In science, energy comes in many fundamental forms and is closely associated with both work and power. So to begin, let us clarify the meanings of work, energy, and power ...
We often think of "work" as something we do to earn money. But when discussing energy, "work" has a more specific meaning.
Work - work is any push or pull that causes an object to change its speed or location
Now that we understand what "work" is, we can better appreciate its relation to energy ...
Energy - energy is something that is exchanged when work is performed. Energy comes in many forms:
Kinetic Energy: energy contained in the motion of an object (spinning wheel or moving car)
Potential Energy: energy contained in the location of an object (brick on a window ledge)
Chemical Energy: energy contained in a possible chemical reaction (burning gasoline)
Conservation of Energy and Heat
In each instance illustrated we find that energy is only transferred when work is performed - the energy itself never disappears! We don't "use up" the energy. We merely convert it to another type of energy. In fact, the universe we live in contains the same amount of total energy now as it did when it was first created. We often say that energy is conserved: it is neither created nor destroyed.
One might wonder then, why we pay money for energy since it is never "used up"? The problem is that in most any transfer of energy, some of the energy is converted to heat. Heat is just another form of energy, but it's a very disorganized form that is not easily redirected to doing useful work.
The production of heat is an inevitable result of friction. For example, when we turn a wheel to make it spin, we deposit some energy into its spinning motion and some into heat. The heat is generated by friction between the wheel and the axle and spreads into the atoms that make up both objects. The heat is eventually spread into the atmosphere and out into the universe.
Question: We sometimes hear people say that the power company"produces" energy. Is this statement really true? Why or why not?
Putting it all together
Now that we understand the relation between work, energy and heat, we can now examine the multiple energy transfer steps of a conventional coal-fired power plant. As illustrated below, the chemical energy in coal or oil is partly converted to heat. This heat is partly transferred to water to produce steam. The kinetic energy of the steam is partly transferred to a turbine (causing it to spin). This causes the steam to lose energy and return it to liquid water which is returned to the boiler. The kinetic energy of the turbine drives an electrical generator and is partly converted into electricity which travels into the community along transmission lines. Near our home, a transformer reduces some of the high voltage current to a lower voltage (and produces some heat in the process) which eventually enters our homes to do work.
In casual conversation, "energy" and "power" are often used interchangeably. However, in fact, power is not the same as energy!
Power - Power is the rate at which energy is being transferred.
Consider a forklift lifting a crate...
In one case, the forklift operator lifts the crate very slowly, taking 5 hours to lift it 20 feet.
In the second case, he lifts the crate rapidly to the same height, taking only 5 seconds.
How is the energy transferred different in each case?
How is the power different in each case?
Power is measured in kilowatts (kW)
Energy is measured in kilowatthours (kWh)
For example, a 60 W light bulb converts energy into light and heat at a rate of 60 watts (0.060 kW). Left on for 1 hour, 0.060 kWh of electrical energy is converted into heat and light.