Electricity

Electricity powers almost everything in the modern world (lights, phones, trains, machines). Behind it all lie a few simple quantities: the current that flows, the voltage that pushes it, and the resistance that holds it back. These are tied together by one elegant rule, Ohm's law. Understanding them explains how every electric circuit works.

Electric current is the rate of flow of electric charge through a conductor. It is measured in amperes (A). Current flows only in a closed loop called a circuit.

To make charge flow, something must push it. This push is the potential difference, or voltage, measured in volts (V). A cell or battery provides this potential difference, like a pump driving water through pipes. The greater the voltage, the harder the charge is pushed.

The relationship between voltage and current was discovered by Georg Ohm. Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the voltage across it:

V = I × R

where V is voltage, I is current, and R is the resistance. So for a given resistance, doubling the voltage doubles the current.

Resistance is the property of a conductor that opposes the flow of current, measured in ohms (Ω). A good conductor (like copper) has low resistance, while an insulator has very high resistance. Resistance increases with the length of a wire and decreases with its thickness.

Components called resistors can be joined in two ways:

• In series (one after another): the resistances add up, so the total is larger and the same current flows through each.
• In parallel (side by side): the total resistance is smaller than any single one. The voltage is the same across each branch. House wiring uses parallel circuits, so each appliance works independently.

Electric power is the rate at which electrical energy is used, measured in watts (W), and given by:

P = V × I

When current flows through a resistance, it produces heat. This is called the heating effect of current. It is useful in electric heaters, irons and geysers. The filament of a bulb also uses this effect: it glows white-hot when current passes through it. This same principle is the basis of the fuse. A fuse is a safety device with a thin wire. If the current gets dangerously high, the wire melts and breaks the circuit. Electrical energy used at home is measured in kilowatt-hours (units).

A semiconductor is a material, such as silicon, whose conductivity lies between that of a conductor and an insulator. Tiny semiconductor devices are the heart of modern electronics, and several of them now light, power and compute for us.

• LED (Light Emitting Diode): a semiconductor device that emits light directly when current passes through it. An LED lamp is highly energy efficient and has a very long lifespan.
• CFL (Compact Fluorescent Lamp): produces light by passing current through mercury vapour, which makes a phosphor coating glow. A CFL is less energy efficient than an LED and has a much shorter life. The mercury also makes disposal a pollution concern.
• OLED (Organic Light Emitting Diode): an LED whose light-emitting layer is a thin organic (carbon-based) film. OLED pixels emit their own light, so no backlight is needed. OLEDs can be fabricated on flexible plastic substrates, which allows roll-up displays embedded in clothing and even transparent displays. A Liquid Crystal Display (LCD), by contrast, is rigid and needs a separate backlight.

Semiconductors also turn sunlight into electricity. Photovoltaics is the technology that converts light directly into electricity using solar cells, and the output is direct current (DC). Solar thermal technology works differently: it uses the Sun's rays to generate heat, and that heat is then used to produce electricity, usually by driving a turbine. The two routes are often confused, so keep the distinction clear: photovoltaic means light to electricity directly, solar thermal means light to heat to electricity.

Computing has its own frontier device. A qubit (quantum bit) is the basic unit of information in quantum computing. Unlike an ordinary bit, which is either 0 or 1, a qubit can exist in a superposition of both states, and qubits can be linked by entanglement. This lets a quantum computer explore many possibilities at once for certain problems.