Report

Prepared for BIO/EES 105 Energy in our World Kenneth M. Klemow, Ph.D. Wilkes University Overview ◦ Energy defined ◦ Forms of energy The physical nature of energy ◦ Energy and Newtonian Laws of Motion ◦ Units of measure ◦ Conversions Terminology pertaining to energy Ability to do work Physicists distinguish between kinetic and potential energy Energy comes in different forms ◦ ◦ ◦ ◦ ◦ ◦ ◦ Radiation Mechanical energy Chemical energy Atomic energy Electromagnetic energy Electrical energy Heat energy Sir Isaac Newton 1642 - 1727 Energy = Force x distance Force = Acceleration x mass Acceleration = Speed / time Speed = Distance / time Speed = distance / time Ways of expressing ◦ ◦ ◦ ◦ Miles / hour Km / hour Feet / second Meters / second Other relationships ◦ Distance = Speed x time ◦ Time = Distance / speed Velocity is a vector: implies speed and direction 1 ft/s = 0.305 m/s 1 mi/h = 0.447 m/s 1 km/hr = 0.28 m/s 1. A car drives 72 miles in 120 minutes. What is its velocity in miles per hour? 2. A person runs at 6 miles per hour. How far can that person run in 10 minutes? ◦ Expressed in miles: ◦ Expressed in feet: 3. How long does it take for that person to run 528 feet? A car is traveling 60 miles per hour. How many feet can it travel in one second? Acceleration = Change in velocity / time ◦ Expressed as distance / time X time ◦ Or distance / time2 Occurs when an object is speeding up or slowing down Units include ◦ ◦ ◦ ◦ Miles / hour2 Km / hour2 Feet / second2 Meters / second2 1 ft/s2 = 0.305 m/s2 1 m/s2 = 3.28 ft/s2 A Kia Rio can accelerate to 30 km / hour in 6 seconds. What is its acceleration? ◦ Express in terms of m / second2 (see Example 2.2 on p. 40) Velocity = Acceleration X Time Problem: ◦ Return to the Kia What After After After After is velocity after 1 second? 3 seconds? 6 seconds? 9 seconds? 12 seconds? Gravity has an acceleration (Agrav) ◦ Metric: 9.8 m/s2 ◦ English: 32 ft/s2 X = (1/2) x A x T2 (see p. 62 of text for derivation) Problem: Imagine you drop a stone from a cliff, and it takes three seconds to hit the water below. How high was the cliff above the water? How fast was the stone moving when it hit the water? Momentum = mass x velocity Force = mass x acceleration Common unit of measure for force: ◦ Newton (N = kg x m / s²) Other relationships ◦ Mass = Force / acceleration (kg=F/a) ◦ Acceleration = Force / mass (A=F/kg) A rock having a mass of 2 kg falls into the water from a cliff. What is the force that it exerts? ◦ Does that force vary if the cliff is 50’ high, as opposed to being 100’ high? Mass is a property of a body (measure of inertia). ◦ Irrespective of its position relative to gravity. ◦ Often expressed as Kg. Weight depends on gravity. An object will weigh more on earth than on moon because gravitational force greater on earth. ◦ Weight often considered to be unit of force, expressed as Kg x Agrav Where Kg is mass and Agrav is acceleration due to gravity. 1. A body will continue to remain at rest or in motion with a constant velocity unless it is acted upon by an outside force. 2. The acceleration of an object is directly proportional to the net force acting on it, and is inversely proportional to its mass (A = F/Kg). 3. For every action force, there is an equal and opposite reaction force. Energy = Force x Distance ◦ Joule (J) = Newton x meter Energy of an apple 1 m from the floor ◦ Some additional measures of energy Foot pound = 1.4 J 1 calorie = 4.187 J 1 BTU = 1054 J Potential energy ◦ Stored energy, able to do work if released. Examples include: Objects placed at an elevation Water behind dam Release energy if they fall Objects placed at mechanical tension Wound up spring Release energy if tension is relieved Chemical bond energy Organic molecules Energy released if combusted ◦ Potential energy due to elevation PEG = weight x height = Kg x Agrav x h Kinetic energy ◦ Energy of motion Examples include: Moving water Moving catapult ◦ Can be expressed mathematically as 1/2 Kg x v2 Rate at which energy is produced, used, or transferred. ◦ Expressed as energy per time ◦ Common units include Watt (J / s) Ft-lb / sec Horsepower 1 hp = 550 ft-lbs / sec 1 hp = 746 Watts Question: A kilowatt hour is a measure of: ◦ ◦ ◦ ◦ ◦ Power Energy Force Acceleration None of the above Power = energy / time Energy = power x time www.belmont.k12.ca.us W = D (KE + PE) Both have two meanings ◦ Conversion Translating between different units of measure Joule <-> Calorie <-> BTU Changing from one form to another Chemical energy -> Thermal energy ◦ Conservation First law of thermodynamics Energy cannot be created or destroyed, only converted Reduce wasteful energy consumption Switch from incandescent to light-emitting diode (LED) 1 1 1 1 1 1 kilowatt hour = 3.60 x 106 J barrel oil equivalent = 6.119 x 109 J ton wood equivalent = 9.83 x 109 J ton coal equivalent = 29.31 x 109 J ton oil equivalent = 41.87 x 109 J quad (PBtu) = 1.055 x 1018 J First law: Energy cannot be created nor destroyed, can only be converted (conservation of energy) ◦ In an isolated system, total energy will always remain constant Second law: No energy conversion is perfect; always get some loss as heat. ◦ Gives direction to a reaction ◦ Get increase in disorder (entropy). In system involving movement, always get loss as friction Thus perpetual motion machines are impossible (yet people still try to invent them) Waste heat given off to environment ◦ Ultimately go off to space Efficiency = energy (work) output X 100 total energy input • Efficiencies can vary from 5% - 95% • In multistep processes, efficiency is the product of efficiency of each step. • Comparative assessments of energy processes / devices typically take great pains to accurately measure efficiency Refer to Table 3.1 on p. 78 of text