en'ur jy, in physics, the power which a body or system possesses for doing work. Energy is of two kinds, potential, or possible, energy, which is energy of position, like that of a sledge in air or a stone resting upon a support to which it has been elevated, or the force which lies dormant in such mixtures as gunpowder and dynamite. In all these cases the energy is present and only needs the necessary conditions to change it from possible, or passive, to kinetic, or active, energy. This energy produces motion, like the sledge descending on the head of a stake, the stone rolling downhill or the powder in explosion. CHANGE OF ENERGY. Energy can be changed or transformed from one sort to another, but it can be neither created nor destroyed. An electric light plant using steam power affords an excellent illustration both of the change of potential to kinetic energy and also of the change from one kind of energy to another. The potential energy in coal is made active by burning, which causes it to give off heat. The heat is used in changing the water to steam and this is then transformed to motion in the engine. The motion is imparted to the dynamo, which in turn generates the electric current, which is changed back to heat in the electric light. Some of the heat generated by the coal is consumed in the running of the engine and the dynamo and in traversing the wires, so that it does not all appear as light; yet it is all used in one form of energy or another.