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GEOL 1302, Week 3 Notes

by: Theresa Nguyen

GEOL 1302, Week 3 Notes GEOL 1302

Marketplace > University of Houston > Geology > GEOL 1302 > GEOL 1302 Week 3 Notes
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Energy is expressed in units of joules (J). Power is the rate that energy is flowing, and it is express in watts (W); 1 W = 1 J/s. Temperature is a measure of internal energy of an object and is fr...
Intro To Global Climate Change
yunsoo choi
Class Notes
Radiation and energy balance
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This 6 page Class Notes was uploaded by Theresa Nguyen on Sunday September 11, 2016. The Class Notes belongs to GEOL 1302 at University of Houston taught by yunsoo choi in Fall 2016. Since its upload, it has received 28 views. For similar materials see Intro To Global Climate Change in Geology at University of Houston.


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Date Created: 09/11/16
Chapter 3: Radiation and Energy Balance Energy  To a physicist Energy is the capacity to do work. - lifting a weight, turning a wheel, compressing a spring  Units of energy are commonly expressed in as joule (J)  Energy in food is expressed in kilocalories (kcal), 1kcal = 4184 J.  Rate of energy movement is referred to as power.  Power is expressed in watts (W).  1 W = 1 joule per second = 1 J/s  So a 60-W light bulb consumes 60 J of energy every second (60 J/s).  Difference between power and energy: - Gallon is measure of quantity of water, like J is measure of quantity of energy - Rate of water flow through a pipe could be Gallons per minute. - Watts is rate of energy flow. How much power to run a human body?  Typical human consumes approximately 2,000 food calories (2,000 kcals)  2,000 kcals x 4,184 J/kcal = 8,368,000 J = 8.37 MJ  One day has how many seconds? - 60sec/min x 60min/hr x 24 hr/day = 86,400 sec/day  Power = W = J/s  8,368,000 J / 86,400 sec = 97 W  Therefore, typical human requires roughly 100 W to power his/her body for one day.  1 horsepower (hp) = 740 W  So 97 W/ 740W = 0.14 hp to run your body for one day. Temperature and Internal Energy  Internal energy refers to how fast atoms and molecules in object are moving.  In a cup of water (liquid) if molecules are moving slowly, then has a low internal energy. - If molecules are moving faster, then has high internal energy.  In a solid, the movements of atoms are approximately fixed by intermolecular forces, however atoms can still move small distances around their “fixed” positions.  The faster the atoms move about their fixed positions, the higher the internal energy.  And the higher the temperature. Kelvin Temperature  Celsius (C) temperature scale used by Climate Scientists, but physicists more commonly use the Kelvin (K) temperature scale.  K = C + 273.15  Therefore, freezing temperature of water is 0°C = 273.15 K. - Boiling point of water is 100°C = 373.15 K - Room Temperature is approximately 22°C = 295 K  Most temperatures in Earth’s atmosphere are between 200 K and 300 K. - The average surface temperature of the Earth is roughly 288K.  Physicists prefer Kelvin scale because temperature expressed in Kelvin is proportional to internal energy. - Thus if the temperature doubles from 200K to 400K, the then internal energy of the object also doubles.  This is not the case when temperature increases from 10°C to 20°C. - Why not?  What is the significance of 0 K? - Absolute zero, where object has an internal energy of 0.  Meaning the molecules of the object are not moving.  Because of the direct relationship between Kelvin temperature and internal energy, we will use Kelvin temperature in future climate calculations. Electromagnetic radiation  Sun is primary source of energy on our planet, and is 150 million km away from Earth. - How does energy from the Sun reach the Earth?  Energy is transported from the Sun to the Earth by what is known as electromagnetic radiation.  Electromagnetic (EM) radiation includes visible light, like the light from a flashlight or the Sun; X-rays that allow us to detect broken bones; microwaves that cook your dinner; radio-frequency waves that bring calls to your cell phone and WiFi to your computer.  EM radiation emanating from a flashlight, or lamp, or WiFi router, or the Sun is really a stream of photons. - Small discrete packages of energy. - As photons travel from point A to point B, a small amount energy is transported.  The wavelength (or energy) of a photon determines how photons interact with the physical world.  Photons with shorter wavelength (higher frequency) have higher energy.  Sun emits VISIBLE radiation, which is NOT absorbed by gases in Earth’s atmosphere.  Earth emits INFRARED radiation, which IS partially absorbed by greenhouse gases in Earth’s atmosphere. Blackbody radiation  Sun and lamp on your desk are emitting photons.  They are not the only things around you that are emitting photons. - In fact, everything around you is emitting photons all the time.  Right now, you are emitting photons, as are the walls of this room, the chairs, the books, everything.  An object emits photons with the wavelength determined by the object’s temperature.  In science we describe idealized objects called a blackbodies. - A blackbody (an opaque and non-reflective body) when held at constant, uniform temperature; emits radiation at a specific spectrum and intensity that depends only on the temperature of the body.  Energy emitted (W) from BB object at room temperature (27°C). - Grey shaded area is 0.4 –0.7 microns or visible radiation.  Thus, all room-temperature objects are emitting photons, but you cannot see these photons, this is the origin of the term blackbody.  