Name the energy stores, describe energy transfers, and state the principle of conservation of energy

What this dot point is asking

SEAB wants you to name the main energy stores, to describe how energy is transferred from one store to another, to state the principle of conservation of energy, and to work with efficiency and wasted energy. The big idea is that energy is never used up; it is only moved from one store to another, though some always ends up in a less useful form.

The answer

Energy stores

Energy is the ability to do work, measured in joules ($\text{J}$). It is stored in different ways:

• Kinetic store: energy of a moving object.
• Gravitational potential store: energy of an object raised up high.
• Chemical store: energy in food, fuel and batteries.
• Elastic (strain) store: energy in a stretched or squashed spring.
• Thermal (internal) store: energy of a hot object.
• Electrical energy and light and sound are forms in which energy is carried or transferred.

Energy transfers

When something happens, energy moves between stores. Some everyday examples:

• A falling ball: gravitational potential store transfers to kinetic store.
• A torch: chemical store in the battery transfers to electrical energy, then to light and heat.
• A car braking: kinetic store transfers to thermal store (the brakes get hot through friction).
• A stretched catapult: elastic store transfers to the kinetic store of the launched object.

The principle of conservation of energy

The principle of conservation of energy says that energy cannot be created or destroyed; it can only be transferred from one store to another, or changed from one form to another. The total amount of energy always stays the same.

So when a ball slows as it rises, the kinetic energy it loses exactly equals the gravitational potential energy it gains (ignoring air resistance).

Useful and wasted energy, and efficiency

In any transfer, some energy goes to a useful place and some is wasted, usually as heat spread out to the surroundings (often through friction). The energy is still conserved, but the wasted part is no longer useful.

Efficiency measures how much of the input ends up useful:

No real device is 100% efficient, because some energy is always wasted as heat.

Examples in context

Example 1. A roller coaster. At the top of the first hill a car has a large gravitational potential store. As it drops, this transfers to a kinetic store, so it speeds up. Friction and air resistance waste some energy as heat, which is why each hill must be lower than the last.

Example 2. A power station. A coal power station transfers the chemical store in fuel to thermal energy (burning), then to kinetic energy (spinning turbines), then to electrical energy. At each step some energy is wasted as heat, so the overall efficiency is well below 100%, often around 40%.

Try this

• Cue. Name the main energy transfer when a stretched rubber band fires a paper pellet. [2 marks] Elastic (strain) store transfers to the kinetic store of the pellet.

• Cue. A motor takes in $400\ \text{J}$ and does $300\ \text{J}$ of useful work. Find the efficiency. [2 marks] Efficiency $= \dfrac{300}{400} \times 100\% = 75\%$.

• Cue. State the principle of conservation of energy. [2 marks] Energy cannot be created or destroyed, only transferred from one store to another, so the total energy stays the same.