Robotics · Electronics basics · Grades 5–6

The electric circuit: a torch

A switch, a battery, a bulb: when does current flow? Does a nail conduct, and what about a ruler?

The situation

A pocket torch has stopped working. You open it: inside are two batteries, a small bulb, a switch and some wires. We lay everything out on the table and connect it again. When does current flow and when does it not? Will the lamp light if a wire is broken somewhere? What if we put a nail between the wires — or a plastic ruler? Why is it dangerous to connect two wires straight to a battery? And how should we connect two lamps so that one keeps shining even if the other burns out?

Topic

Electric circuit: source, conductor, load; current and voltage. A pocket torch: battery, switch and bulb — when the circuit is closed and where the current flows

Key ideas

A closed circuit
Conductor and insulator
Series and parallel

Formulas

source \to conductor \to load \to source

When the circuit is closed, current flows from the battery’s “+” terminal through the wires and the lamp to the “−” terminal. If there is a break anywhere, there is no current.

I = \frac{U}{R}

The current $I$ (amperes) is directly proportional to the voltage $U$ (volts) and inversely proportional to the resistance $R$ (ohms) — this is Ohm’s law, in more detail in Grade 7. Large resistance — small current: graphite makes the lamp glow dimly.

P = U \cdot I

The power of the lamp: 2.5 V · 0.3 A ≈ 0.75 W. If the power is twice the rated value, the filament overheats and breaks.

U = U_{1} + U_{2} (series), I = I_{1} + I_{2} (parallel)

In a series connection the voltage is shared between the lamps; in a parallel connection each lamp gets the full voltage and the current is shared.

Theory

The three parts of a circuit

Every electric circuit has three things: a source (the battery “pushes” the current), conductors (wires — the path of the current) and a load (a lamp, a motor — the place where the current does work). A torch has the same: two batteries, two wires and a bulb. The switch is a “gate” that opens and closes the path.

A closed and an open circuit

Current flows only in a closed circuit: it leaves the battery’s “+” terminal and must return through the wires and the lamp to the “−” terminal. If the path is broken anywhere — at the switch, before the battery, before the lamp, or when the lamp is unscrewed — the current stops in the whole circuit and the lamp goes out. That is why it does not matter where in the circuit the switch is placed.

Conductor or insulator?

Put a nail, a coin or a paper clip into the gap and the lamp lights: metals conduct current well. Pencil graphite conducts too, but its resistance is larger, so the lamp glows dimly. a plastic ruler, an eraser, dry wood, air are insulators: they do not conduct current. Tap water conducts a little — the lamp does not light, but a sensitive instrument shows a current; that is why it is dangerous to touch electrical appliances with wet hands.

How a bulb lights and why it burns out

Inside the lamp is a thin tungsten filament. When current passes through it, it heats to more than 2000 °C and glows. Our lamp is made for 2.5 V and 0.3 A. Two AA batteries (3 V) suit it. Give it 4.5 V or 9 V and the filament overheats — it burns very brightly for a few seconds and then breaks. A short circuit is when a wire connects the two terminals of the battery directly: the current reaches several amperes, the lamp goes out, and the wire and the battery get hot. Never do this with a real battery!

Series and parallel

If two lamps are connected in series (in one loop of the circuit), the voltage is shared between them — both glow dimly, and if you unscrew one the circuit breaks and the other goes out too. If they are connected in parallel (each by its own path to the battery), each gets the full voltage — both are bright, and if you unscrew one, the other keeps shining. That is why the lights in a house are connected in parallel.

About the model

The circuit is calculated by nodal analysis. A lamp is not a simple resistor: the hotter the filament, the larger its resistance — $I \approx 0.3 \cdot {(U / 2.5)}^{0.6}$ A (a typical relationship for real tungsten lamps). Brightness depends on power: if $P$ exceeds the rated value twofold, the filament breaks in 1–2 seconds. The resistances of the samples are approximate typical values (pencil graphite ≈ 6 Ω, water ≈ 40 kΩ). The battery’s internal resistance is taken into account (AA ≈ 0.15 Ω, “PP3” ≈ 1.7 Ω).

Did you know?

The first pocket torch was patented in 1899 in the USA by the inventor David Misell: dry batteries, a bulb and a paper tube. The batteries ran down quickly, so it could give only short “flashes” — hence the American name “flashlight”.
The tungsten filament in a bulb heats to more than 2000 °C when it glows — higher than the temperature at which steel melts (≈ 1500 °C). Tungsten was chosen because it has the highest melting point of all metals — 3422 °C.

Inside the experiment

A real board in a hand-inked isometric drawing: turn it, place the parts, wire the circuit, write and upload the code — current and voltage are computed at once, and a wrong connection burns a part. Inside are missions, theory, a quiz and a guided tour.

More experiments in this section

Breadboard: rows — Grades 5–6
LED and resistor: Ohm’s law — Grade 7
Resistors: the colour code — Grade 7
Voltage divider and potentiometer — Grade 7