Batteries and the mains

1. Every electrical device in a home needs a source that can push a current around its circuit, and there are two sources: batteries and the mains supply.
2. Portable devices such as torches, mobile phones, remote controls and wireless headphones are powered by batteries.
3. A battery stores energy in its chemical energy store, and when the circuit is complete, chemical reactions inside it push charge round the circuit so that energy is transferred electrically to the components.
4. A battery supplies direct current (d.c.), so the charge always flows round the circuit in the same direction, and a single cell typically provides a potential difference of about $1.5V$.
5. As the device runs, the chemical energy store of the battery decreases until the battery is flat and has to be recharged or replaced.
6. Larger and fixed appliances such as kettles, washing machines, ovens and televisions are plugged into the mains supply.
7. The UK mains supplies alternating current (a.c.) with a potential difference of about $230V$ and a frequency of $50Hz$, so the current changes direction back and forth $50$ complete times every second.
8. The energy delivered by the mains comes from power stations and other generators, and it reaches each socket electrically through the cables of the national grid.
9. Whichever source is used, energy enters the device by the same pathway, an electrical transfer, and the device then transfers that energy on to other stores.
10. A rechargeable battery links the two sources, because while a phone charges, energy is transferred electrically from the mains into the chemical energy store of its battery, ready to be transferred out again later.
    1. The phone charger changes the $230V$ a.c. from the socket into the low-voltage d.c. that the battery needs.

Following the energy through a device

1. To describe a device, follow the energy from where it starts to where it finishes, one link at a time.
2. The first link is the source, either the chemical energy store of a battery or the mains supply.
3. The second link is the electrical transfer through the wires to the working part of the device.
4. The third link is what that working part does with the energy, which depends on whether it is a motor, a heating element, a lamp or a loudspeaker.
5. The last link is the useful store or pathway at the end, such as the kinetic energy store of a moving part, the thermal energy store of water, or energy carried away by light or sound.
6. Torch (battery): chemical energy store of the battery $\to$ electrically to the bulb or LED $\to$ carried away by visible light, with some transferred by heating to the surroundings.
7. Electric toothbrush (battery): chemical energy store of the battery $\to$ electrically to a small motor $\to$ kinetic energy store of the vibrating brush head.
8. Wireless speaker (battery): chemical energy store of the battery $\to$ electrically to the loudspeaker $\to$ the speaker cone vibrates and energy is carried away by sound waves.
9. Electric kettle (mains): mains supply $\to$ electrically to the heating element $\to$ by heating to the thermal energy store of the water.
10. Toaster (mains): mains supply $\to$ electrically to the glowing elements $\to$ by infrared radiation and heating to the thermal energy store of the bread.
11. Hairdryer (mains): mains supply $\to$ electrically to a motor and a heating element $\to$ the motor increases the kinetic energy store of the air while the element increases its thermal energy store.
12. Washing machine (mains): mains supply $\to$ electrically to a motor that turns the drum and an element that warms the water $\to$ kinetic energy store of the drum and clothes and thermal energy store of the water.
13. Television (mains): mains supply $\to$ electrically to the screen and loudspeakers $\to$ carried away by light and sound.

Wasted energy in the supply

1. Energy can be wasted before it even reaches the working part of a device, while it is being transferred along the wires.
2. Every cable has some resistance, so as the current flows through it, a small amount of energy is transferred to the thermal energy store of the cable.
3. A long, thin extension lead has a larger resistance than a short, thick one, so it warms up more when a powerful appliance such as a heater is plugged into it.
4. A phone charger feels warm while it is working, because part of the energy taken from the mains is dissipated in its circuits instead of reaching the battery.
5. A battery also has some resistance inside it, so a battery that is supplying a large current, such as the one in a cordless drill, becomes warm as it is used.
6. Each of these transfers leaves less energy for the useful job, and the energy ends up dissipated in the thermal energy stores of the surroundings.

Wasted energy in motors

1. An electric motor is designed to transfer energy into the kinetic energy store of a rotating part, such as a fan blade, a drill bit or a washing machine drum.
2. Not all of the energy transferred to a motor reaches this kinetic energy store, because energy is wasted at several points inside it.
3. In the coils: the current flowing through the copper coils meets resistance, so energy is transferred to the thermal energy store of the coils.
4. In the bearings: the spinning axle rubs against the parts that support it, so work done against friction transfers energy to the thermal energy stores of the axle and bearings.
5. Through vibration: the moving parts vibrate, and energy is carried away by sound waves, which is the hum or whine you hear from a food mixer or vacuum cleaner.
6. Through air resistance: the rotating parts push air aside, transferring energy to the kinetic and thermal energy stores of the air inside the casing.
7. All of these wasted transfers end in the thermal energy stores of the motor and the room, so the energy is dissipated.
8. This is why a vacuum cleaner feels warm after use and why many motors have air vents or a small cooling fan to carry the energy away and stop the motor overheating.

Wasted energy in heating devices

1. A heating device such as a kettle, toaster, iron or oven is designed to increase the thermal energy store of something specific, such as the water, the bread or the food.
2. Inside the device, the current passes through a heating element, a coil or strip of high-resistance metal that becomes very hot.
3. The element then transfers energy by heating to the material in contact with it, and in a toaster or grill also by infrared radiation.
4. Energy is wasted because the element does not heat only its intended target.
5. In a kettle: some energy warms the element and the plastic body, and the hot kettle then transfers energy by heating to the air in the kitchen.
   1. Once the water boils, more energy leaves with the steam instead of staying in the water.
6. In a toaster: much of the infrared radiation misses the bread and warms the toaster casing, the worktop and the surrounding air.
7. In an oven: energy passes through the oven walls and door and warms the kitchen, and each time the door is opened hot air escapes.
8. In an iron: energy is transferred from the hot soleplate to the air and to the handle as well as to the creases in the clothes.
9. Heating devices still transfer most of their energy usefully, because the element is placed in direct contact with, or very close to, the thing it is meant to heat.
10. An electric fan heater is a useful contrast, because warming the air of the room is its whole purpose, so almost all of the energy transferred to it ends up where it is wanted.