Energy Is Conserved

Energy can be transferred from one store to another. It can also be stored in different ways, such as in kinetic, thermal, gravitational, elastic, magnetic, electrostatic, nuclear, or chemical stores. What energy cannot do is appear from nowhere or disappear into nothing.

A system is the object or group of objects you choose to study. For example, you might study a falling ball, a moving car and the road, or a torch and its surroundings.

In a closed system, the total energy stays the same. The distribution of energy can change between stores, but the total amount does not change.

For example, imagine a ball falling in a sealed container. As the ball falls, energy is transferred from the ball's gravitational store to its kinetic store. If the ball then hits the floor and stops, energy is transferred from the kinetic store into thermal stores of the ball, floor and air, and some energy is transferred by sound. If the ball, floor and air are all included in the chosen closed system, the total energy of that system has not changed.

Keep this as the central sentence for the rest of the lesson: the store can change, but the total energy of the closed system cannot.

Useful Transfer And Storage

An energy transfer is useful when it helps the system do the job you want it to do. A useful transfer is not a special type of energy: it depends on the purpose of the device or process.

In a battery-powered toy car, energy is stored chemically in the battery. When the car moves, some energy is transferred electrically and then mechanically to increase the kinetic store of the car. That increase in the car's kinetic store is useful if the purpose is to make the car move.

In an electric kettle, energy is transferred electrically from the mains supply to the heating element and then by heating to the thermal store of the water. The increase in the thermal store of the water is useful if the purpose is to heat the water.

Energy can also be usefully stored for later. Lifting a book onto a high shelf increases its gravitational store. Stretching a spring increases its elastic store. The energy is not doing the intended job at that instant, but it is stored in a way that can be transferred later.

When describing useful transfer and storage, use precise store language:

Closed-System Accounting

Closed-system questions often give a before-and-after story. Your job is to account for the energy without implying that any has vanished.

Suppose a moving trolley and its surroundings are treated as a closed system. Before the trolley hits a soft barrier, 120 J is in the kinetic store of the trolley. After the collision, the trolley is stationary. The energy might be redistributed like this:

The trolley's kinetic store has decreased by 120 J. The other stores in the closed system have increased by 120 J altogether. There is no net change to the total energy of the closed system.

A good written answer does not need a full table unless data are given. It should make the same accounting idea clear in words: identify the starting store, identify the final stores, and say the total energy is unchanged.

Dissipation Into Less Useful Stores

In real system changes, some energy is dissipated. Dissipated energy has not been destroyed. It has been transferred into less useful stores, often spread out in the thermal stores of the surroundings.

The phrase wasted energy can be useful in exams, but it needs careful handling. Wasted energy is energy that is not useful for the intended purpose. It is still part of the total energy of the system if the system boundary includes where that energy has gone.

For example, when a car brakes, the useful aim is to reduce the car's speed. The kinetic store of the car decreases. Energy is transferred mechanically because friction acts in the brakes and tyres. Much of the energy is dissipated into thermal stores of the brakes, tyres, road and surrounding air, with a small amount transferred by sound. The energy becomes less useful because it is spread out and cannot easily be transferred back into the car's kinetic store.

The same idea applies to a bouncing ball. After each bounce, the ball rises to a lower height. This does not mean energy has disappeared. Some energy has been dissipated into thermal stores of the ball, floor and air, and some has been transferred by sound, so less energy remains in the ball's gravitational and kinetic stores.

Use that explanation whenever an example seems to show energy "running out". The useful store may decrease, but the closed-system total is still accounted for.

This is why dissipation is an accounting idea, not a disappearance idea.

Exam Answer Language

For this lesson, the strongest answers use a simple sequence:

1. Name the system.
2. State the starting energy store.
3. Describe the useful transfer or useful increase in a store.
4. Describe any dissipated energy as being stored in less useful ways.
5. State that the total energy in the closed system is unchanged.

Avoid saying "energy is lost" unless you immediately explain where it has gone. Better phrases include:

Here is a model description for a drill making a hole in wood:

The system is the drill, wood and nearby surroundings. Energy is transferred electrically to the drill motor, then mechanically to the rotating drill bit. Some energy is useful because it changes the wood and moves the drill bit. Some energy is dissipated into thermal stores of the drill bit, wood and air, and some is transferred by sound. If the chosen system is closed, the total energy in the system does not change.

This answer stays focused on conservation, useful transfer, storage and dissipation.