- Количество слайдов: 16
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 Newton’s 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. • 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 C·V; – The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W·s. 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 = ½mv²
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 61°Fahrenheit 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 5 h x 60 min/hour x 60 sec/min = 18, 000 s in 5 hours so the total energy used is 100 j/s *18000 s = 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 x 106 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 Kw. H x 11 cents/Kwh = 5. 5 cents every day to run the 100 W light bulb and 0. 15 Kwh 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 you 1. 65 cents/day x 365 days/year = $5. 50.
Types of Energy: kinetic and potential Energy Kinetic energy of a moving object KE=1/2 mv 2 Potential Energy – Energy stored in a system, for example an object of mass m, a distance h above the surface of the earth has a potential energy given by mgh. g is the acceleration due to gravity = 9. 8 m/s 2
More examples of potential energy Another example is a spring, compressed a distance x from its equilibrium point has a potential energy 1/2 kx 2, where k is the spring constant, a property of the spring.
Chemical Energy that is released via chemical reactions. Often times release is through combustion such as energy generation via coal Another example is a battery
Heat Energy associated with the random motions of the molecules in a medium. Measured by temperature • Temperature Scales: • Fahrenheit – based on the height of liquid (often mercury or alcohol) in a glass tube. • Celsius – another scale using height of liquid in a tube • Kelvin-absolute scale – True measure of energy