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Because we see things move and watch them crash, we easily perceive the many manifestations of kinetic energy in the universe. Potential energy and other common forms of energy, however, often elude our senses.
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so, comets or asteroids as wide as a football field still strike land somewhere on Earth every few thousand years, on average, with explosive results.
The deadliest asteroid to hit Earth in recent geological history struck about 65 million years ago off the coast of what is now the Yucatán Peninsula in Mexico. Today, a crater more than 100 miles across is all that remains, much of it under the seafloor in the Gulf of Mexico. The 6-mile-wide asteroid carried 10 million times more kinetic energy than either the Tunguska or Meteor Crater objects. The heat produced by an explosion of that magnitude burned the air itself, as well as most vegetation on the continents. It also threw enough dust into the upper atmosphere to block sunlight for many months. The environmental aftermath, rather than the impact itself, led to extinction of the dinosaurs and most other species on the planet.
Because we see things move and watch them crash, we easily perceive the many manifestations of kinetic energy in the universe. Potential energy and other common forms of energy, however, often elude our senses. One form is crucial to the workings of the universe: nuclear energy. The name derives from the nuclei of atoms, bound together tightly by the strong and weak nuclear forces. During radioactive decay, nuclear bonds in unstable atoms break, releasing a flash of stored potential energy. The atoms eject helium nuclei (alpha particles) and electrons (beta particles), which carry away kinetic energy. Radioactive decay also releases energy in the form of gamma rays--energetic light that heats up its surroundings. Such atomic light warms the interior of Earth and other planets. In space, supernovas forge countless radioactive atoms of nickel and cobalt whose subsequent decay enables the explosions to shine brightly for many months.
Especially energetic breakups occur in processes called nuclear fission and nuclear fusion. Fission, the splitting of heavy atoms such as uranium, releases a million times more energy than the detonation of an equivalent amount of TNT. Under controlled conditions, fission drives nuclear power plants. Uncontrolled fission is the energy source for the atomic bomb. Pound for pound, fusion of hydrogen nuclei into helium is 100 times more energetic still. Our deadliest weapons draw their awesome power from this reaction. So too do the Sun and, for most of their lives, all stars. Indeed, stars are brilliant hydrogen bombs cloaked and contained by hundreds of thousands of miles of gas.
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Conduction, Convection,
and Radiation
Conduction is the transfer of heat from
one part of a system to another through the vibration of stationary
particles. Convection is the transport of heat by physically moving
particles from hot to cold regions. Radiation does what the other
two methods cannot: it carries energy through a vacuum. Vibrations
of atoms and molecules are converted into the packets of electromagnetic
waves called photons--the light reaching us from the rest of the
cosmos.
Convection Convecting
fluids move in circular currents called convection cells, as illustrated
by bubbles in a pot of water coming to a boil (above). Inside the
Sun convection cells carry thermonuclear heat to the surface and
out into space (above, right). High- resolution photographs show
the Sun's surface to be granular and bubbly, like a pot of boiling
soup (above, far right).
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Conduction
In a pot on a stove burner (left), heat is carried by conduction through the burner to the pot by vibrating atoms and molecules. Plastic and wood are poor conductors, so they are often used to make the handles of pots, lids, and cooking utensils. But you use a potholder when grasping the hot handle of cast iron cookware. On a hot, sunny day at the beach (above), you need insulation to keep your feet from being burned by contact with the sand.
Radiation
Of the entire spectrum of wavelengths found in electromagnetic radiation, the kind we feel as heat lies in a range that readily triggers vibrations and random motions in atomic matter. So heat radiation, commonly known as infrared, is actually a form of light. Just as our skin warms when we sit next to a fire (left), interstellar gas warms when heated by radiation from hot young stars. The gas then emits radiation of its own, which we see in so-called emission nebulas (above).
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