Transcript Energy
Energy Adapted From Exploring Engineering Chapter 4, Part 1 Energy Energy Energy is the capability to do work Work = force x distance Distance over which the force is applied Energy Units: SI: joules Mixed SI units: Watt-hours (= 3.6 kJ) English: ft-lbf “foot pound force” Energy Mixed SI units: Watt-hours (= 3.6 kJ) Power How fast work is done or how rapidly the amount of energy possessed by an object changed “Power is defined as time rate of doing work or time rate of change of energy” Power = work/time Power Units: SI: watts (joules/sec) English: Horsepower Kinds of Energy Kinetic Energy Potential Energy Often mechanical energy Some other forms of energy: Magnetic energy Electrical energy Surface energy Chemical energy (a form of potential energy) Internal energy etc. Kinetic Energy Also known as “Translational Kinetic Energy” (TKE) TKE = ½ mv2 (SI units) = ½ mv2/gc (English units) m = mass, v = speed, gc = 32.2 lbm.ft/lbf.s2 Units: ??? Kinetic Energy: Example What is the translational kinetic energy of an automobile with a mass of 1X103 kg traveling at a speed of 65 miles per hour (29 m/sec)? Need: TKE of the vehicle Know: Mass: 1X103 kg, speed: 29 m/sec How: TKE= ½ mv2 SOLVE: TKE = 4.2 x 105 J Anything that has mass and is moving in a line has TKE. Gravitational Potential Energy GPE is the energy acquired by an object by virtue of its position in a gravitational field-typically by being raised above the surface of the Earth. In SI, GPE = mgh in units of joules In Engineering English units, GPE = mgh/gc in units of ft.lbf GPE & Power: Example A person takes 2.0 seconds to lift a 1. kg book a height of 1. meter above the surface of Earth. Calculate the power expended by that person or calculate the energy spent by the person per unit time. Work done = Force x distance = mg x h = 1. x 1. x 9.81 [kg][m/s2][m] = 9.81 [J][m] = 1. x 101 J Power expended = Work done/time = 1. x 101/2.0 [J/s] = 5 Watts Gravitational Potential Energy Mt. Everest is 29, 035 ft high. If a climber has to haul him/herself weighing 200. lbm (including equipment) to the top, what is his/her potential energy above sea level when on the summit. Give your answer in both in joules and in ft.lbf. Gravitational Potential Energy Need: GPE in English and SI units Know: m = 200. lbm = 90.7 kg (“Convert”); h = 29, 035 ft. = 8850. m (“Convert”); g = 32.2 ft/s2 = 9.81 m/s2 & gc = 32.2 lbm ft/s2 lbf (English) and gc = 1 [0] in SI How: GPE = mgh/gc English GPE = mgh SI Gravitational Potential Energy Solve: English … GPE = mgh/gc = 200. 32.2 29,035/32.2 [lbm][ft/s2][ft][lbf.s2 /lbm.ft] = 5.81 106 ft.lbf (3 significant figures) SI … GPE = mgh = 90.7 9.81 8850. = 7.87 106 J A check direct from the units converter: 5.81 106 ft.lbf = 7.88 106 J …OK Potential Energy GPE is NOT the only form of PE. Chemical, nuclear and electromagnetic are other forms of PE For us, chemical and electrical energy are so important that we will reserve extra chapters and lectures to them for later presentation. Thermal Energy Thermal energy, often referred to as heat, is a very special form of kinetic energy because it is the random motion of trillions and trillions of atoms and molecules that leads to the perception of temperature All higher forms of energy dissipate to thermal energy, the ultimate energy sink. The laws of thermodynamics state 1) all energy is conserved and 2) that the thermal energy in the universe, corrected for temperature, always increases. Energy We have defined energy is the capability to do work But energy comes in different guises • Potential, translational kinetic, rotational kinetic, thermal and others Energy can be converted from one form to another • The energy in the Universe is conserved • A “control volume” is a subset of the Universe you construct to isolate the problem of interest. It exchanges energy with the rest of the Universe Energy Conservation : Energy exchanges Energy = F distance is “The Universe” generic equation for System energy Energy is conserved (although it may change form) System energy changes 0 Universe energy changes = 0 Example of a book lying on a table and then falling on ground Energy Conservation Example of a control volume The energy in the room is constant unless we allow exchange with the Universe E.g., a person could walk through the door and add energy A heating duct could also add thermal energy On a winter day, a window could break and the c.v. would lose thermal energy C.V. boundary This class room Insulated walls Door Control volume example Application of Control Volumes The TKE of the vehicle, RKE of the wheels, electrical energy in the lights, thermal energy lost from the radiator, etc. We deduce that the source of all these energies is exactly equal to the loss in chemical (potential) energy in the fuel. Summary: Energy We specifically identified gravitational, potential, and thermal energy We learned that energy is conserved in the Universe, but not necessarily in a control volume. Deficiencies within a control volume mean that energy in leaking in or out of the control volume at an exactly compensating amount.