Скачать презентацию What is Energy Physics Definition The ability Скачать презентацию What is Energy Physics Definition The ability

9e81dbd64b73e6cc3cf7537520bb8323.ppt

  • Количество слайдов: 16

What is Energy? • Physics Definition: The ability to do work • Work: Force 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 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 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 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 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. 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 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 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 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 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, 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 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 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 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 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 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