Solar and Renewable Energies€¦ · Physics 162: Solar and Renewable Energies Prof. Raghuveer...

Physics 162:

Solar and Renewable Energies

Prof. Raghuveer Parthasarathy

[email protected]

Winter 2009

Physics 162: Solar and Renewable Energies R. Parthasarathy Winter 2009

January 15, 2009

Lecture 4: Announcements

• Reading: Wolfson, Chapter 3

• Homework: Problem Set 2. Due Today, Jan. 15

• Homework: Problem Set 3. Due Thurs., Jan. 22

• Quiz (30m) Tuesday Jan. 27

• We encourage you to come to office hours!


HydrocarbonsWhich of these is a molecular structure for natural gas?


CH HH

H

H H H HHCHH

HCH

CH

HCH

CH

CH

HCH

HCH

CH

H

A

B

HydrocarbonsWhich of these is a molecular structure for natural gas?


CH HH

H

H H H HHCHH

HCH

CH

HCH

CH

CH

HCH

HCH

CH

H

A

B

Methane (CH4) (Natural gas)

Nonane (C9H20) (a “typical”molecules in oil (petroleum))

Combustion

• Combustion (burning) of fossil fuels:hydrocarbon + oxygen

→ carbon dioxide + water + Energy


Same reactions apply to biofuels also, as we’ll see.

More specifically...

A bit of chemistry

• Combustion (burning) :#hydrocarbon + #O2 → #CO2 + #H2O + Energy

• “#” indicates the number of molecules involved in the reaction

• E.g. burning methane1 CH4 + 2 O2 → 1 CO2 + 2 H2O + Energy

• Note: The same number of C’s (1), H’s (4), and O’s (4) on each side of the equation – the atoms rearrange what molecule they’re part of.


An oxygen molecule, for example, is composed of 2 oxygen atoms

A bit of chemistry

• Combustion (burning) :#hydrocarbon + #O2 → #CO2 + #H2O + Energy

• E.g. burning nonaneC9H20 + # O2 → X CO2 + # H2O + Energy

• What number must X be?


A. 2B. 9C. 20D. not enough information

Need same number of carbon atoms on each side

A bit of chemistry

• Burning 1 methane molecule, we produce 1 CO2

molecule; burning 1 nonane molecule, we produce 9 CO2 molecules

• Thinking a further about the consequences of this (homework), we can figure out the reason for the following graph:


Carbon content• CO2 emission per energy released:

• Coal is the highest. (A consequence primarily of the high carbon content of coal molecules, and also the somewhat lower energy density of coal.)

• Things we dislike about coal:– lowest energy density

– highest pollution

– highest CO2 emission


Fossil Fuel Problems• What’s problematic about fossil fuels?

• Several things*:– A finite, non‐renewable resource

– Pollution and “local” environmental damage

– Combustion generates greenhouse gases , esp. CO2 (Climate Change)

* Discussed in detail in Phys. 161


The Greenhouse Effect

• Everything emits “electromagnetic radiation”(i.e. light), whether mostly visible light (e.g. the sun) or invisible (e.g. you, Mt. Hood, a butterfly: infrared light). (We’ll discuss this more later.)

• Demo: Thermal camera. Detects infrared. (Amount depends on temperature.)


The Greenhouse Effect• Certain atmospheric gases (e.g. carbon dioxide, CO2) absorb infrared light. These gases are called greenhouse gases.

• Absorption of the Earth’s emitted infrared radiation → increased terrestrial temperaturerelative to what it would be with no atmosphere. (Recall demo: warmer chamber w/ glass, an IR absorber)

• This is the Greenhouse effect – a well understood consequence of Earth’s atmospheric composition.


Climate Change• Fossil fuel combustion→ Increasing atmospheric CO2 → increased greenhouse effect→ increased global temperature (& other climatic changes).

• Physics of climate change is very well understood, and just the above chain of reasoning alone would lead us to worry.– i.e. it’s hard to imagine vastly increasing atmospheric CO2 and not increasing greenhouse warming; rather like putting on a thick sweater and expecting not to be warmer!


Climate Change

• In addition, there is lots of evidence for global climate change, which we’ll very briefly look at.

• The textbook has a very good discussion, if you’re curious; or take Phys. 161.


Carbon Dioxide• Changes in atmospheric CO2

• Is the Earth’s temperature rising?


