Energy is the capacity of a system to do work. That system may be a jet, carrying hundreds of passengers across the ocean. A baby’s body, growing bone cells. A kite, rising on the wind. Or a wave of light crossing a space.
In moving or growing, each of these systems is doing work, and using energy. Every living organism does work, and needs energy from food or photosynthesis. Humans also create machines that do work for them, and that derive energy from fuels.
Some of the many forms that energy takes are:
- Mechanical energy, which includes
- Potential energy, stored in a system.
- Kinetic energy, from the movement of matter.
- Radiant or solar energy, which comes from the light and warmth of the sun.
- Thermal energy, associated with the heat of an object.
- Chemical energy, stored in the chemical bonds of molecules.
- Electrical energy, associated with the movement of electrons.
- Electromagnetic energy, associated with light waves (including radio waves, microwaves, x-rays, infrared waves).
- Mass (or nuclear) energy, found in the nuclear structure of atoms.
One form of energy can be converted to another form. This transfer is based on the law of conservation of energy—one of the laws of thermodynamics.
Humans converted energy from one form to another when they lit the first fire. By burning wood, they released the chemical energy stored in the bonds of the wood molecules, generating thermal energy, or heat. Other examples? A battery generates electrons from chemical reactions, which are used to make electrical energy. A toaster takes electrical energy and converts it to heat. Your leg converts the chemical energy stored in your muscles into kinetic energy when you pedal a bicycle.
Sound is a form of kinetic energy. Molecules of air are vibrated, causing them to move in wave patterns. When these waves hit the eardrum, they make it vibrate too. This vibration energy is turned into electrical energy impulses, which your brain interprets as sound.
Many times, multiple conversions are involved. Consider nuclear power generation. Atoms in the nuclear fuel are split, releasing their nuclear (mass) energy and creating thermal energy. This heat energy is, in turn, captured in the form of steam and used to drive a turbine generator, creating kinetic energy. And, finally, this kinetic energy spins a magnetic field around a conductor, causing a current to flow—creating electrical energy.
To measure energy, we use the "heating value" of the fuel, which indicates how much of a certain fuel is converted to how much heat. Common units of energy are the calorie and Btu.