"Potential energy" comes in many forms: Chemical energy - stored in the bonds of atoms and molecules. It is the energy that holds these particles together. Biomass, petroleum, natural gas, and propane are examples of stored chemical energy.
Stored Mechanical Energy is energy stored in objects by the application of a force. Compressed springs and stretched rubber bands are examples of stored mechanical energy.
Nuclear Energy is energy stored in the nucleus of an atom??the energy that holds the nucleus together. The energy can be released when the nuclei are combined or split apart. Nuclear power plants split the nuclei of uranium atoms in a process called fission. The sun combines the nuclei of hydrogen atoms in a process called fusion. Scientists are working on creating fusion energy on earth, so that someday there might be fusion power plants.
Gravitational Energy is the energy of position or place. A rock resting at the top of a hill contains gravitational potential energy. Hydropower, such as water in a reservoir behind a dam, is an example of gravitational potential energy.
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http://www.eia.doe.gov/kids/energyfacts/science/formsofenergy.html )
SO let's look at gasoline. Base gasoline has a an energy content of approximately 116,090 btu/gallon.
That gasoline, say a gallon of it, represents (chemical) potential energy that can be released.
You hit the starter in your car, the engine fires up, and is producing MECHANICAL energy (kinetic) energy by the controled combustion of that gasoline to 'push' the pistons down in the cylinders.
The product of that combustion is that the engine gets hot. VERY HOT. So hot that you need to circulate coolant around he cylinders and head to absorb that heat and dissapate it via the radiator.
Simple, huh?
As an addendum, ethanol has less potential chemical energy than gasoline:
The energy of ethanol relative to gasoline
A. 76,000 = BTU of energy in a gallon of ethanol
B. 116,090 = BTU of energy in a gallon of gasoline
C. .655 = 2/3 = GGE of energy in a gallon of ethanol. A / B.
D. 1.53 = Gallons of ethanol with the energy of 1 gallon of gasoline. D = B / A.
So you need a gallon and a half of ethanol to get the same energy that a gallon of gasoline yields.
http://zfacts.com/p/436.html