Circuits you can test

Up to now electricity may have felt like something to believe: invisible charge, invisible pushes, invisible resistance. This layer is where you stop believing and start testing. Every idea here can be checked with a battery, a few bulbs, some wire and a cheap multimeter — or with the labs on this page when you do not have the kit to hand.

You will learn to set up a fair test, to put meters in the right places, to read them honestly, and to decide whether your results really support a claim. Then you will use those skills on the big practical questions: why are the lights in your home wired in parallel? Does a bulb obey Ohm's law? What exactly makes an MCB trip when someone plugs a heater into an already-busy extension board? And can a lemon really power anything?

Two rules run through every chapter.

1. Predict before you test. Write down what you expect and why. A surprise is only useful if you had an expectation to be surprised against.
2. Low voltage only. Every hands-on activity here uses cells or batteries of 9 V or less. Mains electricity (230 V in India) appears only as calculations and simulations. The physics is the same; the danger is not.

Suppose a friend claims: "A longer wire makes the bulb dimmer." How would you find out?

The tempting method is to grab a long wire and a short wire, connect each to a bulb, and compare. But if the long wire is thinner, or the battery was fresher for the first test, or you used a different bulb, you cannot tell which change caused the difference. A fair test changes exactly one thing on purpose and keeps everything else the same.

Scientists name the three kinds of variable:

• Independent variable — the one thing you deliberately change (wire length).
• Dependent variable — the thing you measure to see the effect (current through the bulb, or its brightness).
• Control variables — everything else that could affect the result, which you hold steady (battery, bulb, wire thickness, wire material, temperature).

Why repeat readings? Any single reading can be thrown off: a loose crocodile clip, a meter that has not settled, a finger pressing a contact. Take each reading at least three times. If the repeats agree closely, you can trust them. If one is far from the others, it is an anomaly — check the circuit, find the cause if you can, and repeat that reading rather than quietly including it.

Why several values of the independent variable? Two lengths only tell you whether something changes. Five or six lengths, evenly spread, show you the pattern: does the current halve when the length doubles? Is the graph a straight line or a curve? Patterns are what turn a result into an explanation.

Two meters do almost all the work in circuit investigations, and they are connected in opposite ways.

An ammeter measures current, the flow of charge through something. To count what flows through a bulb, the charge must flow through the meter too, so the ammeter goes in series: you break the circuit and let the meter become part of the loop. A good ammeter has a very low resistance so it barely changes the current it is measuring.

A voltmeter measures voltage (potential difference), the energy each coulomb of charge gains or loses between two points. So it connects across a component, in parallel, with one lead on each side. A good voltmeter has a very high resistance so almost no current is diverted through it.

In the water picture: an ammeter is a flow meter spliced into the pipe; a voltmeter is a pressure gauge with one tube before the pump or narrow pipe and one tube after it.

Reading an analogue scale. Older school meters have a needle. First work out what each small division is worth: if the scale goes from 0 to 1 A with 50 divisions, each is 0.02 A. Read with your eye directly above the needle — looking from the side makes the needle appear to sit over a different mark (this is called parallax). If the needle sits between two marks, estimate to about half a division.

Reading a digital display. Digital meters feel exact, but the last digit can flicker. A reading of 0.46 A usually means "somewhere around 0.455 to 0.465 A", and the meter's own accuracy (printed in the manual, often something like ±1% plus a digit or two) adds more. The resolution is the smallest change the display can show: 0.01 A here.

Uncertainty is an honest statement of how far off your answer might be. A simple, widely used estimate is half the range of your repeat readings.

A series circuit is a single loop. Charge leaves the battery, goes through the first bulb, then the second, then back — one path, no choices.

That single path has two consequences you can test. First, the current is the same everywhere in the loop: an ammeter placed before the first bulb, between the bulbs or after the last one shows the same reading. Charge is not used up by bulbs; it is energy that is transferred. Second, the battery's voltage is shared between the bulbs. With identical bulbs it is shared equally.

In a parallel circuit each bulb has its own branch, connected directly across the battery. Charge reaching a junction can go one way or the other, and the branches join again before returning to the battery.

Now the rules flip. Every branch gets the full battery voltage, so each bulb glows as brightly as a single bulb would. The branch currents add up to give the total current drawn from the battery. And because each branch is a separate path, breaking one bulb leaves the others working.

