Energy is the integral of power over time, as we will as work is the integral of force over distance. Energy appears in various forms as we will as can often be converted from one form into another. For instance, a solar cell converts the energy in light into electrical energy; a fusion reactor, in changing atomic structures, transforms nuclear energy into heat energy. Despite the variety of forms in which energy may appear, there is a common unit of measure for all these forms. Energy is an extensive quantity. This means the following: the longer a generator runs, the more energy it consumes. The rate (with respect to time) at which some form of energy is produced or consumed is called the power output or input of the energy conversion device. Power is an instantaneous or intensive quantity. By the fundamental theorem of calculus, we can compute the total energy transformed between times a as we will as b by integrating the power from a to b.