# What is Energy? - Western Kentucky University

What is Energy? Physics Definition: The ability to do work Work: Force applied over a distance (W =f*d) Force: From Newton, force is the product of a mass and its acceleration (F=ma) also known as Newtons second law. But this applies mostly to mechanics, the study of the physics behind an objects motion What is Energy? Thermodynamics: the study of the conversion of heat energy into other forms of energy.

Very important in the production of energy as we will discuss it In themodynamics, work is defined as the quantity of energy transferred from one system to another without a change in its amount of order (called entropy) Units of energy Joules: The work done by a force of one newton traveling through a distance of one meter;

The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol CV; The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol Ws. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules; The kinetic energy of a 2 kg mass moving at a velocity of 1 m/s. The kinetic energy is linear in the mass but quadratic in the velocity, being given by E = mv Everyday examples of the Joule

the energy required to lift a small apple one meter straight up. the energy released when that same apple falls one meter to the ground. the energy released as heat by a quiet person, every hundredth of a second. the energy required to heat one gram of dry, cool air by 1 degree Celsius. one hundredth of the energy a person can receive by drinking a drop of beer. the kinetic energy of an adult human moving a distance of about a hand-span every second.

Power Power: the rate at which work is performed Or, the rate at which energy is transmitted Or the amount of energy expended per unit time Measured in Watts: Other units: HP or horse power BTUs

Horse power Arose as a result of the invention of the steam engine. People needed a way to compare the power of a steam engine to that of the horses it was replacing. Confusing unit there are too many different definitions! BTU BTU: British Thermal Units - an energy unit the amount of heat required to raise the temperature

of one pound of liquid water by one degree from 60 to 61Fahrenheit at a constant pressure of one atmosphere Used in the power, steam generation, heating and air conditioning industries and the energy content of fuels. However, BTU is often used as a unit of power, where BTU/hour is often abbreviated BTU. So you need to watch the context! Back to Watts..

A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts. A 100 Watt light bulb consumes energy at the rate of 100 joules/second. After 1 hour, this light bulb uses 100 watt-hours 1 kilowatt (kw) is 1000 Watts Examples

In a certain room in your house, you use a 100 W light bulb. This light is on for 5 hours every day. How much energy does it use? 1 W = 1 J/s and there are 5hour x 60min/hour x 60 sec/min = 18,000s in 5 hours so the total energy used is 100 j/s *18000s = 1.8 x 10 6 J. Lets assume the same lighting level can be achieved using a 30 W compact florescent bulb. How much energy is used by the compact florescent bulb? Examples

Total energy = 30 j/s x 18000 s = 5.4 x 105 j. So how much energy is saved every day using the compact florescent bulb? Take the difference between the energy used by the two different light bulbs: 1.8 x 10 6 j - 5.4 x 105 j = 1.3 x106 j. Lets look at this in something you might be able to relate to better than joules---dollars! Example continued After 5 hours, our 100 W light bulb uses 500 Watt-hours, or 0.5 Kwh. The 30 W bulb will

use 150 Watt hours or 0.15 Kwh. Assume electricity costs 11 cents/Kwh (average cost in the US in April 2008). So it costs .5 KwH x 11 cents/Kwh = 5.5 cents every day to run the 100 W light bulb and 0.15Kwh x 11 cents = 1.65 cents every day to run the compact florescent. Example continued So in a year, the 100 W light bulb costs you 5.5 cents/day X 365 days/year = \$20.00 and the 30 W bulb costs costs you 1.65 cents/day x

365 days/year = \$5.50. Savings of 14.50/year per 100 W light bulb!

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