Reducing wasted energy

The conservation of energy says energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed.

When energy is transferred, some of it is always dissipated. It spreads out into the surroundings, usually into their thermal store, and is stored in a less useful way. We call this wasted energy.

For example, a phone charger gets warm. Some of the energy transferred electrically heats the charger and the air around it instead of charging the battery.

When surfaces rub together, friction transfers energy to the thermal stores of the parts and the surroundings. This energy is wasted.

Lubrication cuts this waste. Oil or grease between moving parts, such as the gears of a bike or the parts of a car engine, lets them slide past each other more easily, so less energy is dissipated by heating.

Thermal conductivity describes how quickly energy is transferred through a material by conduction. Metals have a high thermal conductivity. Air, foam and wool have a low one, so they make good thermal insulators.

How fast a building cools depends on its walls:

• Thicker walls make it cool more slowly.
• Walls with a lower thermal conductivity make it cool more slowly.

That is why homes use cavity wall insulation, loft insulation and double glazing. They trap air or use materials with a low thermal conductivity, so less energy escapes and heating bills are lower.

This practical compares how well different materials insulate.

1. Stand a small beaker inside a larger beaker.
2. Pack the gap between them with the insulating material you are testing.
3. Fill the small beaker with hot water from a kettle, to the same level each time.
4. Put on a lid with a hole for a thermometer, and record the starting temperature.
5. Record the temperature every 2 minutes for 20 minutes.
6. Repeat with other materials, and with no insulation, using the same volume and starting temperature.

Plot temperature against time for each material. Each line slopes down steeply at first, then levels off as the water nears room temperature. The shallower the curve, the better the insulator.

You can repeat it with different thicknesses of one material too.

• Independent variable: the type (or thickness) of insulating material.
• Dependent variable: the temperature drop of the water in a set time.
• Control variables: the volume and starting temperature of the water, the size of the beakers, the lid and the time.

Hazard: hot water can scald you, so pour carefully and keep the beakers away from the edge of the bench.

Improving accuracy: use a lid on every test, read the thermometer at eye level, and start timing at the same water temperature each time.