Circuit Builder
Drag cells, bulbs, switches and meters onto the board, join the terminals with wires and watch what really happens — the bulb glows, the ammeter reads, and a short circuit warns you.
A circuit works only if it forms a closed loop — an unbroken path from the positive terminal of the cell, through every component, back to the negative terminal. Open the switch or remove one wire and the current stops. Two 10 Ω bulbs in series on a 3 V battery give I = 3 ÷ 20 = 0.15 A, so both glow dimly.
Tap a part to drop it on the board, or drag it. Drag from one terminal dot to another to lay a wire. Tap a switch to open or close it. Keyboard: Tab steps through the parts, arrow keys move them, R rotates, Del deletes, Enter flips a switch.
What you just built
A plain-English reading of your circuit, written fresh every time you change it.
Working
Selected part
Tap anything on the board to change it.
Nothing selected. Tap a component or a wire on the board.
Challenges
Build each one — they tick themselves.
Symbol key
The same symbols your science textbook uses.
A real circuit lab in your browser
Every bulb, meter and wire on the board is solved with Ohm’s law and Kirchhoff’s current rule — so what you see is what a real circuit would do.
Drop the parts
Tap or drag a cell, bulb, switch, resistor, ammeter or voltmeter from the tray onto the board.
Join the terminals
Drag from one terminal dot to another to lay a wire. The loop must close for current to flow.
Watch it work
The bulb’s glow follows the real current, the switch opens the loop and the meters show true readings.
Beat the challenges
Ten tasks from “light one bulb” to “keep one bulb lit when the other is removed”.
A circuit must be a closed loop
Charge can only flow if there is an unbroken path from the + plate of the cell, through every component, back to the − plate. One missing wire, or one open switch, and the whole circuit stops — that is why a single loose connection kills a torch.
I = V ÷ R V = I × R
Bulbs in series
Rs = R₁ + R₂ + …
One path only, so the same current passes through both bulbs and the cell voltage is shared between them. Two 10 Ω bulbs on a 3 V battery give R = 20 Ω and I = 0.15 A — half the current, so both look dim. Unscrew one and both go out.
Bulbs in parallel
1/Rp = 1/R₁ + 1/R₂ + …
Each bulb gets the full battery voltage, so each still draws 0.3 A and glows normally — the battery supplies 0.6 A and runs down twice as fast. Remove one bulb and the other keeps glowing. This is exactly how house wiring works.
Short circuit — why it is dangerous
If a wire joins the two terminals of the cell with no bulb or resistor in the way, R is almost zero, so I = V ÷ R becomes enormous. The cell and the wire heat up fast. Always keep at least one bulb or resistor in the loop.
Ammeter and voltmeter
An ammeter has almost no resistance and is joined in series, in the path of the current. A voltmeter has very high resistance and is joined in parallel, across the component. Swap them around and the circuit misbehaves — try it on the board and read the warning.
Need the numbers only?
This board is the hands-on half of the pair. For step-by-step sums — V = IR from any two values, resistor networks, the colour code and your electricity bill — open the Ohm’s Law & Circuit Calculator. Build it here, then check the arithmetic there.
Circuit Builder — your questions
What is an electric circuit?
An electric circuit is a closed path along which charge can flow. It needs a source such as a cell or battery, connecting wires, and at least one component such as a bulb or resistor. If any part of the path is broken — a loose wire or an open switch — the circuit is open and no current flows.
Why do bulbs in series glow dimmer than bulbs in parallel?
In series the resistances add, so two 10 Ω bulbs on a 3 V battery give 20 Ω and a current of only 0.15 A, and the voltage is shared between them. In parallel each bulb gets the full 3 V and draws its own 0.3 A, so each glows normally — but the battery supplies 0.6 A and runs down twice as fast.
What is a short circuit and why is it dangerous?
A short circuit happens when a wire joins the two terminals of the cell with no bulb or resistor in the path. Resistance is almost zero, so I = V ÷ R becomes very large. The wire and the cell heat up rapidly, which can cause burns or a fire. Always keep at least one component in the loop.
How are an ammeter and a voltmeter connected in a circuit?
An ammeter measures current, so it is connected in series, in the path the current takes, and it is built with almost no resistance. A voltmeter measures potential difference, so it is connected in parallel across the component, and it has a very high resistance. Swapping them disturbs the circuit and gives wrong readings.
Why does one blown bulb switch off all the bulbs in a series circuit?
A series circuit has only one path. When a bulb's filament breaks, that single path is broken, so no current can flow anywhere in the loop and every bulb goes out. In a parallel circuit each bulb has its own branch, so removing or breaking one leaves the others working — which is how house wiring is done.
What are the circuit symbols for a cell, a bulb and a switch?
A cell is drawn as one long thin line for the positive terminal and one short thick line for the negative. A battery is two or more cells drawn in a row. A bulb is a circle with a cross inside it, a switch is a line hinged away from a contact, and a resistor is a plain rectangle.
What is the difference between a series and a parallel connection?
In series the components are joined end to end in one loop, so the same current flows through all of them and the voltage is shared. In parallel they are joined across the same two points, so each receives the full voltage and the currents add up. Series resistance adds; parallel resistance is always less than the smallest resistor.