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updated: 12/4/2012 8:16 AM

Energy comes in a variety of scientific forms

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"If energy can't be created or destroyed, how do we know it exists?" asked a student in Katherine Crawford's fifth-grade class at West Oak Middle school in Mundelein.

Energy, a Greek word that means activity or operation, is the ability to do work. Energy includes thermal, kinetic, chemical, potential, nuclear, electrical and gravitational.

The law of conservation of energy, a law of physics, states that energy can be neither created nor destroyed.

Geza Gyuk, director of the Adler Planetarium, offers this example: "It takes energy to raise a weight from the ground to over your head. The amount of 'potential' energy the weight has is now larger -- potential because it can be converted to the more visible form of kinetic energy by letting the weight fall back down."

In this instance, energy comes from a few sources -- your body, its chemical activities, the nutrition that fuels the chemical activities of the body, and its originating source, the sun.

"The energy the sun radiates comes from fusion of hydrogen atoms," Gyuk explains. "It is a long, long chain, but ultimately most of the energy in the universe comes from two places -- mass being converted into energy (E=mc a la Einstein) or things falling together under the influence of gravity and converting potential energy to kinetic energy."

Let's take a look at a much larger example -- the universe. Energy is being created and destroyed at all times in the form of chemical and nuclear reactions.

"When the universe was young, it consisted almost entirely of hydrogen, with a bit of helium thrown in," Gyuk said. "Now, as time has passed, some of that hydrogen has been converted to helium and other heavier elements.

"In the process, a bit of the matter is destroyed and converted into energy. In truth, it isn't energy that is conserved, but the combination of energy and matter."

Where did all this original energy and matter come from? Some scientists believe the total amount of energy in the universe is exactly zero.

The energy locked up in the mass of objects and the motion of objects is always positive, so how can this be?

"It turns out that gravity, the bending of space, can be thought of as negative energy," Gyuk said. "So if you take the total energy of the space of the universe and then add the energy of everything in the universe, the sum comes to zero."

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