Some Basic Concepts of Energy

1. Some Basic Conceptsof Energy Prepared for BIO/EES 105 Energy in our World Kenneth M. Klemow, Ph.D. Wilkes University

2. Overview of topics • 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

3. What is 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

4. Some basic laws of Newtonian motion Sir Isaac Newton 1642 - 1727

5. Speed • 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

6. Speed: Conversions • 1 ft/s = 0.305 m/s • 1 mph = 0.447 m/s • 1 km/hr – 0.28 m/s

7. Some quick problems • 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?

8. Another problem • A car is traveling 60 miles per hour. How many feet can it travel in one second?

9. Acceleration • 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

10. Helpful conversions • 1 ft/s2 = 0.305 m/s2 • 1m/s2 = 3.28 ft/s2

11. A quick problem • A Kia Rio can accelerate to 30 km / hour in 6 seconds. What is its acceleration? • Express in terms of km / hour2 • Express in terms of m / second2

12. Can calculate velocity by knowing acceleration and time • Velocity = Acceleration X Time • Problem: • Return to the Kia • What is velocity after 1 second? • After 3 seconds? • After 6 seconds? • After 9 seconds? • After 12 seconds?

13. Gravity • Gravity has an acceleration • Metric: 9.8 m/s2 • English: 32 ft/s2

14. Can calculate distance moved as a function of acceleration and time • 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?

15. Momentum and Force • 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 (m=F/a) • Acceleration = Force / mass (A=F/m)

16. Sample problem • 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?

17. Weight vs Mass • 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 m x g (or mg) • Where m is mass an g is acceleration due to gravity.

18. Newton’s Laws of Motion • 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/m). • 3. For every action force, there is an equal and opposite reaction force.

19. Energy (work) • 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

20. Energy can be potential or kinetic • 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 = mgh

21. Energy can be potential or kinetic • Kinetic energy • Energy of motionExamples include: • Moving water • Moving catapult • Can be expressed mathematically as • 1/2 m v2

22. Power • 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

23. Common unit is kilowatt hour • That unit is a measure of: • Power • Energy • Force • Acceleration • None of the above

24. Energy and power • Power = energy / time • Energy = power x time www.belmont.k12.ca.us

25. Work is one way of transferring energy to an object • W = D (KE + PE)

26. “Conversion” and “conservation” two important concepts • 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)

27. Some conversion factors(See Table 3.4 on pp. 86 – 87 for more complete list) • 1 kilowatt hour = 3.60 x 106 J • 1 barrel oil equivalent = 6.119 x 109 J • 1 ton wood equivalent = 9.83 x 109 J • 1 ton coal equivalent = 29.31 x 109 J • 1 ton oil equivalent = 41.87 x 109 J • 1 quad (PBtu) = 1.055 x 1018 J

28. Energy conversions(From Table 2.2 in text)

29. Laws of Thermodyamics • 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).

30. Consequences of Second Law • 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

31. Efficiency an important measure Efficiency = • 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 energy (work) output X 100 total energy input

32. Efficiencies of energy conversion devices and systems • Refer to Table 3.1 on p. 78 of text

33. Example of multistep efficiency

34. Temperature and heat • Based on kinetic energy of molecules • Heat is TOTAL energy of all molecules in a system • Typically measured in Calories or BTUs • Temperature is AVERAGE energy of all molecules in a system • Typically measured in degrees

35. Temperature vs heat within and between systems • Within a system • Increase in heat causes increase in temperature • Between systems • Not related • One system can have higher heat yet lower temperature • Ocean vs duck • Heat can move from one system to another • Only when there is a temperature difference • Move from higher temperature to lower temperature object.

36. Specific heat (c) • Measure of change in temperature as a result of heat absorbed. • Metric system: # joules needed to raise 1 kg of material by 1 oC. • English system: # BTUs needed to raise 1 lb of material by 1oF.

37. Examples of specific heats of different substances

38. Heat of vaporization and heat of fusion • Represent phase changes • Vaporization: liquid <-> gas • Fusion: solid <-> liquid • Can represent large values • For water • Vaporization: 540 kcal/ kg • Fusion: 80 kcal / kg • Heat absorbed or released depending on direction • Important in heat balance at earth’s surface, regulating temperatures of organisms

39. Forms of heat transfer • Conduction • Convection • Radiation

40. Terminology pertaining to modern energy • Renewable vs nonrenewable • Traditional vs new energy • Commercialized vs non-commercialized • Centralized vs distributed generation • On-grid vs off-grid

41. Stages of energy flowfrom http://www.fao.org/docrep/u2246e/u2246e02.htm • Primary energy is the energy as it is available in the natural environment, i.e. the primary source of energy. • Secondary energy is the energy ready for transport or transmission. • Final energy is the energy which the consumer buys or receives. • Useful energy is the energy which is an input in an end-use application.

42. Stages of energy flowfrom http://www.fao.org/docrep/u2246e/u2246e02.htm

43. Energy Carbon reduction CO2 C6H12O6 H2O Carbon oxidation Energy Why is living matter a source of energy?