At room temperature, the object appears black because emitted photons are not visible by humans.  Peak of the emissions spectrum for 300-K object is near 10μm.  Object also emits photons over a range of wavelengths.  This object is 1600K, peak is close to 2 microns.  Can humans “see” this object?  Simple relationship between temperature of an object and wvl of peak emission. Wien’s displacement law 3000 λ max = T  Peak of the emissions spectrum for 300-K object is near 10μm.  What would the answer be if we used °C instead of Kelvin?  What happens as an object heats up?  The peak of its emission spectrum moves to shorter wavelengths.  What is the peak emission for a 1500-K object? - A 3000-K object? A 6000-K object? Blackbody radiation  Visible radiation, human eyes, and chlorophyll, etc.  If eyes unable to see IR, how can you read your textbook? Incandescent lightbulb  85% of photons are emitted in the Infrared wavelengths.  How to make light bulbs more efficient? - Run at higher temperature? - Add halogen gas? - Fluorescent light bulbs –4 to 6 times more efficient. - 12W CFL produces same visible light as 60-W incandescent. - 2007 law phasing out standard incandescent bulbs by 2014. Power emitted by black body  Not only is peak wavelength change with an object’s temperature, but total power emitted also increase with temperature.  This is similar to relationships used by infrared thermometers and by astronomers to infer the temperature of distant stars and planets. Infrared emitted by object  What is room temperature in °F? In °C? In K? - 18°C = 191K  What is the temperature of a living human body? - 37°C = 310K  Simple relationship between total power radiated by a blackbody and its’ temperature. Stefan-Boltzmann equation P 4 α =sT  P/a is the power emitted by a blackbody per unit of surface area, W/m2  σ is the Stefan–Boltzmann constant, with σ = 5.67 ×10−8 (W/m2)/K4  T is the temperature of the object in degrees Kelvin.  If you multiply P/a by the surface area a of the object (in square meters), then you get the total power emitted by a blackbody, in watts.  300K vs 600K object? 4 P = s T a  P is the power emitted by a blackbody Watts (W)  σ is the Stefan–Boltzmann constant, with σ = 5.67 ×10−8 (W/m2)/K4  T is the temperature of the object in degrees Kelvin  a is the surface area of the object (in square meters) Energy balance  One of the cornerstones of modern physics is the 1st Law of Thermodynamics: “Energy is conserved”.  In other words, if an object loses some energy, then some other object must gain the same amount of energy.  Furthermore, because photons are packets of energy, when an object emits a photon, the emitting object’s internal energy must decrease.  Because temperature is a measure of internal energy, the emission of a photon causes the temperature of an object to decrease (and vice versa).  If energy flowing out of an object (energy out) exceeds energy flowing into an object (energy in), then the internal energy (and temperature) of the object decreases.  Change in temperature (or change in internal energy) = energy in – energy out Conservation of money  Think of your checking account.  Money such as your paycheck or birthday check from your grandmother is deposited into your account.  At the same time, money is withdrawn to pay for your cell phone bill. The change in your bank balance is equal to the difference between total deposits and total withdrawals.  Change in bank balance = money in – money out.  If money in = money out, then no change in bank balance that month. Cooking a frozen turkey T 4 P = s a  Preheat oven to 375°F = 191°C = 464 K.  Frozen Turkey 37°F = 3°C = 276 K  Turkey has surface area = 0.1 m2  Oven has surface area = 1 m2  σ = 5.67 ×10−8 (W/m2)/K4  Turkey emitting 33 W.  Oven at 464K emitting 2650 W/m2, but Turkey is only 0.1 m2, so only absorbing 265 W.  Turkey is absorbing more energy than it is emitting and therefore heats up.  We will use concepts covered in this class to develop a simple model of Earth’s climate with which we can begin to understand how humans can alter the climate. Chapter Summary  Energy is expressed in units of joules (J). - Power is the rate that energy is flowing, and it is express in watts (W); 1 W = 1 J/s.  Temperature is a measure of internal energy of an object and is frequently expressed by physicists in units of Kelvin. - The temperature in degrees Kelvin is equal to the temperature in degrees Celsius plus 273.15.  Photons are small discrete packets of energy. - They have a characteristic size, known as the wavelength, which determines how the photons interact with matter.  Photons with wavelengths between 0.4 and 0.7 μm are visible to humans; photons with wavelengths between 0.8 and 1,000 μm are known as infrared.  Photons emitted by room-temperature objects are in the infrared and not visible to humans.  Most objects emit blackbody radiation. - The characteristic wavelength emitted by a blackbody is equal to 3,000/T (where wavelength is in micrometers and temperature is in degrees Kelvin).  The total power emitted per unit area by a blackbody is equal to σT4, where σ = 5.67 x 10−8 (W/m2)/K4 and temperature is in degrees Kelvin.  When a photon is emitted by an object and then absorbed by another object, this process transfers a small amount of energy from the emitter to the absorber.  If the energy received by an object by absorbing photons exceeds the energy lost by emitting photons, then the object’s internal energy increases – and it warms up.  The object cools off if the energy in emitted photons exceeds the energy received by absorbing photons.


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