Copyright © 2007 Dorling Kindersley
Without energy, the world would be lifeless, dark, and still. When something has energy, it can do work and bring about change. Energy produces light and movement. Energy is required to produce heat, to generate electricity, and to overcome forces such as friction.
Speeding cars, whizzing rockets, and wind and waves have KINETIC (motion) energy. A stretched rubber band has potential (stored) energy because of the forces that try to make it return to its original size. Heat is the kinetic energy of particles in materials. Light is radiation energy created by the electromagnetic force.
When something happens, energy is changed from one form into another. When you climb stairs, chemical energy in your food is changed into kinetic energy by your muscles, and into potential energy as you raise your body against gravity. The amount of energy transferred is measured in JOULES.
Energy cannot be created or destroyed; it can only change from one form to another. There is always as much energy after an event as there was before, but some of the energy may be wasted as heat that escapes into the surroundings.
Moving and vibrating objects have kinetic energy. The greater the moving object’s mass, and the higher its speed, the more kinetic energy it has.
The modern international unit of energy is the joule. One joule is the energy used up when a force of one newton moves through a distance of one meter.
The original calorie was an old-fashioned unit of heat energy, equal to just over four joules. A different, larger unit, also called a calorie, is used today give the energy content of foods. This type of calorie is equal to 4,200 joules. An active teenager needs between 2,000 and 2,500 calories of energy a day. The body stores excess calories as fat.