UNIT 2 MECHANICS CHAPTER 6 ENERGY

Transcription

1 UNIT 2 MECHANICS CHAPTER 6 ENERGY

2 Chapter 6A The Nature of Energy Objectives: Discuss the importance of energy State what energy can do and the units in which energy is measured Define mechanical work as it relates to energy Compare and contrast potential and kinetic energy Assignment: Section Review, page 133

3 Introduction Energy moves everything! Humans, animals, plants all require energy to live and carry out life processes Energy drives processes in nature that recycle water Winds and ocean currents circulate because of energy Internal and external sources of energy heat the earth Without energy the universe would cease to exist! Energy holds matter together

4 What is Energy? Energy The ability to do work An object moves a distance because of a force acting on it No one really knows what energy is, only what it does or has the potential to do Scalar quantity, measured in joules Compared to wealth Can be exchanged for work, matter or other forms Work and energy are interchangeable and measured in joules Facet Page 131

5 Potential & Kinetic Energy Energy can be broken into two groups: 1. Potential energy Energy of position or energy of condition Gasoline 2. Kinetic energy Energy of motion All moving matter regardless of size has kinetic energy More mass and greater speed = greater kinetic energy

6 Chapter 6B Classification of Energy Objectives: List the nine forms of energy given in the text Briefly discuss the nature of each kind of energy and state man-made and natural sources for each Compute an object s gravitational potential energy (GPE), given its mass and height Explain the relationship between matter and energy Given the mass of an object, compute the equivalent mass energy according to Einstein s special theory of relativity Assignment: Section Review, page 140

7 Identifying Forms of Energy Energy is classified by how it is sensed or by its principal source In this text (briefly discussed here, more details in future chapters) Mechanical Thermal Acoustic Electrical Magnetic Radiant Chemical Nuclear Mass

8 Mechanical Energy The energy of a system due to its position or its motion Consists of both potential and kinetic energy

9 Mechanical Potential Energy Zero Reference Position The position from which a distance is measured Two types 1. Gravitational Potential Energy (GPE) The potential energy of a system due to its weight and its height above a zero reference height Zero point is below the object Formula: GPE=wh w=weight h=height GPE changes as the height changes Example problem 6-1, page 134

10 Mechanical Potential Energy 2. Elastic Potential Energy (EPE) The potential energy of a system due to an elastic force acting on it and its distance from a zero reference position Examples Rubber bands Springs

11 Mechanical Kinetic Energy Formula: KE=1/2mv 2 m=mass of the system (kg) v=speed (m/s) Kinetic energies of large objects or single particles are easily measured Small molecules and substances at rest are difficult and use a different method to calculate Example problem 6-2, page 135 Read Kinetic Energy vs. Momentum

12 Thermal Energy Remember the kinetic-molecular model of matter? All matter consists of innumerable tiny particles in constant, random motion Every particle has kinetic energy! The sum of all the kinetic energies of a particle is its thermal energy Theoretically ceases at absolute zero (-273K) We measure the changes in thermal energy Heating gaining thermal energy Cooling loosing thermal energy Principal sources: sun and earth

13 Acoustic Energy The transmission of energy through matter by particle oscillations that occur in specific directions Occurs in waves Examples Earthquakes Explosions Bass speakers Submarine sonar equipment Human vocal cords Piccolos Jet engines Dog whistles Bats Medical ultrasound diagnostic equipment

14 Acoustic verses Thermal Energy Acoustic Energy When present, always moves through matter Cannot move through a vacuum Thermal Energy Always present in matter Only moves at different temperatures Causes particles to move in back and forth vibrations (periodic vibrations) when kinetic and potential energies or repeatedly exchanged When transferred the distance the particle move is usually much larger than the size of the particles Particle motion is random in direction and duration Vibrations are approximately the size of the particles

