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Energy: The Quick Tour
Energy is the driving force of the universe.

Energy is the capacity of a system to do work. That system may be a jet, carrying hundreds of passengers across the ocean. A baby's body, growing bone cells. A kite, rising on the wind. Or a wave of light crossing a space.

In moving or growing, each of these systems is doing work, and using energy. Every living organism does work, and needs energy from food or photosynthesis. Humans also create machines that do work for them, and that derive energy from fuels.

Some of the many forms that energy takes are

  • Mechanical energy, which includes
    • Potential energy, stored in a system
    • Kinetic energy, from the movement of matter.
  • Radiant or solar energy, which comes from the light and warmth of the sun.
  • Thermal energy, associated with the heat of an object.
  • Chemical energy, stored in the chemical bonds of molecules.
  • Electrical energy, associated with the movement of electrons.
  • Electromagnetic energy, associated with light waves (including radio waves, microwaves, x-rays, infrared waves).
  • Mass (or nuclear) energy, found in the nuclear structure of atoms.

One form of energy can be converted to another form. This transfer is based on the law of conservation of energy-one of the laws of thermodynamics.

Humans converted energy from one form to another when they lit the first fire. By burning wood, they released the chemical energy stored in the bonds of the wood molecules, generating thermal energy, or heat. Other examples? A battery generates electrons from chemical reactions, which are used to make electrical energy. A toaster takes electrical energy and converts it to heat. Your leg converts the chemical energy stored in your muscles into kinetic energy when you pedal a bicycle.

Sound is a form of kinetic energy. Molecules of air are vibrated, causing them to move in wave patterns. When these waves hit the eardrum, they make it vibrate too. This vibration energy is turned into electrical energy impulses, which your brain interprets as sound.

Many times, multiple conversions are involved. Consider nuclear power generation. Atoms in the nuclear fuel are split, releasing their nuclear (mass) energy and creating thermal energy. This heat energy is, in turn, captured in the form of steam and used to drive a turbine generator, creating kinetic energy. And, finally, this kinetic energy spins a magnetic field around a conductor, causing a current to flow-creating electrical energy.

To measure energy, we use the "heating value" of the fuel, which indicates how much of a certain fuel is converted to how much heat. Common units of energy are the calorie and Btu.

The Rules

Energy always follows the laws of thermodynamics:

Law #1 (Conservation of energy): Total energy in the universe is constant. Energy is never actually created or destroyed. It only changes from one form to another, or moves from one place to another, or is converted to mass.

Law #2 (Entropy): All things (mass and energy) tend toward disorder. So within any given system, energy will move from a state of organization toward a state of randomness. As disorder increases, energy changes into less usable forms. Because of this, any process that harnesses energy always loses some along the way-conversion can never be 100 percent efficient.

Law #3 (Absolute zero): Molecular movement stops at absolute zero (-273°C). Since temperature measures how fast molecules move, there can be no temperature lower than absolute zero.

How Hot Is Hot?

How Hot Is Hot?

Photo by Pacific Northwest National Laboratory

The energy content of a fuel is measured in terms of the heat it can generate.

Calories are defined as the amount of heat needed to raise one gram of water 1°C. Food calories actually refer to kilocalories, or 1,000 calories. If the body takes in more calories that it can use, it will convert and store the excess energy in the form of fat-an example of the law of the conservation of energy.

A common energy heat unit is the British Thermal Unit (Btu). One Btu is the amount of energy required to raise the heat in one pound of water by 1°F. 100,000 Btu is called a Therm. A quadrillion (1015 ) Btu is called a Quad. Average energy contained in or consumed by:

  Btu Calorie
A match 1 252
An apple 400 100,800
Making a cup of coffee 500 126,000
Stick of dynamite 2,000 504,000
Loaf of bread 5,100 1,285,200
Pound of wood 6,000 1,512,000
Running a TV for 100 hours 28,000 7,056,000
Gallon of gasoline 125,000 31,500,000
20 days cooking on gas stove 1,000,000 252,000,000
Food for one person for a year 3,500,000 882,000,000
Apollo 17's trip to the moon 5,600,000,000 1,411,200,000,000
Hiroshima atomic bomb 80,000,000,000 20,160,000,000,000
1,000 transatlantic jet flights 250,000,000,000 63,000,000,000,000
United States in 1999 97,000,000,000,000,000 24,444,000,000,000,000,000
Adapted from the Materials Science and Technology Teacher's Workshop, University of Illinois, Champaign/Urbana

Other heat unit measurements include joules and kilowatthours. One Btu is equal to 252 calories or 1055 joules. One kilowatthour is equal to 3,412 Btus, 859,824 calories, or 3,599,660 joules.

The average American adult uses 3,500 kilocalories of energy per day, active and resting. This is roughly the thermal energy needed for one tubful of hot bath water.

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