In physics, energy is a system's capacity to perform work, which is force acting on an object. Energy has many forms, including kinetic or mechanical energy, potential energy, light energy and heat energy. The basic rules for understanding energy are the laws of thermodynamics.
The first law of thermodynamics is the principle of conservation of energy. The law states that the total amount of energy in the universe is constant. Energy can transform from one form to another, but transformation does not create or destroy energy.
The second law of thermodynamics is the principle of entropy. Each time energy transfers, some energy transforms to an unusable form as the energy moves away from its source. This increases the total disorder of a closed system. Entropy measures the amount of unusable energy in the system.
The third law of thermodynamics states that at absolute zero, all molecular motion stops, causing all processes to stop. Absolute zero is 0 degrees Kelvin or minus 273 degrees Celsius.
The laws of thermodynamics explain why a perpetual motion machine is impossible. Because a machine loses energy to the outside world as energy moves away from its source, the machine eventually has too little energy in usable form to continue operating unless more energy enters the system from outside.