Simple Circuits
A circuit works or it does not, and there is almost never anything in between. One gap anywhere in the loop and the whole thing stops, which makes it the easiest topic in science to test yourself on.
It has to be a loop
Electricity only flows round a complete, unbroken loop.
That is nearly the whole topic, and it is worth saying plainly because so much of what follows is a consequence of it: a current is charge moving all the way round, out of one end of the cell, through every component on the way, and back into the other end, which means the path has to close on itself before anything at all can happen. Break it anywhere and nothing moves anywhere.
The second picture is worth being emphatic about, because a child who quietly assumes that a small gap gives a small current will be baffled by every practical they ever do, and will keep looking for a loose connection that is somehow only half loose. Air does not conduct. A one millimetre gap is as good as a mile.
What each part does
- The cell pushes the current round. It is the store of chemical energy that everything else runs on. Two or more cells joined together are a battery, though almost nobody uses the words that carefully.
- The wires carry the current. They are made of metal because metal conducts.
- The switch is a deliberate gap you can open and close.
- The bulb turns electrical energy into light and heat. It is the reason the circuit exists.
Draw components with symbols, not pictures
A cell is a long line and a short line. A bulb is a circle with a cross. A switch is a line that lifts away from a contact.
Drawing a little sketch of a torch bulb instead of the symbol loses marks, and the symbols take about a minute to learn.
Conductors and insulators
A conductor lets current pass through it, and an insulator does not. Metals conduct, which is why every wire has a metal core. Plastic, rubber, wood, glass and air are insulators, which is why every wire has a plastic coat.
Both jobs are needed at the same time, and noticing that is the thing that makes the pair of words useful rather than a list to memorise: a wire is a conductor wrapped in an insulator, so the current is free to travel the whole length of it and completely unable to leave it on the way. That is why you can hold one safely.
A circuit with a bulb does not light. The cell is new. What would you check, and in what order?
- Is the switch closed? Most non-working circuits are this.
- Is every connection actually touching, including under the crocodile clips?
- Is the bulb screwed in properly, or has its filament broken?
- Is the cell the right way round in its holder?
Follow the loop with your finger and find the gap
Series and parallel
Two bulbs can be joined in two different ways, and they behave completely differently.
In series they sit one after the other on a single loop, so there is only one path, the same current passes through both, and each bulb ends up dimmer than one on its own would be. Take either bulb out and the loop is broken, so the other goes out as well. Old Christmas lights were wired this way, which is exactly why one dead bulb ruined an entire string and left somebody testing forty of them on the living room floor.
In parallel each bulb sits on its own branch. Each gets its own full path back to the cell, so each is as bright as a single bulb, and removing one leaves the other lit. Everything in a house is wired in parallel, which is why switching off a lamp does not turn off the fridge.
The quick test
Trace the loop with a finger. If there is one path, it is series. If the path splits and rejoins, it is parallel.
Common mistakes
1. Thinking current gets used up as it goes round
Picturing the current leaving the cell full and arriving back empty. The same current flows all the way round, the whole way, at every point. What gets used is the cell's stored energy, not the current itself.
2. Believing one wire is enough
Connecting one wire from the cell to the bulb and expecting light. It needs a way back as well, or nothing can circulate. Every working circuit is a ring, never a line with two ends.
3. Adding more cells to make bulbs brighter, without limit
More cells does mean a bigger push and a brighter bulb, right up until the filament melts. The bulb is built for a particular amount, and going well past it produces a very brief flash and then nothing at all.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
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What to learn next
That is the forces and energy cluster complete. Matter and the water cycle is the other Science route on the hub, and it starts from what everything is made of.
For parents
A cheap circuit kit is one of the few science purchases genuinely worth making, but you do not need one to start. Ask your child why a torch stops working when the batteries are loose, and make them explain it as a loop rather than as the torch being broken. Getting to the word gap on their own is the whole lesson.
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