15 Electrical Energy The ability to do work through the action of the electromagnetic force on and by electrical charges Like charges repel Opposites attract Useful sources of electrical energy are mostly manmade Electrical batteries, generators, solar cells, radiothermal generators are just a few examples Natural sources Lightning, electric rays and eels, electrical currents circulating in the earth s magnetic fields in space

16 Magnetic Energy The ability of a magnetic field to do work on magnetic objects and on moving electrical charges All magnetic objects have both a north and south pole unlike electrical which has either positive or negative charges Natural sources Rocks, the earth, large bodies in our solar system Man-made sources Magnets, computer hard drives, wire conducting electricity

17 Radiant Energy Also called electromagnetic energy The combined action of electrical and magnetic energies in the form of wavelike, radiant energy Energy and magnetism usually go hand in hand Visible light is the most common source Others include Ultraviolet light, radar, radio waves, x-rays, microwaves, infrared light, and gamma rays Naturally emitted from stars, black holes, pulsars, and nebulas Artificial (man-made) are used for communication, illumination, imaging and medical purposes

18 Chemical Energy The potential energy stored in the chemical bonds between atoms that is released or absorbed during chemical reactions Depends on the kinds of atoms involved and the bonds formed Most energy is released as thermal, radiant, or acoustic energy Photosynthesis is an exception

19 Nuclear Energy The potential energy stored in an atom s nucleus that is released or absorbed when an atom experiences nuclear fission or fusion Released in two ways Fission Large nuclei with many protons and neutrons can be split into two or more smaller nuclei The sum of the masses of the smaller nuclei is less than the original Energy is released by breaking bonds Fusion Small nuclei are smashed together and form larger ones Energy is released with the formation of a new bond Man-made Nuclear fission reactors for electrical power generation and research, fission and fusion bombs

20 Mass Energy The energy equivalent to matter itself, according to the equation in Einstein s special theory of relativity Largest source of energy in the universe! Energy obtained if you could convert all of an atom s mass into energy Formula: E=mc 2 Example 6-3, page 139

21 Chapter 6C Energy Conservation Objectives: Show how energy can be transformed from one kind into another Give examples of energy conversions Define the efficiency of a process that involves the conversion of energy from on kind into another State the law of energy conservation and explain the conditions under which it is true State the combined energy-matter law of conservation as the first law of thermodynamics Compare and contrast momentum and kinetic energy Classify collisions as elastic, partially elastic, or inelastic Assignment: Section Review, page 145

22 Energy Transformation Read 6.14, page 140 Conservation is never 100%, there is always some energy that escapes the process and is unusable Efficiency The process can be measure by comparing the amount of usable energy produced with the amount available before the transformation Man-made energy transformations are usually 20%-40% efficient A standard incandescent light bulb is only 5% A fluorescent is about 20%

23 First Law of Thermodynamics Also called the Law of Conservation of Energy Energy can never be created or destroyed, only changed in form Pendulums are a clear, simple example No exception has even been observed! Directly related to the Law of Conservation of Matter The total amount of matter and energy in the universe is constant Read 6.16, page 141

24 Collisions and Energy Three kinds of collisions: 1. Elastic (ball bearings, hockey pucks) A collision between two objects in which the momentums and kinetic energies of the colliding objects are conserved No real world collision is perfectly elastic 2. Partially Elastic (car crash, soccer players) A collision between two objects in which momentum is conserved but some of their kinetic energies is lost to other forms of energy (usually heat) during the collision Objects are slightly deformed but rebound Most real world collisions are this type 3. Inelastic (hockey players) A collision between two objects in which the deformation is so severe that they stick together Total momentum is conserved, total kinetic energy is greatly reduced by the amount of energy required to deform the objects

25 Facet Stop! Read page 144

26 TOMORROW!! Vocabulary Quiz Includes all vocabulary throughout the entire chapter, PowerPoints, and board; not just the box at the end. Complete Chapter Review in Class Study for Chapter 6 Test