A small glowing bulb connected by red and black wires to a battery and a toggle switch on a pale oak bench
doodlecircuit

ELECTRICITY, EXPLAINED SIMPLY

Circuits in
plain English.

I = V/R Ohm's law

AI-generated image, not a photograph of a real person or place.

Voltage pushes. Current flows. Resistance holds it back.

Every circuit, from a torch to a house, runs on the same three ideas and one short equation. Here is how they fit together, why your lights are wired in parallel, what UK mains really delivers, and why every plug has a fuse.

Educational only, with the sources linked.

Updated · 6 sources

Try the bulb lab

THE BIG THREE

Push, flow, friction.

One equation links them:
current equals voltage divided by resistance.

VOLTS AND AMPS

Ohm's law

For most materials, current rises in step with voltage. Double the push, double the flow. One ohm is one volt per amp.

See a worked example

SERIES OR PARALLEL

One path or many

In series, resistances add up and share the voltage. In parallel, each branch gets the full voltage.

Try the bulb lab

SAFETY BY DESIGN

Why fuses exist

A fuse is a deliberately weak link that melts when too much current flows, before the wiring can overheat.

How a fuse works
Unmarked white ceramic cartridge fuses and an opened glass fuse showing its thin wire, beside a coil of copper wire on dark slate
AI-generated image, not a photograph of a real person or place.

SPOT IT AT HOME

Four things to notice,
no tools needed.

  1. Watch the parallel wiring

    Switch off one lamp and the others stay at full brightness. That independence is what parallel circuits give you.

  2. Read a rating plate

    Appliances list their power in watts. Divide watts by volts and you get amps: a 2,300 W kettle on 230 V draws about 10 A.

  3. Notice the switch-on moment

    A filament bulb has lower resistance when it is cold, so it briefly draws more current at the moment it is switched on than once it is hot and glowing.

  4. Leave the wiring to the qualified

    Understanding circuits is not the same as working on them. Mains wiring is a job for a qualified electrician working to BS 7671.

SAME PARTS, DIFFERENT LAYOUT

Three resistors,
two answers.

A textbook example: 1 Ω, 6 Ω and 13 Ω
connected to the same 12-volt source.

20.0 Ω 0.600 A

Total resistance

The resistances simply add up. Only 0.6 amps flows, and the 12 volts is shared out: 0.6 V, 3.6 V and 7.8 V across the three resistors.

IN SERIES · ONE PATH

0.804 Ω 14.9 A

Total resistance

Each resistor gets the full 12 volts. The total resistance is lower than even the smallest resistor, so the source supplies about 25 times as much current.

IN PARALLEL · THREE PATHS

Heat

Too much current

Excess electric power turns into unwanted heat. A damaged cord or an overloaded circuit carrying too much current can overheat and even start a fire inside a wall.

THE PROBLEM

Open circuit

Fuses and circuit breakers

A fuse holds a metal strip with a low melting point that melts and permanently breaks the connection. A circuit breaker does the same job but can be reset.

THE FIX

Adding more paths lowers total resistance and raises total current. That is why plugging more and more appliances into one supply draws more current from it, and why circuits need protection.

THE BULB LAB

Series or parallel:
what changes?

Imagine a 12-volt battery and some identical bulbs of 6 ohms each. Pick a layout and see how much current the battery has to supply, worked out with Ohm's law.

A simplified model: real bulbs change resistance as they heat up, so real figures differ. The pattern is what matters.

Bulb lab · 12 V battery

Choose how the bulbs are connected
Current from the battery: one bulb2 A

Total resistance 6 Ω. Current = 12 V ÷ 6 Ω = 2 A. The bulb gets the full 12 V.

Current from the battery: two in series1 A

Resistances add: 6 + 6 = 12 Ω, so the current halves to 1 A. Each bulb gets only 6 V and glows more dimly.

Current from the battery: two in parallel4 A

Each bulb has its own path and the full 12 V. Total resistance falls to 3 Ω, so the battery supplies 4 A.

Current from the battery: three in parallel6 A

Three paths: total resistance is 2 Ω, less than any single bulb. The battery now has to supply 6 A.

Calculated with I = V ÷ R for ideal 6 Ω bulbs.

Three small glowing bulbs in metal holders on an oak board, joined one after another by red wires
AI-generated image, not a photograph of a real person or place.

CIRCUIT FACTS

Five facts that
switch the light on.

  1. Named after a German physicist

    Georg Simon Ohm (1787–1854), a German physicist, was the first to show by experiment that the current in a metal wire is directly proportional to the voltage across it.

  2. A headlight, worked out

    A car headlight drawing 2.50 amps from 12.0 volts has a resistance of 12.0 ÷ 2.50 = 4.80 ohms. When cold, its resistance is lower, so it draws more current at the moment it is switched on.

  3. Resistance spans huge ranges

    A metre of thick copper wire has roughly 0.00001 ohms of resistance. The ceramic insulators that hold up power lines have a million million ohms or more.

  4. UK mains: 230 V at 50 Hz

    The law sets the declared low-voltage supply at 230 volts and 50 hertz. The voltage may run up to 10% above or 6% below (about 216–253 V); the frequency may vary by 1%.

  5. Wiring has a national rulebook

    Electrical installations in the UK follow BS 7671, the IET Wiring Regulations, which the Institution of Engineering and Technology co-publishes with the British Standards Institution.

QUICK QUESTIONS

Circuits, answered.

Short answers to the questions people ask most.

What is the difference between voltage and current?

Voltage is the push that drives electric charge around a circuit, measured in volts. Current is how much charge actually flows each second, measured in amperes (amps). Resistance, measured in ohms, is what holds the flow back.

What does Ohm's law say?

For most materials the current is directly proportional to the voltage: double the push and you double the flow. Written as a formula, current equals voltage divided by resistance (I = V/R), and one ohm is one volt per ampere.

Why are the lights and sockets in a house wired in parallel?

In a parallel circuit each device has its own path and receives the full voltage of the supply, so each one works independently. Switching off one lamp does not dim or switch off the others, which is how buildings are wired.

What voltage is UK mains electricity?

The declared low-voltage supply in Great Britain is 230 volts at 50 hertz. The law allows the voltage to be up to 10% above or 6% below that, roughly 216 V to 253 V, and the frequency to vary by 1% either way.

What does the fuse in a plug actually do?

A fuse is a deliberately weak link: a strip of metal with a low melting point. If too much current flows for too long it overheats and melts, permanently breaking the circuit before the wiring can overheat. UK plugs must contain a fuse made to BS 1362.

Sources

Each figure on this page comes from the sources below.

  1. College Physics 2e, 20.2: Ohm's law, resistance and simple circuitsOpenStax, Rice University
  2. College Physics 2e, 21.1: Resistors in series and parallelOpenStax, Rice University
  3. College Physics 2e, 20.6: Fuses, circuit breakers and thermal hazardsOpenStax, Rice University
  4. Electricity Safety, Quality and Continuity Regulations 2002, regulation 27legislation.gov.uk
  5. The Plugs and Sockets etc. (Safety) Regulations 1994legislation.gov.uk
  6. BS 7671: the national standard for electrical installationsThe IET

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