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What are the main forms of energy, and what does it mean to say energy is conserved?

List the main forms of energy, describe energy transfers, and state the principle of conservation of energy

A focused answer to the O-Level Physics outcome on energy. The main forms of energy, how energy is transferred and transformed, the principle of conservation of energy, and why energy is often wasted as heat.

Generated by Claude Opus 4.87 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to name the main forms of energy, to describe how energy is transferred and transformed in everyday situations, and to state and use the principle of conservation of energy. The big idea is that energy is never created or destroyed, only changed from one form to another, even though some of it is usually wasted as heat.

The answer

The main forms of energy

Energy comes in several forms, all measured in joules ( $\text{J}$ ):

Kinetic energy: energy of a moving object.
Gravitational potential energy: energy due to an object's height.
Elastic potential energy: energy stored in a stretched or squashed spring.
Chemical energy: stored in fuel, food, and batteries.
Thermal (internal) energy: the energy of particles due to their motion.
Electrical energy, light energy, and sound energy: carried by current, light, and sound.
Nuclear energy: stored in the nucleus of atoms.

Energy transfers

Energy is transferred when it moves from one place or form to another. A torch transfers chemical energy in the battery to electrical energy, then to light and heat. A falling ball transfers gravitational potential energy to kinetic energy.

The principle of conservation of energy

Energy cannot be created or destroyed. It can only be transferred from one form to another, or from one object to another, while the total energy stays the same:

\text{total energy before} = \text{total energy after}

Wasted energy

In any real transfer, some energy ends up in forms we did not want, usually heat (and sometimes sound), spread out into the surroundings. This energy is not destroyed; it is just wasted, no longer useful for the job in hand. This is why no machine is perfectly efficient.

Worked example

A child slides down a slide. At the top she has $400\ \text{J}$ of gravitational potential energy. At the bottom she has $300\ \text{J}$ of kinetic energy. (a) State where the missing energy has gone. (b) State the total energy at the bottom.

Step 1: Apply conservation of energy

Energy cannot be destroyed, so the $400\ \text{J}$ at the top must all still exist at the bottom in some form.

Step 2: Account for the difference

She has only $300\ \text{J}$ of kinetic energy, so $400 - 300 = 100\ \text{J}$ has been transferred to heat by friction between her and the slide (and a little to sound).

Step 3: State the total

The total energy at the bottom is still $400\ \text{J}$ : $300\ \text{J}$ kinetic plus $100\ \text{J}$ of wasted heat. No energy is lost; it has only changed form.

Examples in context

Example 1. A roller coaster. At the top of the first hill the car has maximum gravitational potential energy. As it races down, this transfers to kinetic energy, then back to potential on the next rise. A small amount is lost to friction and air resistance as heat, which is why each hill is a little lower than the last.

Example 2. A mobile phone. A phone stores chemical energy in its battery, transfers it to electrical energy to run the circuits, and then to light (screen), sound (speaker), and a great deal of heat. The phone warms up because some electrical energy is always wasted as heat, exactly as conservation of energy predicts.

Try this

Q1. State the principle of conservation of energy. [2 marks]

Cue. Energy cannot be created or destroyed, only transferred from one form to another; the total energy stays the same.

Q2. Name the energy transfers in a battery-powered torch. [2 marks]

Cue. Chemical energy in the battery to electrical energy, then to light energy and heat.

Q3. A pendulum swinging slowly comes to rest. Explain where its energy has gone. [2 marks]

Cue. Its kinetic and potential energy are gradually transferred to heat (and a little sound) by air resistance and friction at the pivot.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA ball is thrown straight up into the air, rises, and falls back down. Describe the energy changes (a) as it rises and (b) as it falls, ignoring air resistance.

Show worked answer →

(a) As it rises it slows down, so kinetic energy decreases. This energy is transferred to gravitational potential energy, which increases as the ball gets higher.

(b) As it falls it speeds up, so gravitational potential energy is transferred back to kinetic energy.

Ignoring air resistance, the total energy stays constant; energy is only changing form between kinetic and gravitational potential.

Markers reward kinetic to potential on the way up, potential to kinetic on the way down, and the statement that the total energy is conserved.

Original4 marks(a) State the principle of conservation of energy. (b) A light bulb is supplied with electrical energy. Most of the energy is given out as heat rather than light. State whether energy has been destroyed, and explain your answer.