Everything thatís a "thing" has some energy associated with it! Energy comes in all kinds of different forms and can be converted from one form to another. Iíll list some different kinds of energy and give some examples of some things, both living and non-living, which have this kind of energy.
(But first a warning- these ítypesí of energy are not all ídifferentí. Energy can be in several of these categories at once. See below.)
1) Kinetic energy. This is the energy of something thatís moving. A car moving down the road has quite a lot of kinetic energy. The actual amount of energy depends on the frame of reference, however. If you are in the car, in your frame of reference, the car is not moving, and therefore does not have kinetic energy, while viewed by an observer on the sidewalk, the car has quite a bit. (The internal parts of the car are moving with respect to each other, so the carís total kinetic energy never goes to zero in any frame). A cat jumping through the air has kinetic energy.
2) Gravitational potential energy. Something thatís way up high has much more gravitational potential energy than something thatís lower. Drop a ball from your hand and it will speed up, trading its gravitational potential energy for kinetic energy on its way down. A bird has to expend energy to fly to a great height because it must raise its gravitational potential energy (and also increase its kinetic energy too).
3) Chemical potential energy. This is the energy thatís stored in some molecules which can be transformed into other forms of energy when chemical reactions happen. Sugar contains lots of calories which can be put to use by an organism to do mechanical work or to heat it up. Gasoline has lots of chemical potential energy stored up inside it to help make cars go. Batteries store energy in the molecules that make them up. (Chemical potential energy really consists of electrical and magnetic and kinetic energy of the electrons etc. in the molecules and atoms.)
4) Thermal energy. Heat something up, and it will have more energy than if it is cold. All living things have some thermal energy, and many of them deliberately make thermal energy by causing chemical reactions to take place (although some cold-blooded organisms do not regulate their temperatures). Only at absolute zero (Kelvin) does an object have no thermal energy. (Thermal energy consists of kinetic and potential energies of the little bits of material- atoms, molecules, etc.)
4) Electrical energy. This is transmitted along power lines. As current flows through an object with a different voltage on either side, it either deposits energy (as in a toaster) or takes energy away (as in the case of a battery). Maybe electric eels store up electrical energy, but I suspect that itís mostly a chemical reaction that takes place whenever the eel needs its electricity.
6) Magnetic energy. Magnetic fields have energy in them. If you force two magnets together that repel each other, it takes energy to do that, which is stored in the magnetic field. If the magnets attract each other, you get energy out of the magnetic field when the magnets are brought together. I canít think of an organism which uses this in a big way, however (some creatures do have little bits of magnetized material in their cells, though, and may be sensitive to the Earthís field).
7) Mechanical potential energy. Compress a spring and it stores energy which can be released later. On a microscopic scale, this really takes the form of electrical and magnetic energies of the atoms making up the material.
8) Energy carried by sound. Sound waves consist of regions of air which move back and forth together, carrying both kinetic energy and mechanical potential energy (and usually end up as thermal energy when the sound is absorbed). Many living and non-living things make sounds.
9) Energy is carried by light. The sun makes quite a lot of light which comes to Earth (it has lots of thermal energy too). The sun gets its energy from the nuclear fusion of hydrogen to make helium atoms. Some living things make light too -- fireflies and many aquatic creatures glow in the dark.(Light is an electromagnetic wave, so this is just a form of electrical and magnetic energy.)
10) Nuclear energy -- the sun works by fusing very light atoms together to make heavier ones, releasing energy in the process. Also, very heavy atoms can decay or be split, releasing energy, in a process called fission. Uranium and Plutonium are good fissile materials, and both are used to generate electrical power and to make the nastiest weapons known to mankind.
11) The energy of just being matter. Einstein found out that E=mc^2, relating the amount of energy an object has (when itís at rest) to its mass. This energy isnít always available to convert into other kinds of energy because certain numbers (like the total number of protons+neutrons) have to add up to the same number before and after all reactions. If you add some antimatter, you can release this energy.
So everything thatís a "thing" has some energy!
Doubtless I am leaving something out,some other descriptions of different categories of energy. Can you think of other kinds of energy and some examples of things that have lots of that particular kind of energy? Are those kinds really the same in some way as some of the kinds we have described here?
Tom J. (with quibbles from Mike W.)
(republished on 07/25/06)