2011. The greenhouse effect. The greenhouse effect. The greenhouse effect in your car. The greenhouse effect in your car

10/24/2011 The greenhouse effect EM radiation so far • Spectrum of EM radiation emitted by many objects may be approximated by the blackbody spectru...
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10/24/2011

The greenhouse effect

EM radiation so far • Spectrum of EM radiation emitted by many objects may be approximated by the blackbody spectrum • Blackbody spectrum (plot of emitted intensity versus l)depends only on temperature • Temperature determines a) Total emitted EM power  S-B law: P = s a e T4 b) Shape of spectrum range of l, l peak  1/T

First part of today’s lecture • Apply BB spectrum and S-B law to understand green house effect/global warming • What determines the temperature of the earth’s surface? • Why do greenhouse gases in the atmosphere cause it to heat up?

Lecture 19 : Greenhouse effect Begin static electricity

Reminders: HW 8 due Monday 31st at midnight Extra evening class tomorrow, 7pm, G1B31 Reading quiz in a moment

The greenhouse effect.

Rest of lecture • Begin static electricity

Your car is sitting in the bright sun. The inside of the car gets much hotter than the air next to it. Why?

What are green house gases? CO2, H2O and others

Why does the temperature of the earth‟s surface depend on the presence of green house gases in the atmosphere? Greenhouse gases insulate the earth, increase its temperature (Without them, life would not survive.) Exactly how does the temperature change now we are adding more CO2? Details of this get complicated But the basic GH effect can be understood using just the BB spectrum and SB law a. The car absorbs light energy from sun better than the pavement. b. Sunlight causes chemical reactions in car materials that give off heat. c. Electrical appliances such as clock that run all the time in car causes it to heat up. d. The windows let in energy but do not let it escape. e. None of the above make sense, must be different explanation.

The greenhouse effect in your car

The greenhouse effect in your car

EM energy gets into car but can‟t get out? How does that happen?

BB spectra of emitted EM radiation from sun and car

• We all know visible light travels easily through glass (windows!) • But remember this? The glass blocked the IR from the hot plate, so we could not detect the IR from the hot plate.

UV Visible power

IR

sun 40 C inside car wavelength • Remember effect of T on shape of BB spectrum • Sun is very hot (5800K) and emits most power as visible light, which goes through the car window. • The interior of the car absorbs this energy and heats up. • Inside of car is less hot than sun (310 K) so emits most power in the IR, which cannot go out through the glass. • Extra energy is trapped inside car making it hotter than the surrounding environment

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How does the green house effect affect the temperature of the earth? 1) Power in: Visible light energy from hot sun hits the earth and is absorbed 2) Power out: The cool earth (at temp. T E) radiates power out mainly as IR radiation 3) If earth remains at a constant temperature, it must have constant thermal energy:

Power in from sun = Power out from earth, radiated into space.

How does the green house effect affect the temperature of the earth? Power in from sun = Power out from earth, radiated into space. First lets calculate the power arriving at the earth from the sun: If Re is the radius of the earth, what is the area of the earth absorbing sunlight?

Since power out depends on T E, we can calculate temperature of earth!

a) b) c) d)

How does the green house effect affect the temperature of the earth? Power in from sun = Power out from earth, radiated into space.

2pRe pRe2 4pRe2 4/3 p Re3

How does the green house effect affect the temperature of the earth? Power in from sun = Power out from earth, radiated into space. Now lets calculate the power radiated from earth at temperature T E:

Power in from sun = 1380 W/m2 x pRe2 x 0.7 = 1.22 x 1017 W

If Re is the radius of the earth, what is the area of the earth that emits EM radiation (mainly IR)? a) 2pRe b) pRe2 c) 4pRe2 d) 4/3 p Re3

solar power/m2 at earth area of earth absorbing sunlight fraction of sunlight absorbed by earth

How does the green house effect affect the temperature of the earth? Power in from sun = Power out from earth, radiated into space.

How does the green house effect affect the temperature of the earth?

From Stefan Boltzman law:

How hot would earth be if no greenhouse gases (f = 1)? (All the IR radiated from earth lost directly to space)

Power out from earth to space = s x TE4 x surface area of Earth X f

Power in from sun

Fraction of radiation that actually makes it through atmosphere to space.

= Power out from earth, radiated into space.

1380 x pRe x 0.7 = s X TE4 X 4pRe2 X f  TE4 = (1380 W/m2 X 0.7) / (4 X s X 1) W/m2

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TE = sqrt (sqrt ( 4.26 x 109 K4)) = 255 K = -18 C ~ 0 F

If there are no greenhouse gases in the atmosphere, f = 1. What temperature of earth would this imply?

• Water would freeze! Life would be impossible! • The surface of the earth is not this cold, because the atmosphere contains greenhouse gases.

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The green house effect around the earth

The green house effect around the earth • Why doesn‟t all of the radiated IR go directly into space? • Certain types of molecules in the atmosphere called “greenhouse gases” absorb IR and send it back towards earth. Carbon dioxide, H2O, but not oxygen or nitrogen.

