# Forms of Energy

> Energy comes in many forms and changes from one to another, but the total never changes.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/forms-of-energy/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The toast that started with the Sun
Noah's breakfast toast popped up golden brown. His older cousin, a physics student, asked him: "Where did the energy to make that toast come from?"

"The power point," said Noah.

"And before that?"

The power station burned coal. The coal was made from ancient plants squashed underground for millions of years. Those plants grew by soaking up sunlight. And the Sun made that light by fusing atoms in its core.

So Noah's toast was cooked, in a long chain of steps, by sunlight from millions of years ago. Energy never appeared from nowhere. It just kept **changing form**.

**In short:** Energy is the ability to cause change, measured in joules (J). It comes in forms such as kinetic, gravitational, elastic, chemical, thermal, electrical, light, sound and nuclear, and it is never made or destroyed, only changed from one form to another.

## What is energy?

**Energy** is the ability to do work or cause change. You need energy to move, heat things up, make light or sound, and keep your body alive.

Energy is measured in **joules (J)**. Lifting an apple 1 metre takes about 1 J. Food labels often use **kilojoules (kJ)**: 1 kJ = 1000 J.

## Forms of energy

| Form | What it is | Example |
|---|---|---|
| **Kinetic** | Energy of a moving object | A rolling ball, wind |
| **Gravitational potential** | Stored because of height | A book on a high shelf |
| **Elastic potential** | Stored in a stretched or squashed object | A drawn bow, a trampoline |
| **Chemical potential** | Stored in the bonds of chemicals | Food, fuel, batteries |
| **Thermal (heat)** | Energy of particles moving and vibrating | A hot cup of tea |
| **Electrical** | Carried by moving charges | Current in a wire |
| **Light (radiant)** | Carried by light waves | Sunlight, a torch |
| **Sound** | Carried by vibrations through a material | Music, a drum |
| **Nuclear** | Stored in the nucleus of atoms | The Sun, nuclear reactors |

### Key idea: Two big groups
- **Kinetic** types are energy of **motion**: kinetic, thermal, sound, light, electrical.
- **Potential** types are **stored** energy: gravitational, elastic, chemical, nuclear.

## The law of conservation of energy

> Energy **cannot be created or destroyed**. It can only be **transformed** from one form to another, or **transferred** from one object to another.

The **total** amount of energy always stays the same.

## Energy transformations

We show changes with an arrow diagram.

- **Torch**: chemical (battery) → electrical → light + heat
- **Car engine**: chemical (petrol) → kinetic + heat + sound
- **Solar panel**: light → electrical
- **Kicking a ball**: chemical (in muscles) → kinetic
- **Bouncing ball** falling: gravitational potential → kinetic
- **Hydroelectric dam**: gravitational potential (water) → kinetic → electrical
- **Plants**: light → chemical (photosynthesis)

### Watch out: Energy is not "used up"
When you say a battery is "used up", the energy has not disappeared. It has changed into light, heat and sound, which spread out into the surroundings. Spread-out heat is hard to use again, but it still exists.

## Sankey diagrams

A **Sankey diagram** shows energy flowing through a device. The **width** of each arrow shows how much energy goes that way. The useful output goes straight on. Wasted energy bends off to the side.

*Diagram: Sankey diagram of a light bulb: 100 joules of electrical energy in, 20 joules light out straight ahead, 80 joules heat bending downward*
*A filament bulb wastes most of its energy as heat.*

**Mind map**

```text
Energy
  Kinetic types
    Kinetic
    Thermal
    Sound
    Light
    Electrical
  Potential types
    Gravitational
    Elastic
    Chemical
    Nuclear
  Conservation
    Not created
    Not destroyed
    Only transformed
  Showing it
    Arrow chains
    Sankey diagrams
```

### Year 11: Work and power
**Work** is done when a force moves an object. It is energy transferred by a force:

$$W = Fs\cos\theta$$

where $s$ is the displacement and $\theta$ is the angle between force and motion. If you carry a bag horizontally, gravity does no work on it because the force (down) is at 90° to the motion.

**Power** is how fast energy is transferred:

$$P = \frac{W}{t} = \frac{E}{t} \qquad \text{unit: watt (W)} = \text{J/s}$$

A 60 W bulb transfers 60 J every second. Also useful: $P = Fv$ for an object moving at constant speed.

### Year 12: Mass is energy
Einstein showed that mass itself is a form of energy:

$$E = mc^2$$

with $c = 3.0 \times 10^8$ m/s. Because $c^2$ is so large, a tiny mass holds a huge amount of energy. In the Sun, about 4 million tonnes of mass turn into energy every second. In nuclear fission and fusion, the products have slightly less mass than what went in. That "missing" mass is released as energy.

## Quick summary

- Energy is the ability to cause change. Unit: joule (J).
- Main forms: kinetic, gravitational, elastic, chemical, thermal, electrical, light, sound, nuclear.
- **Conservation of energy**: energy is never created or destroyed, only changed.
- Show changes with arrow chains: chemical → electrical → light + heat.
- Sankey diagrams use arrow width to show amounts of energy.
- "Wasted" energy usually ends up as heat spread into the surroundings.

## Quiz

Try these questions from the chapter quiz.

1. The unit of energy is the:
   - A. Watt
   - B. Joule
   - C. Newton
   - D. Volt
   - Answer: B. Joule. Energy is measured in joules (J).

2. A stretched rubber band stores:
   - A. Chemical energy
   - B. Kinetic energy
   - C. Elastic potential energy
   - D. Nuclear energy
   - Answer: C. Elastic potential energy. Stretched or squashed objects store elastic potential energy.

3. A battery stores energy as:
   - A. Kinetic energy
   - B. Chemical energy
   - C. Light energy
   - D. Sound energy
   - Answer: B. Chemical energy. The energy is stored in chemicals inside the battery.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/forms-of-energy/#quiz