Conservation and Dissipation of Energy

Energy is one of the most tightly controlled quantities in physics. It moves between stores endlessly, but the total never changes.

Energy cannot be created or destroyed. It can only be transferred usefully, stored, or dissipated. This is the principle of conservation of energy.

"Dissipated" means spread out to the surroundings in a way that is no longer useful. When a phone battery runs down, its chemical store has not vanished; the energy has been transferred to light, sound and, mostly, the thermal store of the phone and the air around it.

This principle underpins the whole of energy at GCSE. Whenever you describe or calculate an energy change, the energy you start with must be fully accounted for by the stores you end with.

A closed system is one in which no energy enters or leaves. It is a useful idea because it makes conservation of energy easy to state precisely.

In a closed system there is no net change to the total energy. Energy is only transferred between the stores inside the system.

A well-insulated flask of hot water is close to a closed system over a short time: the energy stays inside, moving between the thermal stores of the water, the flask and any air trapped above it, but the total remains almost constant.

No real system is perfectly closed, because some energy always finds a way to the surroundings. Treating a system as closed is a modelling choice that lets you apply conservation cleanly, then account for the small losses separately.

When a device transfers energy, only some of the output does the intended job. The rest is dissipated, or "wasted", almost always ending up in a thermal store of the surroundings.

Take an ordinary filament lamp. Its job is to produce light, but most of the electrical energy supplied is transferred to the thermal store instead.

For every 100 J of electrical energy supplied, only about 10 J leaves as light; the other 90 J is dissipated to the surroundings as thermal energy. The wasted energy is not destroyed, but it is spread out so thinly that it can no longer be used. Reducing this waste is what the rest of this lesson is about.

Two methods on the specification reduce the energy dissipated by a system: lubrication and thermal insulation.

• Lubrication — adding oil or grease between surfaces that rub together reduces friction. With less friction, less work is done against it, so less energy is dissipated to the thermal store as the surfaces slide. This is why engines and hinges are oiled.
• Thermal insulation — surrounding an object with a material that transfers energy slowly reduces the energy dissipated by conduction. Loft insulation, cavity-wall insulation and the double walls of a vacuum flask all work this way.

Lubrication tackles energy wasted through friction in moving parts; thermal insulation tackles energy wasted through conduction to the surroundings.

Both methods keep more of the input energy in the useful store, which raises the efficiency of the device (the subject of the next lesson).

How quickly a building loses energy through its walls depends on the material and the thickness of those walls.

A material with a higher thermal conductivity transfers energy by conduction at a higher rate. (The definition of thermal conductivity is not required.)

This gives two ways to slow the rate at which a building cools:

Factor	To slow cooling…	Why
Thermal conductivity of walls	Choose a lower value	Lower conductivity means a lower rate of conduction
Thickness of walls	Make them thicker	Energy must conduct through more material, slowing the rate

So a house built with thick walls of low-conductivity material cools more slowly and needs less energy to keep warm. Cavity walls filled with insulating foam combine a low-conductivity material with extra thickness to reduce the rate of energy transfer.

(Separate Physics only) Required practical 2 is assessed in GCSE Physics but not in Combined Science.

This practical investigates how effective different materials are as thermal insulators, and the factors that affect insulation.

Apparatus

• A small can or beaker to hold hot water
• Different insulating materials to wrap around it (for example bubble wrap, felt, newspaper, cotton wool)
• A thermometer, a lid, a stopwatch and a measuring cylinder

Method

1. Pour a fixed volume of hot water into the can and record the starting temperature.
2. Wrap the can in one layer of the first insulating material and fit the lid.
3. Record the water temperature every minute for a fixed time, such as 10 minutes.
4. Repeat with each material, using the same volume and starting temperature of water each time.
5. The smallest temperature drop identifies the best insulator.

To study the effect of thickness, repeat with increasing numbers of layers of the same material and compare the temperature drops.

Controlling the variables is what makes the comparison fair, and conservation of energy explains the results.

Variable	Role	Why
Insulating material (or thickness)	Independent variable	It is what you deliberately change
Temperature drop over the time	Dependent variable	It measures how much energy the water lost
Volume and starting temperature of water, time	Control variables	Kept the same so the comparison is fair

The lid reduces energy lost by evaporation and convection so that the test focuses on conduction through the wrapping. The energy leaving the water is not destroyed; it is dissipated to the thermal store of the surroundings, and a good insulator simply slows that rate.

Worked example — a 0.20 kg sample of water cools from 80°C to 72°C over the test. Using $\Delta E=mc\Delta \theta$ with $c=4200$ J/kg°C:

$\Delta E=0.20\times 4200\times (80-72)=0.20\times 4200\times 8=6720\text{J}$

So 6720 J were dissipated to the surroundings. A better insulator gives a smaller temperature drop and therefore a smaller energy loss over the same time.

1. Saying energy is "lost" or "used up"

Energy is dissipated, not destroyed. Wasted energy is transferred to the thermal store of the surroundings, where it spreads out and becomes hard to use.

2. Confusing lubrication and insulation

Lubrication reduces energy wasted through friction between moving surfaces. Thermal insulation reduces energy wasted through conduction to the surroundings. They address different transfers.

3. Trying to define thermal conductivity

The specification does not require the definition of thermal conductivity. You only need that a higher conductivity gives a higher rate of energy transfer by conduction.

4. Ignoring wall thickness

The rate of cooling depends on both the thermal conductivity and the thickness of the walls. Thicker walls slow the transfer even if the material is the same.

5. Not controlling variables in the practical

Use the same volume, starting temperature and timing for every material. Changing more than one factor at a time makes the comparison of insulators unfair.