Stores and pathways

1. An energy store says where energy is being held: the thermal store of a warm object, the kinetic store of a moving one, the chemical store of a battery or a fuel, the elastic store of a stretched spring, and the gravitational store of a raised mass.
2. Energy is never held in a pathway. A pathway is the mechanism that moves energy from one store to another, and there are four of them: electrical working, mechanical working, heating and radiation.
3. Every mains device begins in the same way. The supply drives a current through the device, so energy is transferred into it along the electrical working pathway.
4. A complete description of a device names three things: the store that empties, the pathway used, and the store or stores that fill.
5. For a device plugged into the mains the store that empties is far away and out of sight, so the description starts at the supply doing electrical work on the device itself.
6. One device usually fills several stores at the same time, and only some of that filling is the filling the user wanted.

Filling a thermal store

1. An electric kettle is described like this: the mains supply does electrical work on the heating element, and the thermal store of the water fills, so the temperature of the water rises.
2. The kettle also fills the thermal store of its own body and of the surrounding air, and more energy leaves with the escaping steam. Those transfers are wasted, because the user wanted hot water and nothing else.
3. A hair dryer carries out two transfers at once. Electrical working on the element fills the thermal store of the air, and electrical working on the motor fills the kinetic store of the fan blades.
4. Both of those transfers are useful in a hair dryer, since warm air is wanted and moving air is needed to carry it to the hair. The wasted part is the energy filling the thermal store of the casing, together with the noise of the motor.
5. An electric iron, an electric oven and a toaster all follow the kettle's pattern, with electrical working filling the thermal store of the soleplate, of the oven air, or of the bread.
6. An immersion heater is the same transfer stretched over hours, filling the thermal store of the water in a tank so that the water is still hot when it is wanted.

Filling a kinetic store

1. A food mixer is described like this: the supply does electrical work on the motor, the kinetic store of the rotating blades fills, and the blades then do mechanical work on the food.
2. Not all of the input reaches the blades. Energy dissipated in the resistance of the motor windings fills the thermal store of the motor, and friction at the bearings fills the thermal store of the bearings and the air around them.
3. The mixer is also noisy, and the energy carried away by those sound waves finishes in the thermal store of the surroundings once the walls and the air absorb them.
4. A desk fan makes the same transfer, filling the kinetic store of the blades and then the kinetic store of the moving air, which is the output the user actually wants.
5. A washing machine fills two different stores in turn: the kinetic store of the drum while it tumbles and spins, and the thermal store of the water while its element heats the wash.
6. An electric drill fills the kinetic store of the rotating bit, and that store then does mechanical work on the wood or masonry, filling the thermal store of the material and of the bit. This is why a drill bit is hot to the touch after use.
7. A vacuum cleaner fills the kinetic store of a fan, and the moving air it creates carries the dust. Its motor and its rushing air both fill the thermal store of the surroundings as well.

Light and sound

1. A filament lamp is described in two steps. Electrical working fills the thermal store of the filament, and the hot filament then transfers energy to the surroundings by radiation, part of which is visible light.
2. The useful output of a lamp is the light radiated. The energy filling the thermal store of the bulb, the fitting and the air around them is wasted, because the user wanted a lit room and not a warm one.
3. An LED lamp does the same job while filling a thermal store far less, which is why an LED stays cool in its fitting and a filament lamp cannot be touched.
4. A television takes energy in by electrical working, and its useful outputs are the light radiated from the screen and the sound waves from its speakers. The thermal store of its circuit boards and case fills too, which is why the back of a television is warm.
5. A loudspeaker fills the kinetic store of its vibrating cone, and the cone then sets the air vibrating so that sound waves travel out. Its coil has resistance, so the thermal store of the speaker fills as well.
6. Light and sound both end up in the thermal store of the surroundings once walls, furniture, air and people have absorbed them.

Charging a battery

1. A phone on charge is the everyday case where the store that fills is neither thermal nor kinetic. The supply does electrical work on the phone and the chemical store of the battery fills.
2. The transfer runs the other way once the charger is unplugged. The chemical store of the battery empties and does electrical work on the screen, the processor and the speaker.
3. Both the charger and the phone become warm during charging, so part of the transfer fills the thermal store of the charger, the battery and the room rather than the chemical store.
4. A cordless drill on its base, an electric toothbrush in its stand and a rechargeable torch all fill a chemical store in exactly the same way.
5. A battery is therefore a store and not a source, because the energy held in it was put there by an earlier transfer from the mains.

Useful and wasted transfers

1. In any device the transfer that fills the store the user wanted is the useful transfer, and every other transfer counts as wasted energy.
2. The same store can be useful in one device and wasted in another. The thermal store of the surroundings is the whole point of a fan heater and a plain nuisance in a laptop.
3. No device transfers all of its input usefully, and three separate reasons appear in real appliances.
   1. Every part that carries a current has some resistance, so a thermal store fills wherever charge flows.
   2. Every moving part meets friction at its bearings and drag from the air, so a thermal store fills wherever something turns.
   3. Every device that makes light or sound sends some of it in directions nobody is using.
4. Wasted energy is not destroyed. It spreads into the surroundings and becomes shared between so many particles that it cannot be collected and used again.
5. Wasted transfers can be reduced but never removed. Lubricating the bearings cuts friction, thicker copper windings cut the resistance of a motor, and a smoother fan blade cuts air resistance.
6. Deciding which transfer is useful always starts from what the device is for, so identical physics is judged differently in a kettle, a fan and a lamp.