Energy: The Quick Tour
Fuels are at the heart of the matter.
We cannot create new energy that is not already present in the universe. Instead, we take materials in which energy is stored, change their state, and harness the energy that escapes. In particular, energy can be harnessed from fuels.
Energy must be captured, concentrated, and converted to do useful work. Sometimes, converting energy is-literally-as simple as falling off a log. When you stand on top of the log, you embody stored mechanical energy called potential energy. When you fall off the log, this potential energy is transformed into kinetic energy, which is another form of mechanical energy. This kinetic energy takes the form of your motion through the air, plus the vibrations that rattle your joints when you land.
Many natural processes convert energy. Photosynthesis is the process used by plants to capture and transform the radiant energy of the sun into the chemical energy they need to grow. Through digestion, animals capture and "burn" the chemical energy in food, water, and air, to fuel their own cell growth (chemical energy) and movement (kinetic energy). Combustion, a thermochemical reaction, is used to convert the stored chemical energy in wood to heat energy, and the stored chemical energy in gasoline to the kinetic energy of a moving car.
Nuclear reactions-including fission and fusion-release the mass energy in atoms. Albert Einstein showed that mass and energy are both forms of the same thing. Energy can become mass, and mass can become energy. The energy in a mass of m kilograms, if it were all converted to energy, can be calculated with the formula E = mc2, where c is the speed of light. The mass energy in even a tiny amount of matter is enormous. Mass energy can be released in three ways: matter-antimatter reactions, nuclear fission, and nuclear fusion.
The biggest "fuel" of all is the sun. Virtually all energy sources on earth came originally from the sun's radiant energy. The sun's hydrogen gases have been going through massive nuclear fusion reactions for billions of years, releasing all the heat and light energy that our solar system depends on. Radiation from the sun makes the wind blow and heats the surface of the earth. It makes water evaporate and condense, creating water cycles. Hydroelectric and wind power convert the movement of wind or water to electricity, when the water or wind spins a turbine that turns a magnet that generates an electrical current.
All these reactions change one form of fuel into different substances, and also release energy in the form of light, heat, or motion. But one form of energy never converts totally into another single form, which means that some energy is always "lost" in the conversion process. Often, energy loss takes the form of heat escaping into the air.
Consider riding a bicycle: chemical energy stored in your leg muscles is converted to motion when you pedal. Muscle power is fueled by chemical nutrition in food. But of the calories eaten that day, only about 18 percent is used to move your legs. More calories are converted into the body heat created by exertion, and others go into producing sweat to cool you off. The heat (thermal energy) you generate escapes into the air.
Of course, other energy conversions happened before you fell off the log. You ate food, which was stored as chemical energy in your muscles. And you used some of that chemical energy to get on top of the log. Backing up further, the food you ate required radiant (solar) energy from the sun, which was converted into chemical energy by plants through photosynthesis. You, in turn, took in this energy by eating the plants directly, or eating meat from animals that had eaten the plants.
Through photosynthesis, plants use sunlight to make carbon molecules such as sugar from carbon dioxide and water. When plants need energy, the sugar is converted back to carbon dioxide and water, releasing the energy previously stored. We eat plants to get this stored energy. Plant material from millions of years ago, under continuous heat and pressure, makes up the fossil fuels we use so widely today.
One of the ways mass (nuclear) energy can be released is through matter-antimatter reactions. Ordinary matter is made of atoms, which contain positive protons and negative electrons. Antimatter would be made from negative protons and positive electrons. When matter meets antimatter, each and every anti-particle will combine with its corresponding ordinary particle, and they will annihilate each other-only energy will remain. Antimatter existed at the time of the Big Bang when the universe was formed. But none of it could be left in our corner of the universe now, because it would already have combined with the ordinary matter here, and both would have disappeared.
Nuclear fission reactions also release mass energy. Some substances are made from very big, unstable atoms. They fall apart easily, and each of the smaller pieces becomes a new atom of a different substance. For instance, uranium atoms fall apart easily. They might break into two smaller atoms, like radium and lead. When a big atom falls apart, the pieces always have less mass than the original atom. The extra mass turns into energy.
Nuclear fusion reactions are a third way to release mass energy. Nuclear fusion means combining atoms to make bigger atoms by colliding them together at very high speeds. When the nuclei of two atoms combine, they form a bigger atom, and some of the mass is turned into energy. The amount of energy released is enormous, bigger than the energy from a fission reaction. Colliding atoms at high speed requires tremendous heat. But when it works, the energy released is even greater.
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