Temperature Measurements• Last ≈200 years: direct temperature measurements• More distant past? We don’t have direct measures

• But we have many proxies that allow us to reconstruct past temperature

Rise: ≈ 0.7 °C over the last 100 years


Temperature Proxies• Proxies for temperature

– Want processes that are well‐understood

– Want records that overlap w/ modern records, so we can verify their accuracy

• Many proxies for temperature– Tree rings (width, density depend on T)

– Coral reefs (similarly, annual deposits)

– Lake sediment layers (thickness, composition)

– Pollen (tells species present)

– Isotope ratios from ice cores


Temperature: 1000 Years• Past 1000 years from many independent proxies

– Good agreement with each other!

– Good agreement w/ instrumental record

– Last few decades: unusually warm

black: instrumental record


Rise: about 0.7 °C over the last 100 years

Temperature: Further back• Reconstruct more distant history: ice core data

Interglacials; Ice Ages

Note scale: Ice ages, w/ 2 miles of ice over Chicago: ΔT only ≈ 7°C colder !

same source as text 14‐06: Petit et al., 1999


Temperature: Further back• Small changes in average temperature can correlate with large effects on the planet

• “But the weather often changes by 10 degrees in a day!” – But weather and climate aren’t the same

• Weather: Local, short‐timescale phenomena. Lots of variability

• Climate: Regional & global, long‐timescale phenomena.

• Don’t use your “intuition” about weather to assess impacts on climate


Temperature and CO2• We have T, CO2 data. Let’s plot them together...

Now:

385 ppm


A strong correlation

Are we to blame?• Reasons for attributing climate change to humans

– Basic physics of the greenhouse effect + known CO2increase due to fossil fuel use

– “Fingerprints” of changes due to greenhouse effect, e.g. stratospheric cooling. (Which wouldn’t occur, e.g., from variations in the sun’s output.)

– Lack of correlation between recent temperature and variations in solar activity, volcanic dust (next slide), but very good correlation before ≈1950

– Climate modeling. (Many independent models.) E.g. start modeling the past; does it predict the present?


Models: simulate climate w/ & w/o including anthropogenic forcings. Good agreement w/ only natural forcing up to ≈1950, then natural forcing insufficient to match observations.

Are we to blame?

• Earth’s climate is rapidly changing

• Basic physics: well understood– i.e. It would be very surprising if dumping lots of CO2 into the atmosphere did not lead to warming!

• Mid 1990’s: Most scientists conclude “a discernible human influence on climate” (IPCC* Second Assessment Report)

* Intergovernmental Panel on Climate Change: 100s of scientists from many fields, & policymakers. Very good reports, summaries assessing climate knowledge. (2007 Nobel Prize). [See text box 13.1]


Are we to blame?

• By 2001: Becomes clear that “most of the warming... is attributable to human activities”(IPCC 3rd Assessment Report); Independent report from U. S. National Academy of Sciences agrees.

• 2007: IPCC 4th report “Warming is unequivocal...”


Projections: Temperature

• The global temperature is already rising, and this will continue. How much will it rise?

• According to the IPCC and others, the present CO2

concentration already commits the planet to a long‐term* warming of ≈ 2.4 °C, with a 90% probability that it’s between 1.4 and 4.3 °C.

• This is the “best case” for the future; if we don’t reduce CO2 emission, it will be worse.


* takes ≈ 100 yrs. to stabilize

Consequences of Climate Change• Changes in temperature (not uniform over globe)

• More extreme temperature events

• Changes in precipitation, weather

• Sea level rise

• Ocean circulation changes, e.g. the “thermohalinecirculation” that keeps N. Europe warm is strongly dependent on temperature

• And more...


What can we do?

• Carbon capture & sequestration– Prevent the release of CO2 to the atmosphere

– Difficult, expensive

• “Planetary engineering” – manipulating climate– Very risky

• Alternative Energy Sources!

• Using less energy!


Next...

What is energy?


break + clicker registration

Clicker registration

• Notes student (from roster) associated with each clicker (serial number)

• Just click what’s indicated (two letters) when your name scrolls by

• We can repeat this later if needed.

• Starting last time: Clicker participation grade –i.e. no penalty for wrong answers.


Energy• Power is the rate of energy usage (or production)

• What is energy?

• Colloquially, “the ability to do things”

• Technically, a more precise (though similar) definition

• We’ll look at various physical concepts that are “simple.”


