Flip a light switch. Charge your phone. Start a car. Every one of those actions relies on the same handful of circuit rules — and once you see them, you can't unsee them.
An electric circuit is simply a complete loop that lets electric charge flow from a power source, through components, and back again. That's it. But to understand how circuits work — why some bulbs glow brighter, why batteries die, why a broken wire stops everything — you need three quantities: voltage, current and resistance.
The push
The electrical pressure that drives charge around the circuit, measured in volts (V).
The flow
The rate at which charge actually moves past a point, measured in amps (A).
The opposition
How much a component restricts the flow of charge, measured in ohms (Ω).
What Exactly Is a Circuit?
CLOSED LOOP REQUIREDA circuit needs three things to work: an energy source (like a battery), a conductor (like copper wire) to carry charge, and something for the charge to do — light a bulb, spin a motor, heat a wire. But the most important rule is that the path must form a complete, unbroken loop.
If there's a gap anywhere, the charge has nowhere to go and the current stops instantly. This is called an open circuit — exactly what happens when you flip a light switch off. When the switch closes, the loop is complete again and current flows.
Think of voltage as water pressure, current as the litres per minute flowing through a pipe, and resistance as a narrow section of pipe that slows the flow. The analogy breaks down eventually, but it's a solid first mental model.
A simple circuit, step by step
- Battery provides voltage — it creates an electrical pressure difference between its two terminals.
- Charge leaves the negative terminal and travels through the wire.
- It passes through a component (say, a bulb), which converts electrical energy into light and heat.
- The charge returns to the positive terminal, completing the loop.
- The battery keeps pumping charge around, so the process repeats continuously.
Ohm's Law: The Formula That Ties It All Together
V = IRVoltage, current and resistance aren't independent. They're linked by one simple equation that forms the backbone of every circuit calculation.
Rearranged, this gives you two more useful forms:
Try it below: drag the voltage and resistance sliders, and watch the current respond — along with the brightness of the bulb in the diagram.
Notice how current rises with voltage and falls with resistance — exactly as Ohm's law predicts. The bulb glow updates too.
Worked example
A 12 V battery powers a bulb with a resistance of 24 Ω. What current flows?
- Start from the formula: I = V / R
- Substitute the values: I = 12 V ÷ 24 Ω
- Solve: I = 0.5 A
| Voltage (V) | Resistance (Ω) | Current (A) | What changed |
|---|---|---|---|
| 12 | 24 | 0.50 | Baseline case |
| 24 | 24 | 1.00 | Voltage doubled → current doubled |
| 12 | 48 | 0.25 | Resistance doubled → current halved |
| 24 | 48 | 0.50 | Both doubled → current unchanged |
Series vs Parallel Circuits
TWO WAYS TO WIREComponents can be connected in two fundamentally different ways, and the difference changes everything about how the circuit behaves.
Series circuits
In a series circuit, components are connected one after another in a single loop. The same current flows through every component — there's only one path, so nothing can split off. The total resistance is simply the sum of each component's resistance.
- One path for current — break it anywhere and the whole circuit stops.
- Voltage is shared between components; they each get a fraction of the total.
- Total resistance: R_total = R₁ + R₂ + R₃ + …
- Example: old-style fairy lights — one bulb fails, the whole string goes dark.
Parallel circuits
In a parallel circuit, each component is connected directly across the power source, so each gets the full voltage. The current splits between the branches, then rejoins. Total resistance is actually lower than the smallest individual resistance, because there are more paths for charge to take.
- Multiple paths — one branch can fail while the others keep working.
- Voltage is the same across every branch.
- Total resistance: 1/R_total = 1/R₁ + 1/R₂ + …
- Example: household wiring — turn off one light, the rest stay on.
| Feature | Series | Parallel |
|---|---|---|
| Current | Same through all components | Splits between branches |
| Voltage | Shared between components | Same across all branches |
| Total resistance | Adds up (R₁ + R₂ + …) | Less than any single branch |
| If one fails | Whole circuit stops | Others keep working |
| Everyday use | Fairy lights, simple sensors | Home wiring, phone chargers |
Power and Energy in Circuits
WATTS AND KILOWATT-HOURSCircuits don't just move charge — they transfer energy. The rate of that transfer is called power, measured in watts (W). The formula combines voltage and current:
Combining this with Ohm's law gives two more handy forms: P = I²R and P = V²/R. These explain why a resistor gets hot, why power lines use high voltage to reduce losses, and why your electricity bill is measured in kilowatt-hours (energy = power × time).
Everyday power examples
- A 60 W bulb running on 120 V draws 0.5 A of current.
- A phone charger delivering 5 V at 2 A is supplying 10 W of power.
- An electric kettle drawing 10 A at 240 V is consuming 2,400 W — which is why it heats up so fast.
Where You'll See Circuits in Action
Once you know the basics, circuits are everywhere — and the same rules apply whether you're looking at a torch or a national grid.
- Torches: a battery, a bulb and a switch in a simple series loop.
- Home lighting: wired in parallel so each room can be controlled independently.
- Phone chargers: convert mains AC to low-voltage DC using complex circuits, but the output obeys Ohm's law.
- Car electrics: the 12 V battery powers everything from headlights to the starter motor, all in parallel.
Mains electricity can kill. Never experiment with household wiring unless you're trained and qualified. Use low-voltage batteries and safe components for all your learning experiments.