Energy. on this world and elsewhere. Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434)

Transcription

1 Energy on this world and elsewhere Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) Course web site available at click on classes and find Physics or at Lecture #5 September 5, 2017

2 Announcements Tentative date for the first quiz is Thursday, September 14th. Homework - Read Chapter 3 of the Class Notes - Read Chapter 3 of Feynman s book.

3 Work F m m d Work is defined by: W = Force x Distance Think of it as a way to either create kinetic energy, or store potential energy.

4 Work F m m d Recall the definition we heard (quoted from the text Ostiek and Bord) that energy was: That which is transferred when work is done. Well if energy is transferred, where does it appear? In the case shown above, it must appear as the kinetic energy of the mass! W = F d = ΔKE Here I am explicitly assuming that the surface above is frictionless.

5 What if there is friction? #1 #2 #3 m F m f m d s We start by applying work, and we can see the energy in the block s kinetic energy. Once the block stops, we don t see any kinetic energy. Where did it go? FRICTION! Thermal energy!

6 Heat Random kinetic energy at a microscopic level

7 Well organized energy can turn into random kinetic energy that we see as heat Examples: Kinetic energy turns to heat through friction. Potential chemical energy turns into heat through burning. Potential energy in the atomic nucleus is turned into heat through nuclear reactions. Radiation (i.e. from the sun or a fire) is absorbed and turns into heat.

8 Brownian Motion - evidence of random kinetic energy Atoms in gases and liquids move about in a random fashion. Small particles such as dust and smoke particles collide with the randomly moving atoms, which, of course are too small to see. During any particular time interval, it is likely that there will be more collisions on one side of the particle than the other. The result is that the small particle moves about in its own random pattern, due to its interactions with the invisibly small atoms.

9 Example of heat being created through release of chemical potential energy Consider the combustion of methane, the primary component of natural gas: CH 4 + 2O 2 CO 2 +2H 2 O + energy What happens is that the CO 2 and the H 2 O molecules come flying out of the reaction with extra energy.

10 Can a gas do work? Or to put the question differently, can we somehow make use of all this random kinetic energy (heat energy) that is associated with the gas atoms and molecules? Yes... but to understand how, we need to remind ourselves of the the concept of pressure.

11 Pressure results from the force of gas atoms and molecules colliding with a surface. Pressure

12 Pressure Pressure is defined as follows: Pressure = Force Area To find the force on a surface, just multiply the pressure times the area of the surface. Air pressure is roughly 15 lbs. per square inch. That 15 lbs represents the weight of a column of air going all the way up to the top of the atmosphere.

13 The Ideal Gas Law P V = n RT P = Pressure V = Volume T = Temperature n = amount of gas, measured in moles R = is a constant

14 Changing temperature at constant pressure V = n R P T If heat is added, thus raising the temperature, while keeping the pressure constant, the volume will increase.

15 Using a gas, and heat, to do work... the concept of a heat engine Work = (force on piston)(distance piston moves)

16 Random kinetic energy has been transformed into gravitational potential energy The internal energy of the gas has been turned back into well organized energy. Here, gravitational potential energy

17 Energy and Power

18 Energy and Power Energy The capacity to do a certain amount of work Power The rate at which you are doing work An amount of money (say that you have saved) Or if energy were money... A wage (As in, for example, $20.00 per hour) You CANNOT do a unit conversion between energy and power!!!! You need a formula to convert between the two.

19 Power consumed (or the rate of energy consumption) Power = Energy Time Power = Energy used over some time period The length of that time period

20 Power produced (or the rate of energy production) Power = Energy Time Power = Energy output during some time period The length of that time period

21 SI unit for Power Power = Energy Time Joules = = Watt (W) seconds page 49 of class notes

22 Work and Power Let s return to our earlier example, and throw in a few more assumptions... F = 30 Newtons d = 3 meters m = 20 kilograms time during which force is applied = 2 seconds F m m d W = Force x Distance = 30 Newtons x 3 meters = 90 N-m = 90 Joules

23 Work and Power What is the rate at which energy is expended? F m m d Power = Energy Time 90 Joules = = 45 Joules 2 seconds seconds = 45 Watts As the example above implies, the SI unit for power is the Watt, which is 1 Joule per second.

24 Working with Units

25 Computing energy using power and time How much energy does the body burn in two hours? Express your answer in Joules. Energy = Power x Time Energy = 100 Watts x 2 hours Energy = 100 (J/s) x 2 x (60 minutes) Energy = 100 (J/s) x 2 x 60 x (60 seconds) Energy = 720,000 Joules

26 Computing power using energy and time A person burns 2000 kilocalories in a day. What is the rate at which they use energy? That is, how much power are they consuming? Power = Energy Time = 2000 kilocalories Day Power = 2000 x (4184 Joules) 24x60x60 seconds 8,368,000 Joules = 86,400 seconds Power = 96.9 Joules/second = 96.9 Watts

27 Getting a concrete sense of human energy output Power = Energy Unit time = Joules second = Watts A person burns energy at the rate of about 100 Watts Athletes can maybe go to a few hundred watts output

28 Working with Units

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