Some comments on “simplicity”

• (Don’t bother taking notes on this.)

• I try to stress that a lot of what we do is “simple.” This does not mean that it’s “easy”or obvious.

• “Simple” and “easy” are not the same thing!

• Huh?


Some comments on “simplicity”• This...

• ... is a map of biochemical interactions in a cell. It is very complicated.

• The political and economic factors that led to World War I are complicated.

• Many people think, mistakenly, that most of science is like this: very complicated, dizzying relations that require knowing lots of “stuff” to figure out.



• This is not true! ... and this is something I very much want to convey in this course.

• One can understand a lot about science and technology by understanding a few “simple”concepts – but understanding them very well.



• It is sometimes difficult to grasp simple concepts. This is what I mean when I say “simple” is not the same as “easy.”

• But this difficulty is not due to missing or not knowing some sort of complex relations, but is rather due to not absorbing the simple relations.

• People sometimes find this hard to accept!



• So when I talk about things being simple, what I'm hoping is that you will see that we don’t fill boards full of equations or memorize lots of facts, but that thinking carefully about a few things gets us very far.

• I am hoping that you realize “hey – this is simple” in the sense that you realize that, even without a background in science, you can reason about deep and important things.


Energy and Work• Energy is...

• ... the capacity to do work

• What is work?

• Again, we have an intuitive grasp of this, e.g. pedaling a bicycle uphill requires work.

• Technically, a more precise definition: Work = Force x Distance (where the force is applied in the same direction as the direction of motion)

• We’ll define force in a moment...


Work•Work = Force x Distance

• For example: If I drag a box, pulling with 10 lbs. of force for 5 feet, I have performed 10×5 = 50 ft. lbs. of work.

• Ft. lbs. is a unit for measuring work.

Force

motion


Work• Work = Force x Distance

• In SI units: If I apply 4 Newtons (N) of force over 3 meters (m) I have performed 12 N m of work. (Newton is the SI unit of force.)

• The SI unit of work is the Joule (J). 1 N m = 1 J (definition).

(So above, 12 J of work.)

• With 12 J of energy, I can do 12 J of work.

• Note: same units for work, energy

1 N ≈ 5 lb.


WorkPhysics 162: Solar and Renewable Energies R. Parthasarathy Winter 2009

• Work = Force x Distance

• A compressed spring exerts force.

• Suppose a compressed spring is sandwiched between two boulders, and the boulders don’t move at all.

• Does the spring do any work?

A. yes

B. noZero distance, so zero work

istockphoto.com



• Suppose I am sandwiched between two boulders, pushing. The boulders don’t move, but I get tired.

• Am I doing any work? What seems reasonable?(Not obvious)

istockphoto.com



• Suppose I am sandwiched between two boulders, pushing. The boulders don’t move, but I get tired.

• Am I doing any work? What seems reasonable?

A. I must be; I’m getting tired.B. No: zero distance, so zero workC. “B” is true, so there’s zero work associated with the boulders. But perhaps things in my body are moving, and I’m doing work on them?

Work• Work = Force x Distance

• Answer: I’m doing zero work on the boulders.• It feels like I’m “doing work,” but from a physics point of view I’m doing zero work on the boulders. (It feels like work for physiological reasons... movement of “motor proteins” in your muscles doing work on other proteins, pulling, letting go, pulling, letting go...)

• So be careful about your “intuition” about work, energy, etc.


Force• What is force?

• Let’s pause, and look at our air cart... Cart on a track, very little friction.

• Sitting still... no force, and no motion

• I’ll tap the cart, briefly applying a force, after the tap ends, there is no force. At this point what do you think will happen?


A. The cart will stop

B. The cart will move steadilyTry it...

Force• With no force, the cart moves steadily, at a constant velocity (speed).

• Newton’s first law of motion: A body at rest tends to stay at rest unless a force acts on it; a body moving at constant speed tends to stay at constant speed unless a force acts on it.


Friction• It took people thousands of years to realize this (until Isaac Newton in the 1680’s). Why? Because in typical experience, the object will stop. Why?

• Because friction provides a force that changes the speed of the object, slowing it down. (We’ll see what friction is later.)

• With experiments controlling friction, and thinkingabout what would happen if friction were reduced to zero, we (Newton) realized that in the absence of all forces, speed remains constant.


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