Ask a Physics Student: What is Energy?

Today we continue our “Ask A Science Student” series. For the previous response, click here.

Ask a Science Student, Part 2: Physics

By Matt Radford ’16

Energy has many different forms. Electrical, thermal, and mechanical kinetic energy are all forms associated with moving objects. Potential energy is dependent on an object’s position in a field, such as apples hanging in a tree, or an object’s position relative to its parts, such as a drawn longbow. An apple also has digestible energy, usually measured in calories. Other units of energy commonly found in day-to-day life include the watt-hour, which probably shows up on your electricity bill, and the British Thermal Unit, which is likely mentioned on any air conditioning unit.

To define energy, physicists use the unit of the joule.  The work done on an object by a force of one Newton over a distance of one meter is equivalent to transferring one joule of energy to the object. Another measure of the energy of a system is its mass. Einstein’s famous says that mass and energy are just two sides of the same mass-energy coin. However, the observed amount of matter in our universe – stars, black holes, galaxies – doesn’t account for nearly enough of the mass-energy required for our universe to behave the way it does. “Dark” energy is the makes up the rest of the universe, and it contributes to the expansion of the universe. It is less dense than ordinary matter, but it is of roughly uniform density across space, and it accounts for over two-thirds of the universe’s mass-energy.

In describing energy, physicist Richard Feynman said that “it is important to realize that in physics today, we have no knowledge of what energy ‘is.’ We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. It is an abstract thing in that it does not tell us the mechanism or the reason for the various formulas.”

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