Power in from Sun = Power out from earth, radiated into space

1380 W/m 2 x pRe2 x 0.7 = s x T E4 x 4pRe2 x f Power radiated from surface

Solve to get TE4 = (1380 W/m 2 x 0.7)/(4 x s x f) TE = sqrt (sqrt (4.26 x 109 /0.61 )) = 289 K = 16 C ~ 61 F

• Only about 61% of IR radiated from surface of earth makes it to space (f = 0.61) • However to keep a stable temperature at the surface, Pout = Pin , so when we add GH gases, earth must radiate more power from surface to ensure that the same amount makes it through the atmosphere • From SB law – surface will radiate more power if TE rises. • How much does TE rise due to naturally occurring GH gases?

Very close to actual value of 287 K Naturally occurring greenhouse gases keep the planet habitable.

When we increase the concentration of GH gases in the atmosphere by burning fossil fuels etc, what happens? a. b. c. d. e.

Less IR reflected back to earth, (lower f value) Less IR reflected back to earth, (higher f value) More IR reflected back to earth, (lower f value) More IR reflected back to earth, (higher f value) None of the above

When we increase the concentration of GH gases in the atmosphere by burning fossil fuels etc, what happens?

To maintain the SAME amount of EM power reaching outer space after the man-made increase in GH gases, what changes must occur at the surface of the earth? a. b. c. d. e.

After the concentration of greenhouse gases has gone up due to burning fossil fuels etc, the total power emitted by earth to space, a. goes up b. goes down c. stays the same

More EM power must be emitted  higher surface temperature More EM power must be emitted  lower surface temperature Less EM power must be emitted  lower surface temperature Less EM power must be emitted  lower surface temperature Neither emitted power or surface temperature changes

If we look at Earth from space with temperature probe and measure amount of IR radiation coming off of earth to measure temperature of earth, what temperature do we measure? a. 255K

b. 273 K

c. 295 K

d. 2000 K

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Now add clouds to the simulation. What is their effect?

Static electricity

a. Clouds decrease temperature because they reflect part of sunlight back to space. b. Clouds increase temperature because they absorb IR radiation from earth. c. Clouds increase temperature because the sunlight is reflected then absorbed by the greenhouse gases in the atmosphere. d. a or b, e. a or c

GH gases: - Transmit all EM power from sun at visible l - Partially reflect EM power from surface of earth at IR l - Surface temperature must rise in presence of GH gases to maintain power balance

Atoms, Electrons and Ions

Source of electricity: Electric charges Everything (earth, you, the table etc) made of tiny particles called atoms Atoms are made up of 3 even tinier particles:

Electron “cloud”. Negatively charged Occupies most of volume of atom

The atom

Electrons, neutrons and protons Particle

Charge

Mass

Electron

-e

9.11×10-31 kg

Proton

+e

1.67×10-27 kg

Neutron

0

1.67×10-27 kg

Nucleus Contains protons and neutrons Positively charged 99.9% of mass is concentrated in tiny nucleus,

e = 1.6 × 10-19 Coulombs Coulomb (C) is the unit of charge

Atoms have equal number of protons and electrons, so total charge is zero. They are electrically neutral

Static electricity: What happens when charges are stationary Electricity (and electric currents): What happens when charges (usually electrons) are moving

The same is true of most ordinary objects (made of atoms.) You can pull electrons off atoms. This leaves 2 charged particles : - Unbound (free) electrons (negatively charged) - Positively charged ions (ion = atom with unequal number of protons and electrons)

atom

+e +ve ion

Electrostatic force between charged particles

Electric Hockey Simulation!

-

-

F

-

F

F

+

Consider 2 „point‟ charges, A and B. What force does charge A feel?

qA

r

Forceof B on A = kqAqB r2

Place charge (B) 2cm from charged puck (A). See charged puck fly away Now place charge (B) 1 cm away from charged puck (A). Compared to previous situation force on A will be: a. half as large, b. same size, c. twice as large, d. four times larger e. something else.

qB

Observed behavior: - Force depends on qA and qB : More charge, more force - Force depends on distance between them (r) : More distance, less force Coulomb’s Law:

Forceof B on A = kqAqB r2

opposite charges attract

like charges repel F

-e Free electron

• Describes the force between 2 point charges • k is Coulomb constant = 8.99 x 109 N m2/C2 • qA and qB are amount of charge in coulombs (C ) • r is separation in m 1 Coulomb = 6 x 1018 electron charges!

Place charge (B) 1cm away from charged puck (A) as in previous Q. Add a second charge to B, right on top of first. Compared to previous question, force on A is: a. ½,

b. same,

c. x 2,

d. x 4,

e. something else.

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B

A electron

-

proton

+

0.001 m

What is force between them? Calculate and write down answer. Forceof B on A = kqAqB r2

-

k is Coulomb constant = 8.99 x 109 N m2/C2 e = 1.6 x 10-19C

B

A electron

proton 0.001 m

+

What is force between them? Calculate and write down answer. Forceof B on A = kqAqB r2

Force between particles = What does the minus sign mean? a) Force on electron points to left b) Force on electron points to right

Summary of electrical materials:

Summary of electrical materials:

In a conductor (think: metal), free electrons can move around

In an insulator, electrons remain bound to atoms

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-

+-

+

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