Energy The Word vs. The Physics Concept

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Derick Barnett
7 years ago
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1 ENERGY

2 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.

3 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

4 Work and Direction Only one component of force does work The component parallel to the distance traveled F Work F = F s =F horizontal s θ s Work gravity = 0 Work N = 0 Work friction = F friction s

5 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

6 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) KE = 1 2 m v2 Gravitational Potential Energy (PE grav ) Drop an object from a height; it pulls on Earth as it falls mg y Therefore it has energy (greater height more energy) PE grav = m g h

7 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

8 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

9 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

10 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

11 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 Work = F d 1 Joule = 1 Newton 1 meter 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

12 Power A system which does work Releases its energy by exerting a force Power The rate at which a system does work Power = work time Measured in Watts (W) or horsepower (hp) A weak force Does work slowly Puts out a small power A strong force Does work quickly Puts out a large power

13 Conservation of Energy Systems do work by exerting a force Newton's Third Law: An equal and opposite force is exerted on the system An equal and opposite work is done on the system If a system's energy is increasing: 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

14 Conservation of Energy Example Object of mass m falling from height h At top At middle At bottom PE grav = m g h PE grav = 1 2 m g h PE grav = 0 h KE = 0 KE = 1 2 m g h 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 2 mv 2 v = 2 g h

15 Elastic PE (springs) Whether spring is stretched or compressed, there is a positive amount of PE stored in the spring When block is released, PE spring is converted to KE k

16 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

17 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% (!)

18 Energy on Earth Most forms of energy on Earth originate as sunlight and end up as heat in the atmosphere Sunlight Food Chain Photosynthesis Ground Heat, Water Cycle and Weather Solar Energy Devices Oil, coal, wood, etc. Hydroelectric, Wind Power, Wave Power Used Energy (Heat)

19 Stored Chemical Energy is Scarce Plants and animals are converted into oil and coal These are nicknamed fossil fuels The geological processes take millions of years Human oil and coal usage 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

20 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

21 Future Energy Strategy Energy Source Pros Cons Fossil Fuels Solar / Wind / Wave Biofuels Nuclear Infrastructure in place Relatively inexpensive in the short-term Abundant, renewable Can often be produced onsite; No need to transport Can use current oil infrastructure Doesn't add new CO 2 Produces large power No CO 2 emissions Relatively little remaining Release of CO 2 global warming Requires large initial investment Sun must be shining / wind must be blowing Difficult to mass produce at this time Converter kits required Dangerous (meltdown) Produce radioactive waste

22 Electric Vehicles and Hybrids About 25% of energy consumption goes to transportation Mostly combustion of oil-based fuels Electric vehicles and hybrids decrease consumption of oil 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 miles without recharging Gas / electric hybrid vehicles have better energy efficiency than traditional gas-only vehicles without the limitations

23 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

In science, energy is the ability to do work. Work is done when a force causes an

What is energy? In science, energy is the ability to do work. Work is done when a force causes an object to move in the direction of the force. Energy is expressed in units of joules (J). A joule is calculated