltage (in volts, V) and is always connected in parallel across the component being measured.

Drawing circuits clearly and accurately — with straight lines meeting at right angles — is an assessed skill at KS3 and GCSE.

What is the relationship between voltage, current and resistance?

Ohm's Law (met at KS3, explored more fully at GCSE) states:

Voltage (V) = Current (I) × Resistance (R)

Or rearranged: I = V ÷ R and R = V ÷ I.

This means:

A higher voltage drives a bigger current through the same resistance.

A higher resistance reduces the current for a given voltage.

Worked example: A 12 V supply is connected to a resistor of 4 Ω.

Current = V ÷ R = 12 ÷ 4 = 3 A

If a second identical resistor is added in series (total R = 8 Ω):

Current = 12 ÷ 8 = 1.5 A — adding resistance reduces current.

If the second resistor is added in parallel, the total resistance falls below 4 Ω — total current increases.

Where are series and parallel circuits used in real life?

Most practical electrical systems use parallel circuits because components can be controlled independently and one failure does not disable the rest. Series circuits are still found in:

Christmas lights (modern sets are parallel; vintage sets were series — one broken bulb killed all)

Simple torch circuits — one switch controls everything in the loop

Overcurrent fuses and circuit breakers — always in series, so a blown fuse breaks the whole circuit

Home mains wiring is entirely parallel: every socket and light is on its own branch from the mains. This is why your toaster and kettle can both run at 230 V simultaneously, and why you can switch off the kitchen lights without powering down the whole house.

Frequently asked questions

What is the main difference between a series and a parallel circuit?

In a series circuit, there is only one path for current to flow, so all components share the same current and share the supply voltage between them. In a parallel circuit, there are multiple branches, so each branch receives the full supply voltage and the current splits between the branches. Parallel circuits are used in home wiring; series circuits are used in simple switching arrangements.

Why does the current stay the same in a series circuit?

Current is the flow of charge (electrons) around a circuit. In a series circuit there is only one path, so every electron that passes one component must pass all the others. Charge cannot pile up or disappear between components, so the rate of charge flow (current) is identical at every point in a series circuit.

Why does adding components in parallel reduce total resistance?

Adding a parallel branch gives the current an extra path to flow through. More paths means less overall opposition to current — like opening more checkout lanes in a supermarket reduces the queue. As a result, the total resistance of a parallel combination is always less than the resistance of any individual branch.

How do you measure current and voltage in a circuit?

Current is measured with an ammeter connected in series (in line with) the component, so all the current flows through it. Voltage is measured with a voltmeter connected in parallel (across) the component or section of circuit you are testing. Swapping them over is a very common mistake — an ammeter in parallel would short-circuit the component.

Why are homes wired in parallel rather than series?

Parallel wiring ensures every appliance receives the full mains voltage (230 V in the UK), regardless of how many other devices are switched on. It also means that switching off or unplugging one appliance does not affect any of the others. In a series home circuit, every bulb would dim when you plugged in a kettle, and a blown fuse would take out the entire house.

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