Coaching Centre
Forms of energy
What energy is, how it is measured in joules, the difference between kinetic and potential energy, the main forms of energy (gravitational, elastic and chemical potential, kinetic, thermal, light, sound, electrical and nuclear), and the law of conservation of energy.
A single Tim Tam contains about 400 kilojoules of energy. That's enough, in theory, to lift a 50 kg student about 800 metres straight up, roughly the height of the tallest building in the world. In practice your body wastes most of it as heat, but the point stands: there is a surprising amount of energy packed into ordinary things.
What is energy?
Energy is the ability to make things happen: to move something, heat it up, light it up or change it. Scientists measure energy in joules (J), named after the English brewer and scientist James Joule. One joule is a small amount: about the energy needed to lift an apple one metre. Larger amounts use kilojoules (1 kJ = 1000 J) and megajoules (1 MJ = 1 000 000 J). Food labels in Australia show energy in kilojoules; an average adult needs about 8700 kJ a day.
All the many forms of energy fall into two big groups:
- Kinetic energy is the energy of things that are moving.
- Potential energy is stored energy, waiting to be released.
Kinetic energy
Anything moving has kinetic energy: a running dog, a spinning wind turbine, air molecules flying about. The heavier an object and the faster it moves, the more kinetic energy it has. Speed matters most: doubling your speed gives you four times the kinetic energy. That's why a crash at 60 km/h is so much worse than one at 30 km/h, and why car safety features such as crumple zones and airbags are designed to absorb kinetic energy gradually.
Several forms of energy are really kinetic energy of particles:
- Thermal (heat) energy is the kinetic energy of the particles in a substance. The hotter something is, the faster its particles move or vibrate.
- Sound energy is carried by vibrations passing from particle to particle through a solid, liquid or gas. A drummer's kinetic energy makes the drum skin vibrate; harder hits make bigger vibrations and louder sounds.
- Electrical energy is carried by charged particles (usually electrons) moving through a conductor.
Light energy is a little different. Light (along with infrared, ultraviolet, microwaves and radio waves) is electromagnetic radiation, which can travel through empty space. That's how energy from the sun reaches Earth.
Potential energy
| Type | Stored by | Examples |
|---|---|---|
| Gravitational potential energy (GPE) | An object's height above the ground. More mass and more height mean more GPE. | A diver on a 10 m platform, water behind a dam, a rollercoaster at the top of the first hill. |
| Elastic potential energy | Stretching or squashing something springy. | A stretched rubber band, a drawn bow, trampoline springs, a wind-up toy, a pole vaulter's pole. |
| Chemical potential energy | The bonds holding atoms together in substances. Chemical reactions release it. | Food, petrol, natural gas, wood, batteries, explosives. Your cells release it from glucose by respiration. |
| Nuclear energy | The nucleus at the centre of each atom. | Uranium in a nuclear reactor; hydrogen fusing in the sun. |
Gravitational potential energy can be calculated: GPE = mass × 9.8 × height (in joules, with mass in kg and height in metres). A 50 kg student at the top of a 3 m ramp has 50 × 9.8 × 3 = 1470 J of GPE.
Biomass energy is chemical energy stored in living or recently living things. Plants capture it from sunlight by photosynthesis. Burning wood releases it as heat. Sugar from crops such as sugar cane can be fermented into ethanol and burned as a biofuel: Australian E10 petrol is 10% ethanol.
Energy is never used up
The law of conservation of energy says energy can't be created or destroyed. It can only be transferred from one object to another or transformed from one form into another. The total amount stays the same.
So where does energy "go" when a ball stops bouncing or a car stops? It hasn't vanished. It has been transformed, mostly into heat, which spreads out into the surroundings so thinly that it's hard to use again. When people talk about "using up" energy, they really mean turning it into forms that are no longer useful to us.
- With no friction, drop in from 3 m. Where does the skater have the most GPE? The most kinetic energy? Does the total bar ever change?
- Find the speed at the bottom. Now try 80 kg. Does a heavier skater go faster?
- Add some friction. Where does the energy go? What happens to the total?
- Double the start height from 2 m to 4 m. How much does the speed at the bottom go up?
A 40 kg skater drops in from 2.5 m on a frictionless ramp. How much kinetic energy does the skater have at the bottom?
GPE = mass × 9.8 × height = 40 × 9.8 × 2.5 = 980 J. At the top the skater isn't moving, so kinetic energy = 0 J.
At the bottom, height = 0, so GPE = 0 J. Energy is conserved and there's no friction, so all 980 J must now be kinetic energy.
At a height of 1.25 m, GPE = 40 × 9.8 × 1.25 = 490 J, so kinetic energy = 980 − 490 = 490 J. Total is still 980 J.
Test yourself
Which has gravitational potential energy?
Height above the ground gives GPE.
The unit of energy is the
Energy is measured in joules (J).
What type of energy is stored in food?
It is released by respiration in your cells.
A ball is thrown straight up. At its highest point it has
Its kinetic energy has been transformed into GPE.
The law of conservation of energy says energy
"Conserving energy" at home is a different idea: wasting less.
A 2 kg rock sits on a 5 m cliff. Its GPE is
2 × 9.8 × 5 = 98 J.
Is the main energy kinetic or potential?
A bungee jumper leaps off a bridge. Put the main energy forms in order.
Energy is measured in . Moving objects have energy. Stored energy is called energy. A stretched spring has potential energy, and petrol has potential energy. Energy cannot be created or .
A ball is dropped from 2 m and bounces back to only 1.5 m. A student says, "Some energy was destroyed in the bounce." Explain why the student is wrong, and describe what actually happened to the energy.
The student is wrong because of the law of conservation of energy: energy cannot be created or destroyed, only transformed or transferred.
At 2 m the ball has gravitational potential energy. As it falls this is transformed into kinetic energy. When it hits the floor the ball squashes, storing elastic potential energy, which pushes it back up, becoming kinetic energy and then GPE again. But not all of the energy goes back into the bounce: some is transformed into sound (the bounce noise) and heat, warming the ball and floor slightly. That energy spreads into the surroundings, so the ball has less GPE at the top and only reaches 1.5 m. The total energy is still the same; it is just in less useful forms.