What is energy?
This is both a simple and complex question. Energy is in everything – it is often described as ‘the ability to do work’.
Almost all food energy comes originally from sunlight. The chemical elements that make up the molecules of living things pass through food webs and are combined and re-combined. At each link, some energy is stored, but much is lost along the way in the form of heat into the environment.
James Joule
James Joule (1818–1889), an English physicist who developed the first law of thermodynamics.
Let’s look at a few examples of energy use:
When we eat food, our body uses (chemical) energy embodied in the food to move around.
When we switch on the TV, electricity (kinetic energy) is used to create the picture on the screen and the sound.
Most of the electricity produced in the world comes from the chemical energy released in the burning of coal, oil or gas.
Every time something gets warm, cools down, moves, grows, makes a sound or changes in any way, it uses energy. What about a piece of paper sitting on a desk not moving? The paper still has energy – it is just not using it. Science classifies energy into two categories – kinetic (moving) and potential (stored) energy.
Kinetic or moving energy
Potential or stored energy
Electrical energy – the movement of electrical charges. Everything is made of atoms, which in turn are made of a positive nucleus surrounded by negative electrons. Applying a force can make some of the electrons move. This includes both electricity we use and lightning.
Gravitational energy – objects within the gravitational field of the Earth will fall towards the Earth. The amount of stored energy depends on its mass and height above the Earth (a pen sitting on your desk contains less gravitational potential energy then a person on top of a building).
Radiant energy – electromagnetic energy that moves in waves. This includes visible light, X-rays and radio waves.
Elastic energy – objects that are stretched or squashed have had a force applied to them and store energy from that force. Examples include springs and rubber bands.
Sound energy – a wave that moves out from a source as a result of molecules on an object vibrating.
Chemical energy – stored in the bonds that hold atoms together, and when the bonds are broken, energy is released (and will become kinetic energy
Motion/kinetic energy – the energy an object possesses because of its motion. It is reliant on the mass of an object and the velocity at which it moves. Imagine being hit by a ping pong ball being rolled along the ground (low mass and velocity) compared with a cricket ball that has been hit with a bat (high mass and velocity). The cricket ball will hurt more when it hits you, that is, when it comes to rest.
Nuclear energy – the energy stored in the nucleus (centre) of an atom. Energy is released when either the nucleus is split apart (called nuclear fission) or when nuclei are combined (nuclear fusion). Power plants that use nuclear power do so by the fission of uranium atoms.
Energy can change between potential and kinetic. Water at the top of a waterfall has stored potential energy but as the water begins to fall, it changes from potential to kinetic energy. It is this process that we utilise when we create energy from hydropower – we harness the kinetic energy of the water for our own use.
Huka Falls on the Waikato River
Waterfalls are often formed along faultlines especially if one side along the fault has been uplifted. The Huka Falls are particularly spectacular, as the water has to push its way through a very narrow gorge.
How is energy measured?
In the International System of Units (the SI system), the unit of energy is the joule. The specific heat capacity (or just specific heat) of a material is defined as the amount of heat required to raise the temperature of one gram (g) of the material one degree Celsius (ºC). It takes 4.18 joules (J) to raise the temperature of 1 g of water 1ºC (at a temperature of 25ºC). One kilojoule (kJ) equals 1,000 joules (J) and is the amount of the heat required to raise the temperature of 239g of water by 1ºC. As an example, a piece of buttered toast contains about 315 kilojoules, which gives you enough energy to ride your bike for 10 minutes or run for 6 minutes.
Nature of science
To communicate in science, we need to use the correct terms, vocabulary and conventions to explore and describe things.
