course webpage: course grading policy you will want to get mastering physics and an iclicker
TRANSCRIPT
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Course Webpage: http://panda.unm.edu/Courses/Thomas/Phys161fa10/P161Syllabus.htm
Course Grading Policy
You will want to get Mastering Physics and an iClickerRegistration info on the webpage
Key Points:
Seven Midterms (Drop lowest score)No makeup examsScantron but NOT multiple choiceModel Exams Posted at least 2 weeks before midterm0.1% of missed exam points for each problem done/ clicker quiz
(up to a maximum of 50%)
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Chapter 17 Young&FreedmanSome things are hot, some things are cold.
Heating (usually) causes expansion.
In thermal contact, two objects (eventually) reach the same temperature.
Daniel Gabriel Fahrenheit (1686–1736)Fahrenheit temperature:
The zero point was determined by placing the thermometer in brine: a mixture of ice, water, and ammonium chloride, a salt. This is a frigorific mixture.
96 degrees, was the level of the liquid in the thermometer when held in the mouth or under the armpit of his wife.
Celsius used ice/water and water/steam for 0 and 100.
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Gases also expand on heating.
Demo
Kelvin temperature
€
TF =9
5TC + 32°
TK = TC + 273.15 Units: K
The ‘size’ of 1K = 1°C
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What is the approximate boiling temperature of water, in K?
A] 100 K
B] 212 K
C] 273 K
D] 373 K
E] 485 K
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Solids also (usually) expand on heating.
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If I heat this metal annulus,The hole will:
A] get smaller
B] get bigger
C] stay the same size
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Last time: Heat = Energy (that is transferred because of a difference in temperature).[The temperature difference may be VERY small!]
“Specific Heat”, c, is the amount of heat needed to raise the temperature of a mass of material a degree (or a Kelvin)
Q=mc
“Latent heat” (of vaporization, or of fusion) is heat per mass of material needed to boil (vaporize) or freeze (fuse).
Today: equilibration of water & icemechanisms of heat transfer
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Mixing Ice & Water
Mechanisms of heat transfer
1. Conduction 2. Convection (flow of fluid, like air)3. Radiation
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Two square (in cross-section) glass rods connect very large copper blocks held at
different temperatures.
Which rod conducts heat faster?
A] Rod A
B] Rod B
C] They conduct heat at the same rate
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Radiation.
Hot objects emit light (glowing red hot, white hot, light bulb filaments, etc.)
€
dQ
dt= AeσT 4 e = emissivity, e=1 is a ‘BLACK BODY’
Room temp: infrared emission Very hot: white emission VV hot: blue
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We can measure (approximately) the temperature of an object by looking at
its black body spectrum. (This assumes that emissivity is
independent of wavelength, which is often nearly true.)
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If an object can emit EM radiation (light, or infrared or UV etc.),then it must also absorb EM radiation.
Consider an object in a vacuum box with perfectly reflective walls at temperature T. In thermal equilibrium, the object is emitting
radiation:
Since it does not change temperature, it must absorb the same amount of radiation.
So e= emissivity = absorptivity!Black objects heat up faster in the sun, but cool off faster at night! €
dQ
dt= AeσT 4
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The ideal gas law:
A] Works with T (temp) in celsius or Kelvin, but you need to use a different R for each.
B] Works with T in °C, °F, or K, but you need to use a different R for each.
C] Works with T in °C or K, but if you use °C you need to use the “gauge pressure”, not absolute pressure
D] Works only and exclusively with T in Kelvin.
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An ideal gas at 200 K occupies 2 liters at a pressure of 1 atm.
If the gas is compressed to 1 liter of volume, what will be its temperature?
A] 200 K
B] 400 K
C] 800 K
D] There is not enough information to determine this!
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An ideal gas at 200 K occupies 2 liters at a pressure of 1 atm.
If the gas is compressed to 1 liter of volume, and the pressure is 2 atm, what will be its temperature?
A] 200 K
B] 400 K
C] 800 K
D] There is not enough information to determine this!
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20 liters of Argon are in thermal equilibrium with 20 liters of Helium. (These are monatomic gases, Mar=40, Mhe=4.)
Which molecules have more kinetic energy, on average?
A] Argon
B] Helium
C] Both have the same
Which molecules are moving faster, on average? HELIUM
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MON Aug 30
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W= Work Done BY a Gas =
€
pdVV1
V2
∫Memorize!
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When gas does work, it loses internal energy
(unless energy is added, via heat.)
When it does negative work, it gains internal energy
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A gas in a piston is taken from state 1 to state 2. The outside pressure is higher than the pressure in the cylinder.
For which path does the gas do the largest positive work? A
For which path does the gas do the most negative work?(I.e. for which path is the most work done ON the gas)
Or chooseE] no path does work of this sign
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In the isobaric process shown, W is: +
A] +
B] -
C] 0
D] cannot determine
In the isobaric process shown, U is: +
In the isobaric process shown, Q is: +
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In the isochoric process shown, W is: 0
A] +
B] -
C] 0
D] cannot determine
In the isochoric process shown, U is: -
In the isochoric process shown, Q is: -
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In the isothermal process shown, W is: +
A] +
B] -
C] 0
D] cannot determine
In the isothermal process shown, U is: 0
In the isothermal process shown, Q is: +
Let’s do this quantitatively.
Wed Sept 1
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On expanding isothermally from 2L to 4L, an ideal gas does 6J of work, as the pressure drops from 2 atm to 1 atm.
By how much must it expand to do an additional 6J of work?
A] It must expand an additional 2L, to reach 6L
B] It must double again, to 8L
C] It must increase four-fold, to 16L
D] It can do no more work, as it has reached 1 atm.
E] Cannot determine without knowing T
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In the mystery path process shown, W is: D
A] larger than for an isothermal process from A->B
B] smaller than for an isothermal process from A->B
C] 0
D] cannot determine
In the mystery path process shown, U is: 0
In the mystery path process shown, Q is: D
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How much work is done when an ideal gas is expandedfrom V1 to V2 at constant pressure?
A] 0
B] nRT2
C] nRT1
D] nR(T2-T1)
E] it depends on whether is it monatomic or diatomic.
What is the difference in internal energy of the gas at points 1’ and 2 ? A
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For the cyclic process shown, W is: D
A] 0, because it’s a loop
B] p0V0
C] - p0V0
D] 2 p0V0
E] 6 p0V0
For the cyclic process shown, U is: A
For the cyclic process shown, Q is: DFor ONE cycle:
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Fri sept 3
For the constant pressure ideal gas process shown, The change in internal energy U of the gas is
A] nCv
B] nCp
C] 0
D] cannot determine
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If the gas is monatomic and ideal, for the constant pressure process shown, the change in internal energy can also be expressed as:
A] pV
B]
C]
D] €
12 pΔV
€
32 pΔV
€
52 pΔV
(use a paper & pencil)
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A non-ideal gas is taken from A to B. The gas does 3 J of work; 6 J of heat is added to the gas.What is the internal energy of the gas at B?
A] 3 J
B] 9 J
C] 10 J
D] cannot determine, since gas is non-ideal
E] the numbers given are not physically possible
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A non-monatomic ideal gas is taken from A to B. The gas does 3 J of work; 6 J of heat is added to the gas.What is the internal energy of the gas at B?
A] 3 J
B] 9 J
C] 10 J
D] cannot determine, since gas is non-monatomic
E] the numbers given are not physically possible