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 September 3, 2013

2 Announcements Reading thus far: - - Chapters 1 and 2 of the class notes, Chapter 3 of Feynman s The Character of Physical Law. Clicker use will begin next Tuesday, September 10th. I will check in with the book store and send out an later today with advice on how to best proceed buying books and a clicker.

3 Primary Energy Sources The sources from which we extract the energy we use Oil Coal Natural gas Nuclear Biomass Hydroelectric Wind Geothermal Solar

4 Diversity of primary energy sources for Includes 0.44% of total from wind. the production of electricity -,$+*)('&%$#"!! )#*+%, Includes 2.4% of total from wind. (#!"#$%"& '"( "'#!"# 80"& &# -./%0+&+1#% % 5 $%#!$1&+"% 6&+1#% % Looking at these pie charts, we can see how shifting energy consumption to the electric grid facilitates using many different primary energy sources. Note wind has quadrupled in fractional contribution in five years.

5 There is a strong correlation between GDP and energy consumption

6 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

7 The power we generate The power we consume American s per capita A person burns energy consumption is energy at the rate 100x100 Watts of about 100 Watts = 10 kilowatts The standard of living is closely coupled to per capita energy use. Pre-industrial societies were definitely energy constrained. Multiple authors have suggested that the historical quest to harness energy was a contributing factor to slavery.

8 Energy intensity tends to improve with time. Energy intensity is the average power consumed per unit of GDP

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10 Some things in nature do not change. Conservation Laws

11 Conservation of charge The total amount of charge in the universe (or some defined isolated space) does not change. Charge comes in positive and negative varieties. If it disappears from one place, it shows up somewhere else. Electrons are negative Protons are positive Neutrons, as the name suggests, are neutral.

12 Conservation of charge Particles can change into different particles, but the charge needs to go somewhere. In Beta Decay, depicted below, a neutron changes into a proton, emitting a W - that then turns into an electron. This is the type of reaction we detected with a geiger counter during one of our earlier classes. Feynman diagram for the weak decay of a neutron n p + e W - - Neutron decay note charge is conserved at each vertex νe

13 Conservation laws are local Imagine two people observing two simultaneous events that are separated in space. When people are in different reference frames, events that are simultaneous for one person are NOT necessarily simultaneous for the other person. If charge is going to be conserved, it must be conserved LOCALLY. That is, you cannot have charge disappearing one place and reappearing someplace that is spatially distant.

14 Conservation of angular momentum Mentioned briefly by Feynman in reference to earlier stuff in the book, more specifically, the law of equal areas in equal times having to do with planetary motion. Its why a top keeps spinning upright...

15 Conservation of angular momentum Kepler discovered that when planets orbit the sun, they... sweep out equal areas in equal times. This is another example of the conservation of angular momentum.

16 The Conservation of Energy

17 The Conservation of Energy

18 Conservation of energy Adding up the different places the blocks can be. When the kid hides some blocks in the toy box: When the kid hides some blocks in the sink:

19 The Conservation of Energy Energy can neither be created nor destroyed, but can only be converted from one form to another.

20 Balls rolling on hills Maximum potential energy (for this system, anyway)

21 Balls rolling on hills A mix of potential energy and kinetic energy

22 Balls rolling on hills Maximum kinetic energy given our initial conditions

23 Balls rolling on hills A mix of potential energy and kinetic energy

24 Balls rolling on hills Maximum potential energy (for this system, anyway)

25 Quantifying the conservation of energy for balls rolling on hills Kinetic energy: KE = (1/2)mv2 Gravitational potential energy: PE = mgh (Here g = 9.81 m/s 2) Time KE i + PE i = KE f + PE f initial Before final After

26 The Conservation of Energy Energy can neither be created nor destroyed, but can only be converted from one form to another.

27 Standard International (SI) Units It will often be convenient to do calculations using SI units, but they are certainly not the only units we will be using.

28 A specific example Initial position of bowling ball Final position of bowling ball h = 2m If its mass is 2 kg, what is the kinetic energy of the bowling ball as it moves through the lowest point (where h=0)?

29 How much kinetic energy does it have at the bottom? Total energy before: Total energy after: KE i + PE i = 0 + mgh KE f + PE f = ½mv Setting: total energy before = total energy after... mgh = ½mv 2 = KE f KE f = mgh = (2kg)(9.81 m/s 2 )(2m) = 39.2 Joules

30 How much kinetic energy does it have at the bottom? Total energy before: Total energy after: KE i + PE i = 0 + mgh KE f + PE f = ½mv Setting: total energy before = total energy after... mgh = ½mv 2 = KE f KE f = mgh = (2kg)(9.81 m/s 2 )(2m) = 39.2 Joules If we wanted, we could also solve this for the velocity: 39.2 Joules = ½mv 2

31 Homework Read Chapters 1 and 2 of the class notes. Read Chapter 3 of Feynman s The Character of Physical Law. Watch for instructions (today) on books and clickers.

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