An electric circuit is a closed loop that allows electric current to flow from an energy source, through components, and back again. Current is measured in amperes (A), voltage in volts (V), and resistance in ohms (Ω). Electricity and circuits is a core KS3 physics topic, typically taught in Year 8 or Year 9.
What is electric current?
Electric current is the flow of electrically charged particles — in most circuits, these are electrons flowing through a metal wire. Electrons carry a negative charge and flow from the negative terminal of a battery, through the circuit, to the positive terminal.
Conventional current (used in circuit diagrams) flows in the opposite direction — from the positive terminal to the negative terminal — because this convention was established before scientists knew electrons were the charge carriers.
Current is measured using an ammeter, which must be connected in series (in line) with the component it is measuring. Current is measured in amperes (A) or milliamperes (mA): 1 A = 1,000 mA.
What is voltage (potential difference)?
Voltage, more precisely called potential difference, is the energy transferred per unit charge between two points in a circuit. It is the "push" that drives current around the circuit. A battery creates a potential difference between its terminals.
Voltage is measured using a voltmeter, which must be connected in parallel (across) the component it is measuring. Voltage is measured in volts (V).
A helpful analogy: imagine current as water flowing through a pipe, and voltage as the water pressure pushing it. A higher pressure (voltage) drives more water (current) through a narrow pipe (resistance).
What is resistance?
Resistance is the opposition to the flow of current in a circuit. All components have some resistance. Resistance is measured in ohms (Ω).
Factors affecting resistance in a wire:
- Length — a longer wire has greater resistance (more collisions between electrons and metal ions).
- Cross-sectional area — a thicker wire has lower resistance (more pathways for electrons to travel).
- Material — copper has lower resistance than nichrome; silver has even lower resistance than copper.
- Temperature — for most metals, resistance increases as temperature increases.
Ohm's Law
Ohm's Law links current, voltage, and resistance for components that obey it:
Voltage (V) = Current (A) × Resistance (Ω)
V = I × R
Rearranging: I = V ÷ R and R = V ÷ I
Worked example
A bulb is connected to a 6 V battery and a current of 0.5 A flows through it. Calculate its resistance.
R = V ÷ I = 6 ÷ 0.5 = 12 Ω
Now suppose a second, identical bulb is added in series. Each bulb still has a resistance of 12 Ω, so the total resistance is 12 + 12 = 24 Ω. The current flowing is now:
I = V ÷ R = 6 ÷ 24 = 0.25 A
The bulbs glow more dimly because less current flows.
Circuit symbols
Standard circuit symbols are used in circuit diagrams to represent components. Key symbols to know for KS3:
| Component | Description |
|---|---|
| Cell | One long thin line + one short thick line |
| Battery | Two or more cells in series |
| Switch (open) | A gap in the line with a lever |
| Bulb (lamp) | A circle with an X inside |
| Resistor | A rectangle |
| Variable resistor | A rectangle with an arrow through it |
| Ammeter | A circle with A inside |
| Voltmeter | A circle with V inside |
| Diode | A triangle pointing right with a bar |
| LED | A diode symbol with two arrows pointing outward |
Series circuits
In a series circuit, all components are connected in a single loop — there is only one path for current to flow.
Rules for series circuits:
- Current is the same everywhere in the circuit: I₁ = I₂ = I₃
- Voltage is shared across components: V_total = V₁ + V₂ + V₃
- Total resistance increases with each component added: R_total = R₁ + R₂ + R₃
- If one component fails (e.g., a bulb breaks), the circuit is broken and all components stop working
Old-style Christmas lights were wired in series — if one bulb failed, all lights went out. This is a classic series circuit problem.
Parallel circuits
In a parallel circuit, components are connected on separate branches — there are multiple paths for current to flow.
Rules for parallel circuits:
- Voltage is the same across each branch: V₁ = V₂ = V₃ = V_supply
- Current splits between branches; the total current entering a junction equals the total leaving: I_total = I₁ + I₂ + I₃
- Total resistance decreases as more branches are added (each branch provides an extra pathway for current)
- If one component fails, current can still flow through the other branches — other components continue to work
Household electrical circuits are wired in parallel. Each socket receives the full mains voltage (230 V in the UK) and can be switched on or off independently without affecting other appliances.
Series vs parallel: a comparison
| Property | Series circuit | Parallel circuit |
|---|---|---|
| Current | Same throughout | Splits at each junction |
| Voltage | Shared across components | Same across each branch |
| Effect of adding components | Current decreases (more resistance) | Current increases (less total resistance) |
| Effect of one failing | All components stop | Others continue to work |
| Common use | Some warning circuits | Household wiring, most electronics |
A worked example: designing a home lighting circuit
A bedroom has three light bulbs (each 60 W, needing 230 V). Should they be wired in series or parallel?
If wired in series: the 230 V mains voltage would be shared between three bulbs, giving each only about 77 V — insufficient for full brightness. Worse, if one bulb failed, all three would go out.
If wired in parallel: each bulb receives the full 230 V. Each can be switched independently with its own switch. If one fails, the others remain lit.
Conclusion: parallel wiring is correct for household lighting. This is why all UK mains circuits use parallel wiring.
According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should be taught to construct and interpret circuit diagrams, investigate series and parallel circuits, and describe current, voltage, and resistance. BBC Bitesize KS3 Physics covers Ohm's Law, series and parallel circuits, and circuit symbols with worked examples.
Frequently asked questions
What is the difference between current and voltage?
Current is the flow of electric charge (electrons) around a circuit, measured in amperes (A). Voltage (potential difference) is the energy transferred per unit charge — the "push" driving the current — measured in volts (V). You need voltage to drive current: without a voltage source such as a battery, no current flows.
What happens to the current and voltage in a series circuit?
In a series circuit, the current is the same at every point. The voltage is shared across the components — the component with the greater resistance gets a larger share of the voltage. The total voltage equals the sum of the individual voltages across each component.
What happens to the current and voltage in a parallel circuit?
In a parallel circuit, the voltage across each branch is the same as the supply voltage. The current splits between the branches — branches with lower resistance carry more current. The total current from the supply equals the sum of the currents in all branches.
How do you use Ohm's Law to calculate resistance?
Rearrange V = IR to get R = V ÷ I. Measure or note the voltage across the component in volts and the current through it in amperes, then divide. For example, if a component has 9 V across it and 3 A through it, its resistance is 9 ÷ 3 = 3 Ω.
Why are household circuits wired in parallel rather than series?
Parallel wiring ensures each appliance receives the full supply voltage (230 V in the UK) regardless of the other appliances. If one device fails or is switched off, the other circuits are unaffected. In a series circuit, the voltage would be shared and a single fault would cut all power — completely impractical for home use.
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