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ENERGY Energy – The Word vs. The Physics Concept ● The word “energy” is used loosely – As a commodity – “I paid my energy bill today” Does “energy” come through pipes like water? – As a mood or behavior – “He played with a lot of energy” – ● In physics, “energy” is a specific measurable quantity – Units: Joules ( J ), calories ( cal ), kilowatt-hour ( kWh ) Exists in many forms: – Chemical, electrical, gravitational, kinetic, etc. – Energy and Force: “Work” ● The energy of a system is made up of two ingredients: – – ● How much force the system can exert on another system How large a distance that force can be exerted for To emphasize the importance of “distance of action”, physicists invented a concept called “work” Work = F | | d ● F|| = force exerted by system d = distance force acts for In physics, energy is the ability to do work Work and Direction ● Only one “component” of force does work – The component parallel to the distance traveled F θ s Work F = F || s =F horizontal s Work gravity = 0 Work N = 0 Work friction = − F friction s Different Forms of Energy ● Force can be thought of as a way of converting energy from one form to another – – ● There are many different types of forces This is why energy takes many different forms Devices and machines use forces to convert energy into a desired form Kinetic → Light Pressure → Kinetic A Couple Common Forms of Energy ● “Kinetic Energy (KE)” – – A moving object will exert a force on anything it runs into Therefore it has energy (faster speed → more energy) 1 2 KE = m v 2 ● “Gravitational Potential Energy (PE – – mg y )” grav Drop an object from a height; it pulls on Earth as it falls Therefore it has energy (greater height → more energy) PE grav = m g h Energy: Storage vs. Usage ● Energy storage – ● A physical system contains energy which can be released at a chosen time Energy usage – – Energy is converted to a form where it can not easily be stored and released again Examples: Light from a TV set, Heated air molecules Chemical Energy ● A form of energy which is stored inside molecules – ● Actually, in the energy of electron orbits in the molecules Chemical energy is the most common stored form – – Fuels (coal, oil, wood, etc.) Release energy by burning – – Batteries Release energy by pushing electric current – – Food Release energy by digestion Friction and Heat: “Used” Energy ● Heat energy (or thermal energy) can be viewed at the microscopic level – ● It's really the total KE of individual molecules and atoms When friction and/or air resistance occur – – – KE of large objects is converted to heat energy Heat energy cannot be easily converted back to KE The energy is said to be “used” or “lost” Energy Example: Regenerative Braking ● “Traditional” Car Brakes – – – ● Operate based on friction KE of car → Heat energy “Wastes” energy, which originally came from burning fuel Regenerative Brakes – – – – – Operate based on electromagnetism KE of car → Stored energy in battery Energy can be re-used later! Car must have big enough battery to handle the energy Hybrid car batteries store about 5 times more energy than a traditional car battery Quantifying and Measuring Energy ● How do we compare one form of energy to another? – ● Example: a gallon of gasoline vs. a speeding car ( Chemical ) ( Kinetic ) Units 1 Joule = 1 Newton 1 meter Work = F d 1 J = 1 N⋅m ● Burning one gallon of gasoline releases 125 MegaJoules! ● The KE of an average car on the highway is 1 MegaJoule Power ● A system which does work – ● Releases its energy by exerting a force Power – – Power = The rate at which a system does work Measured in Watts (W) or horsepower (hp) A strong force A weak force ● ● Does work slowly Puts out a small power ● ● Does work quickly Puts out a large power work time Conservation of Energy ● Systems do work by exerting a force – – – ● If a system's energy is increasing: – ● Newton's Third Law: An equal and opposite force is exerted on the system An equal and opposite work is done on the system Another system's energy is decreasing an equal amount Systems can exchange energy – – But never create or destroy it! The total energy in the universe is conserved Conservation of Energy Example ● Object of mass m falling from height h At top PE grav = m g h KE = 0 h At middle 1 PE grav = m g h 2 1 KE = m g h 2 At bottom PE grav = 0 KE = m g h So gravity is a force that converts PE into KE! We can also calculate how fast the object will be moving at the bottom: KE = m g h = 1 m v2 2 v = 2 g h Elastic PE (springs) Whether spring is stretched or compressed, there is a positive amount of PE stored in the spring When block is released, PEspring is converted to KE k Simple Machines ● Work is the product of force and distance – – ● Conservation of Energy: Work input = Work output So we can exert large forces over short distances – – Converted from small forces over large distances Requires the use of “machines” Lever Pulley Efficiency and Energy “Loss” ● Force converts energy from one form to another – – Friction and air resistance cause energy “losses” A “perpetual motion machine” is an unattainable ideal Efficiency = ● useful energy output total energy input Example: Car – – – – – Sources of energy “loss”: Friction between engine parts Air resistance on car body Hot exhaust (not all energy in fuel used) Overall efficiency: Only about 20% (!) Energy on Earth ● Most forms of energy on Earth originate as sunlight and end up as heat in the atmosphere Sunlight Photosynthesis Food Chain Oil, coal, wood, etc. Ground Heat, Water Cycle and Weather Solar Energy Devices Hydroelectric, Wind Power, Wave Power “Used” Energy (Heat) Stored Chemical Energy is Scarce ● Plants and animals are converted into oil and coal – – ● Human oil and coal usage – – ● These are nicknamed “fossil fuels” The geological processes take millions of years At a rate thousands of times faster than production rate This presents a problem for the future! Biofuels – – Modern technology can produce oil-like fuels from plants and algae in a relatively short period of time Use of biofuels to replace fossil fuels increases each year Non-Solar Energy ● These forms of energy do not originate as sunlight: ● Nuclear Energy – two different processes: – – – ● “Fission” of heavy elements (uranium, plutonium, etc.) “Fusion” of light elements (hyrdogen, helium, etc.) Fission is much easier technologically than fusion Geothermal Energy – – Heat energy trapped inside the Earth Drives geological activity on Earth Future Energy Strategy Energy Source Fossil Fuels Pros Cons Infrastructure in place Relatively little remaining Relatively inexpensive in the short-term Release of CO – global Abundant, renewable Solar / Wind / Wave Biofuels Can often be produced onsite; No need to transport Can use current oil infrastructure Doesn't add new CO Nuclear 2 warming Requires large initial investment Sun must be shining / wind must be blowing Difficult to mass produce at this time 2 Converter kits required Produces large power Dangerous (meltdown) No CO emissions Produce radioactive waste 2 Electric Vehicles and Hybrids ● About 25% of energy consumption goes to transportation – ● Electric vehicles and hybrids decrease consumption of oil – – – ● Mostly combustion of oil-based fuels Electricity: more efficient and cheaper energy source than oil However, most electricity still comes from coal (fossil fuel!) Using solar energy with an electric vehicle is emission-free Electric Vehicle Limitations (due to large, costly batteries) – – – Speed: Only very expensive electric cars can handle freeway Range: Usually about 40-60 miles without recharging Gas / electric hybrid vehicles have better energy efficiency than traditional gas-only vehicles without the limitations Summary ● ● ● Energy allows a system to exert a certain force over a certain distance Energy comes in many forms Energy is the underlying concept for much of physics; it will resurface often