The law of conservation of energy: The total amount of energy in the universe is constant, although energy can be transformed from one form to another. The basic unit of energy is the joule. A watt is one joule per second. Thus a 100 watt electric bulb uses 100 joules per second of electric energy. That energy must come from somewhere and must be paid for. One pays for electric energy in kilowatt-hours which is 1,000 watts being delivered per hour. Thus a kilowatt-hour is 3,600,000 joules, since "kilo" means a thousand, and there are 3,600 seconds in an hour. At 10 cents per kilowatt hour, a joule is really cheap. 10 joules would lift a one kilogram weight one meter.
Schemes for powering cars or houses that require violating the law of conservation of energy can't work. Here's an example.
Several times I have received in email the following idea for powering cars with hydrogen from water. Run the car on hydrogen obtained by splitting water by electrolysis using the car's generator to get the electricity.
Here's why it won't work. The amount of energy you get from burning hydrogen and running a generator produces at most the amount of energy required to replace the energy used to split the water. There would be none left over to power the car. Actually, you would get considerably less energy than is needed to get more hydrogen to replace that burned. Most likely it would be about 20 percent.
Splitting water to get hydrogen is not an original source of energy. Rather it is a way of tranforming energy into a form more usable for a certain purpose. For example, nuclear energy can be used to split water (H2O) into hydrogen and oxygen. The oxygen is released into the air, and the hydrogen is liquefied and used to run cars in the same way gasoline is used to run cars. [The process is quite inefficient compared to running the car directly with a small nuclear reactor. Unfortunately, a nuclear reactor in a car would kill the occupants. Ships can use nuclear reactors, because they can afford the tons of shielding required to keep the neutrons away from the crew.] Solar electricity can also be used to split water, but solar energy is expensive.
What are the sources of energy that have been used?
These terms are often confused, because in ordinary language they are sometimes synonyms. However, the distinction is important for science and engineering and is simple to state. Power is the rate at which energy is generated or used. The basic unit of energy is the joule, and the basic unit of power is the watt. A watt is one joule per second. Thus a hundred watt light bulb is using 100 joules of energy per second, turning the electric energy into light and heat. A 1,000 megawatt power plant is generating a billion joules per second. It gets that energy from burning fuel or from splitting atoms of uranium and plutonium. To get 1,000 megawatts it actually uses 3,000 megawatts from the fuel. The leftover 2,000 megawatts is inevitably produced as leftover heat as shown by the second law of thermodynamics.
A variety of units are used for power and energy. Kilowatts and megawatts are a thousand watts and a million watts respectively. A horsepower is 746 watts by James Watts's optimistic definition, but few horses can put out that power for long. Energy is measured in kilowatt-hours. A kilowatt-hour is therefore 3,600,000 joules. Another unit of energy is the British thermal unit or BTU, sometimes used in engineering. It is the amount of energy needed to raise one pound of water one degree Farenheit.
This is a very sketchy exposition. An elementary physics book will do much better.
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