Electronics you can see.

Everything around you is made of atoms, and every atom contains tiny particles called electrons. Electrons carry a negative electric charge. In metals like copper, some electrons are only loosely attached to their atoms, so they can drift from atom to atom. We call them free electrons.

Electric current is simply charge on the move. A battery pushes electrons out of its negative (−) end and pulls them back in at its positive (+) end. For that to happen there must be a complete loop of conducting material from one end to the other. That loop is a circuit.

Open a switch anywhere in the loop and the flow stops everywhere at once. The electrons don’t drain away. They are still sitting in the wire, waiting for a push.

How fast do electrons move?

Surprisingly slowly. In a typical wire, electrons drift along at less than a millimetre per second. The light still comes on instantly because the wire is already full of electrons: pushing at one end moves them all at once, like pushing a marble into a tube already full of marbles. The animations on this page speed electrons up so you can see them.

A note on direction

Circuit diagrams usually show current flowing from + to −. This conventional current was agreed before anyone knew about electrons. Electrons actually travel the other way, from − to +. Both descriptions give the same answers, so don’t let it worry you. The blue dots on this page are electrons.

Three quantities describe almost every circuit. A water pipe is the classic way to picture them.

They are tied together by Ohm’s law:

V = I × R

Rearranged, it answers any question with two known values: I = V ÷ R and R = V ÷ I. More push means more current. More resistance means less current. Example: a 9 V battery across a 450 Ω resistor gives 9 ÷ 450 = 0.02 A, which is 20 milliamps (mA).

There are two basic ways to connect two parts to one battery.

• Series: one after another, in a single loop. The same current passes through both, and the battery’s voltage is shared between them. Resistances add up: R_total = R₁ + R₂. If one part breaks, the loop is broken and everything stops.
• Parallel: side by side, each on its own branch. Every branch gets the full battery voltage, and the branch currents add up at the battery. If one branch breaks, the others keep working.

Your home is wired in parallel. That is why every socket gets the same voltage and switching off the TV doesn’t switch off the fridge.

Power is how fast electrical energy is being used, measured in watts (W):

P = V × I

Combine it with Ohm’s law and you get two handy versions: P = I² × R and P = V² ÷ R. In a resistor, that power turns into heat. That is why resistors are sold with power ratings such as ¼ W or 1 W, and why a phone charger feels warm.

Energy is power used over time. Electricity companies bill in kilowatt-hours (kWh): 1,000 watts running for one hour. On Indian electricity bills, one “unit” is one kWh.

Resistors are too small to print numbers on, so their value is painted on as coloured bands. On the common four-band type:

• Bands 1 and 2 are digits.
• Band 3 is the multiplier: how many zeros to add.
• Band 4 is the tolerance: how far the real value may be from the printed one. Gold (±5%) is the most common.

The ten digit colours run in rainbow order: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Hold the resistor with the gold or silver band on the right and read from the left.

A capacitor is two metal plates separated by a thin insulator. Connect it to a battery and electrons pile up on one plate while the same number leave the other. No current crosses the gap, but charge builds up, and so does a voltage across the plates. Think of a small tank that fills up and can then empty again.

Capacity is measured in farads (F). One farad is huge, so real parts are usually marked in microfarads (µF), nanofarads (nF) or picofarads (pF).

Charging through a resistor is fast at first and then slows down, because the fuller the capacitor gets, the less push is left to fill it. The speed is set by the time constant:

τ = R × C

After one τ the capacitor reaches about 63% of the battery voltage. After five τ it is above 99%, which engineers usually call “full”.

Where capacitors are used

• Smoothing the bumpy output of power supplies.
• Timing, for example making an LED blink at a steady rate.
• Quick bursts of energy, such as a camera flash.
• Filtering unwanted noise out of audio and radio signals.

A diode is a one-way valve for current. Connected one way round (forward), it conducts once the voltage across it passes a small threshold, about 0.6–0.7 V for an ordinary silicon diode. Connected the other way (reverse), it blocks. Phone chargers use diodes to turn alternating current into one-way current.

An LED (light-emitting diode) is a diode that gives off light when current flows forward. Its threshold, the forward voltage, depends on the colour. The longer leg is the + side (the anode).

An LED has almost no resistance once it turns on, so connected straight to a battery it would draw too much current and burn out. Always add a resistor in series to limit the current:

R = (V_supply − V_LED) ÷ I

For a full worked version with resistor power, warnings and every voltage in the loop, open the Circuit lab.

A transistor lets a small current control a much larger one. The common NPN bipolar transistor has three legs: the base (the control), the collector and the emitter (the main path).

A small current into the base allows a current about β times bigger to flow from collector to emitter. β (beta, or gain) is often 100 or more for small transistors.

I_C ≈ β × I_B

The transistor has three working states:

• Off (cut-off): no base current, no main current.
• Partly on (active): main current follows base current. This is how amplifiers work.
• Fully on (saturated): the main current is as large as the load allows. The transistor acts as a closed switch.

DC (direct current) flows one way at a steady voltage. Batteries, USB ports and solar cells give DC.

AC (alternating current) reverses direction many times a second, following a smooth wave. Wall sockets give AC: 230 V at 50 cycles per second (50 Hz) in India, Europe and much of the world, and 120 V at 60 Hz in North America.

AC is used for power grids because a transformer can easily step its voltage up for long-distance transmission and back down for homes. Most electronics then convert it to low-voltage DC inside their chargers.

Peak versus RMS

An AC voltage keeps changing, so which number is “the” voltage? The quoted figure is the RMS value: the DC voltage that would heat a resistor equally. For a sine wave, RMS = peak ÷ √2. So 230 V mains actually swings up to about 325 V and down to −325 V, fifty times a second.

Put two resistors in series and the voltage splits between them in proportion to their resistance. Tap the point in the middle and you get a smaller voltage out:

V_out = V_in × R₂ ÷ (R₁ + R₂)

Equal resistors give exactly half. A potentiometer, like a volume knob, is a divider whose tap slides along one resistor.

Now replace R₂ with a sensor whose resistance changes. A light-dependent resistor (LDR) has high resistance in the dark (around a megaohm) and low resistance in bright light (around a kilohm or less). The output voltage now tracks the light level, and a microcontroller can read it. Automatic street lights work on this idea.

Go further in Circuit lab 02: real thermistor maths, what a load does to the output, and how to choose the fixed resistor.

Digital circuits use only two voltage levels: 0 (low, near 0 V) and 1 (high, often 3.3 V or 5 V). A logic gate takes one or two of these inputs and produces an output by a fixed rule. Each gate is built from a handful of transistors working as switches.

You have already met two of them. Two switches in series light a bulb only if both are closed: that is AND. Two switches in parallel light it if either is closed: that is OR.

From gates to computers

An XOR gate adds two binary digits: 0+0=0, 0+1=1, 1+0=1, and 1+1 gives 0 with a carry of 1. That carry is exactly what an AND gate outputs. Chain enough of these “adders” together and you can add any numbers. Every calculator and computer is built up from gates like these.

Ten questions covering every chapter. Pick an answer to see the explanation straight away. Your first answer to each question counts; you can try again afterwards. For a mixed set with calculations, try the Challenge.