Heating as the purpose

1. Some appliances are built so that the heating effect is the entire point of the device. The working part is a heating element, a length of wire chosen to have a high resistance.
2. Elements are usually made of nichrome, an alloy of nickel and chromium, because it has a high resistance, keeps its strength when red hot and does not oxidise away quickly.
3. The element is fed by thick copper conductors of very low resistance, so nearly all the energy is dissipated in the element and almost none in the leads that reach it.
4. The element is often wound into a tight coil, so that a great length of resistance wire fits into a small space and a large resistance sits exactly where the heat is wanted.
5. In every device of this kind the energy arrives along the electrical working pathway and fills a thermal store: the water, the air, the food, or the metal of the tool itself.

Devices that heat water

1. An electric kettle carries a coiled element rated at about $3kW$, either sitting in the water or hidden in the base plate below it. The element dissipates energy and the thermal store of the water fills until the water reaches $100{}^{\circ }C$.
2. Because the element touches the water directly, energy passes into the water by conduction and is then spread through the whole kettle by convection currents, so the water heats evenly rather than only in the layer against the element.
3. An immersion heater is a long sealed element lowered into a hot water tank. It fills the thermal store of the stored water slowly, so that hot water is ready hours later.
4. An electric shower heats the water as it flows past the element, so the energy must be dissipated very quickly. This is why a shower is one of the highest-current appliances in a house, drawing tens of amperes and needing its own thick cable and its own circuit.
5. A dishwasher and a washing machine each hide an element that fills the thermal store of the water in the drum, because grease is removed and detergents work far better hot than cold.
6. In all of these the heating effect is the advantage, because the useful output of the appliance is hot water and nothing else.

Heating food and air

1. A toaster holds flat nichrome ribbons behind a wire mesh. They dissipate energy until they glow dull red, and the energy reaches the bread mainly as infrared radiation, filling the thermal store of the bread.
2. An electric oven ring holds a heavy element under a metal or ceramic surface. Energy passes by conduction into the pan and then into the food, so the thermal store of the food fills.
3. A hair dryer sets a nichrome element in front of a small fan. The element fills the thermal store of the air, and the fan drives that hot air over wet hair so the water evaporates faster.
4. A soldering iron dissipates energy in an element inside its shaft, filling the thermal store of the metal tip until the tip is hot enough to melt solder onto a joint.
5. A car rear-window demister is a set of thin conducting strips baked into the glass. A current in the strips fills the thermal store of the glass, and the condensation or ice on the surface then evaporates or melts.
6. An electric blanket, a heated towel rail and a fan heater all place resistance wire where the warmth is wanted, and a fan heater is the one case where the thermal store of the surroundings is the intended destination.

Light from a filament

1. A filament lamp contains a very thin coiled tungsten wire. Its resistance is high and its surface small, so the energy dissipated in it raises its temperature to roughly $2500{}^{\circ }C$.
2. At that temperature the filament emits enough visible light to be useful, so the heating effect is the means by which the lamp makes light at all.
3. Tungsten is chosen because its melting point is about $3400{}^{\circ }C$, high enough for the wire to be run white hot without melting.
4. The glass envelope is evacuated or filled with an unreactive gas such as argon, so the hot filament cannot react with oxygen and burn through.
5. Most of the energy dissipated in the filament leaves as infrared radiation rather than visible light. A $60W$ filament lamp becomes too hot to touch, while an LED lamp of the same brightness draws only about $8W$ and stays cool.
6. The filament lamp therefore appears on both sides of the balance sheet. The heating is essential to its operation and is also the reason it has been replaced by LEDs in almost every home.

A wire meant to melt

1. A fuse is the one component in a circuit designed to destroy itself. Its wire is thin and made of a metal of low melting point, so it has a higher resistance than the cable it protects.
2. In normal use the current is small enough that the fuse wire runs barely warm and nothing happens to it.
3. If a fault drives a current larger than the fuse rating through it, the rate at which energy is dissipated in the thin wire rises steeply and its temperature climbs past its melting point.
4. The wire melts and leaves a gap, so the circuit is broken, the current stops, and the appliance and its cable are saved from overheating.
5. The fuse works only because it is the deliberately weakest link. It is built to reach its own melting point well before the insulation of the cable reaches the temperature that would damage it.
6. This is the sharpest example of the effect being useful and destructive at the same moment, because here the destruction is the safety feature.

When heating is unwanted

1. In most electrical devices heat is not the intended output, so any energy that ends up in a thermal store is wasted energy.
2. A phone charger feels warm in use. The energy dissipated in its windings and switching components never reaches the phone, so less of the energy drawn from the mains ends up in the chemical store of the battery.
3. A laptop dissipates energy in its processor, its graphics chip and its power supply. It needs a fan and a metal heat sink to move that energy out, and the fan itself draws energy that does no computing.
4. An electric motor in a drill, a washing machine or a fan dissipates energy in the resistance of its copper windings, so the windings run hot and the motor delivers less to the turning parts than the supply delivers to the motor.
5. A transformer dissipates energy in the resistance of its coils, which is why a large one carries cooling fins or is immersed in oil, and why a small one in a plug-top adaptor feels warm to the hand.
6. A long connecting cable dissipates energy along its whole length, energy that warms the cable instead of reaching the appliance plugged into its end.
7. Unwanted heating brings four separate penalties: a smaller fraction of the input is transferred usefully, running costs rise, components age faster at high temperature, and there is a fire risk wherever the energy cannot escape.
8. Semiconductors suffer first. A processor that overheats slows itself down or shuts down to protect itself, and a battery repeatedly run hot loses capacity permanently.
9. Heating turns dangerous when the energy is trapped. A coiled extension lead is a hazard because the coiled turns insulate one another, so the energy dissipated in the copper cannot escape and the plastic can soften and melt at a current the same lead would carry safely when unwound.
10. A radiator covered by clothes, an appliance pushed hard against a wall and a laptop used on a duvet all fail in the same way, because the surfaces that should be transferring energy to the air are blocked.
11. Designers answer with fans, heat sinks, ventilation slots, cooling fins, thermal cut-outs and low-resistance conductors. All of them cost money and space, and none of them removes the dissipation entirely.