Every socket, fan point and light point in your home is connected in parallel across the live and neutral wires that come from the meter and MCB board. Put the bench results next to what a home needs and the reason is obvious:

• Every appliance gets the full 230 V. A 1500 W geyser is designed to work at 230 V. In series with a fan and a TV it would get only a share of the voltage and barely warm the water.
• Each appliance can be switched independently. Turning off the bedroom light should not turn off the fridge. In series, one open switch would break the only loop and switch everything off.
• One failure does not black out the house. A burnt-out tube light breaks its own branch only.
• Adding an appliance does not dim the others. Each branch draws its own current at 230 V.

The price, as the bench showed, is that the currents add up in the shared wires. That is the whole reason fuses and MCBs exist, and it is the subject of Chapter 8.

Ohm's law says that for some conductors, current is proportional to voltage: double the voltage, double the current. The ratio V ÷ I is then constant, and we call that constant the resistance, R.

That is a claim about the world, so let us test it. The classic method uses a fixed resistor (say one marked 10 Ω), a variable supply or a battery pack you can change from 1.5 V to 9 V in 1.5 V steps, an ammeter in series and a voltmeter across the resistor.

• Independent variable: voltage across the resistor.
• Dependent variable: current through it.
• Controls: the same resistor, kept at room temperature (switch off between readings), same leads and meters.

Now repeat the experiment with a small filament bulb instead of a resistor — say a 6 V, 0.5 A torch bulb. At first the numbers look familiar. Then something odd happens: doubling the voltage does not double the current. The current keeps rising, but more and more slowly.

Calculate V ÷ I for each row and the reason jumps out: the bulb's resistance is not constant. It climbs as the voltage goes up.

The filament is a very thin coil of tungsten wire. As more current flows it gets hotter — at full brightness a filament runs at well over 2000 °C. In a hot metal the atoms vibrate harder, so the drifting electrons collide with them more often, and the resistance rises. Between cold and white-hot, a tungsten filament's resistance typically rises by a factor of roughly 10 to 15.

So a filament bulb is non-ohmic. Its I–V graph starts steep near the origin (low, cold resistance) and bends over as it gets hotter (high resistance). The fixed resistor's graph was straight because it stayed at roughly room temperature.

Parallel wiring means the currents of everything switched on in a circuit add up in the wires they share. Those wires are sized to carry a certain current safely. Push more than that through them and, by P = I²R, they heat up — slowly at first, then enough to soften insulation and start a fire.

A fuse is a deliberately weak link: a short, thin wire that melts when the current exceeds its rating, breaking the circuit before the house wiring overheats. A miniature circuit breaker (MCB) does the same job but can be reset. It has two trip mechanisms: a bimetal strip that heats and bends on a moderate overload (tripping after seconds or minutes) and an electromagnet that snaps it open almost instantly on a huge current, such as a short circuit.

In many Indian homes, lighting and fan circuits are protected by 6 A or 10 A MCBs, and power circuits for sockets, geysers and ACs by 16 A or larger ones (the exact sizes depend on the wiring and the electrician's design).

A short circuit is a path with almost no resistance connected across a supply, so the current takes the short cut and skips the components it was meant to go through. Because I = V ÷ R and R is tiny, the current becomes enormous — limited only by the resistance of the wires and the supply itself.

On the bench, a short circuit is a bare wire accidentally touching both terminals of a battery. In a home, it is a live wire touching neutral inside a damaged cable or appliance. The mains supply can deliver hundreds or even thousands of amps for a moment, which is why the MCB's magnetic trip exists: it opens the circuit in a few thousandths of a second.

A battery cell needs three things: two different metals and an electrolyte between them, a liquid or paste that can carry charge as ions. A lemon's acidic juice or a potato's watery flesh can be the electrolyte. Push in a strip of zinc (a galvanised iron nail is coated in zinc) and a piece of copper (a copper wire or strip), and a chemical reaction at the zinc releases electrons that flow through an outside wire to the copper.

The fruit is not the source of the energy — the zinc is, as it slowly dissolves. The fruit is the electrolyte that lets the reaction happen. A typical lemon cell with zinc and copper gives about 0.9 V, and a current of at most around 1 mA. That is thousands of times too little for a torch bulb, but a red LED needs only about 1.8–2 V and a few milliamps to glow faintly — so several fruit cells in series can light one, dimly, in a dark room.

The fruit chain showed that cells in series add their voltages: each cell lifts the energy of every coulomb a little more, like pumps stacked one after another. That is why a TV remote uses two 1.5 V AA cells for 3 V, and why a rectangular 9 V battery is six small 1.5 V cells stacked inside one case.

Cells in parallel keep the same voltage as one cell but share the job of supplying current. With two identical cells side by side, each supplies half the current, so the pair can deliver more current and lasts roughly twice as long. Power banks and electric vehicle battery packs use both tricks: groups in parallel for capacity, groups in series for voltage.