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Temperature and Ideal Gas 1 Everything is made of atoms In gases the molecules dont interact with each other. Simple How does the atomic (molecular) nature.

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Presentation on theme: "Temperature and Ideal Gas 1 Everything is made of atoms In gases the molecules dont interact with each other. Simple How does the atomic (molecular) nature."— Presentation transcript:

1 Temperature and Ideal Gas 1 Everything is made of atoms In gases the molecules dont interact with each other. Simple How does the atomic (molecular) nature of a gas explain its properties? Air in your tires? How hot is hot? How cold is cold?

2 Temperature 2 Heat is the flow of energy due to a temperature difference. Heat always flows from objects at high temperature to objects at low temperature. The quantity indicating how warm or cold an object is relative to some standard is TEMPERATURE (T). T does not depend on quantity of a substance. A cup and a thimble of boiling water both have same T. When two objects have the same temperature, they are in thermal equilibrium.

3 3 The Zeroth Law of Thermodynamics: If two objects are each in thermal equilibrium with a third object, then the two objects are in thermal equilibrium with each other. There is no heat flow between objects in thermal equilibrium

4 Temperature Scales 4 Fahrenheit scale Water boils * 212 F Water freezes * 32 F Absolute zero F (*) Values given at 1 atmosphere of pressure.

5 Temperature Scales 5 Fahrenheit scaleCelsius scale Water boils * 212 F100 C Water freezes * 32 F0 C Absolute zero F C (*) Values given at 1 atmosphere of pressure.

6 Temperature Scales 6 Absolute or Kelvin scale Fahrenheit scale Celsius scale Water boils * K 212 F100 C Water freezes * K 32 F0 C Absolute zero0 K F C (*) Values given at 1 atmosphere of pressure.

7 7 The temperature scales are related by: Fahrenheit/ Celsius Absolute/ Celsius

8 8 Example (text problem 13.3): (a) At what temperature (if any) does the numerical value of Celsius degrees equal the numerical value of Fahrenheit degrees? (b) At what temperature (if any) does the numerical value of kelvins equal the numerical value of Fahrenheit degrees?

9 Question Which is smaller, a change of 1 o F or 1 o C? A)1 o F B)1 o C C)they are the same

10 Molecular Picture of a Gas Atoms and molecules are the basic units of matter We want to explain thermal Properties in terms of atoms and molecules. How many atoms?

11 Molecular Picture of a Gas 11 We can specify the amount of a substance by giving its mass or by the number of molecules (or atoms) it has. If we know the mass of a molecule we can go from one description to the other A golf ball weighs 1.6 ounces. I have a 20 lb box of golf balls I have a box of 200 golf balls Totally equivalent

12 Molecular Picture of a Gas 12 If a sample contains a single substance (element or compound) the number of particles in the sample is N = M/m. N equals the total mass of the sample (M) divided by the mass (m) of the atom (or molecule) The number density of particles is N/V where N is the total number of particles contained in a volume V.

13 13 One mole of a substance contains the same number of particles as there are atoms in 12 grams of 12 C. The number of atoms in 12 grams of 12 C is Avogadros number. It is convenient to have a standard number to facilitate this going back and forth from the two descriptions. Since we deal with human size numbers (gms) this will involve a very large number of atoms

14 14 A carbon-12 atom by definition has a mass of exactly 12 atomic mass units (12 u or 12 amu). 12g = 12u N A u = 1g/(6x10 23 ) =1.66x g =1.66x kg This is the conversion factor between the atomic mass unit and kg (1 u = kg). N A and the mole are defined so that a 1 gram sample of a substance with an atomic mass of 1 u contains exactly N A particles. A mole of O 2 has mass 32 gm., of water m=18 gm Why 12 gm of Carbon?

15 15 Example (text problem 13.37): Air at room temperature and atmospheric pressure has a mass density of 1.2 kg/m 3. The average molecular mass of air is 29.0 u. How many air molecules are there in 1.0 cm 3 of air? The total mass of air in the given volume is:

16 16 Example continued:

17 Question Which contains more atoms, 5 mol. of helium (mass He =4amu) or 1 mol of neon (m Ne =20amu) A) Helium B) Neon C) both have same number of atoms 17

18 Question Which contains more atoms, 1 mol of helium or 1 mol of Steam (water) A) Helium B) water C) both have same number of atoms 18

19 Decrease the volume Increase the pressure 19 Constant T: P ~1/V

20 Increase the number of molecules Increase the pressure 20 Constant V,T: P ~ N

21 We also know that, as you drive, tire pressure increases with T 21 Constant V,N: P ~ T

22 Absolute Temperature and the Ideal Gas Law 22 Constant P: V ~T

23 Absolute Temperature and the Ideal Gas Law 23 Experiments done on dilute gases (a gas where interactions between molecules can be ignored) show that: For constant pressure Charles Law For constant volume Gay-Lussacs Law

24 24 Boyles Law For constant temperature For constant pressure and temperature Avogadros Law

25 What Temperature do we use? 25 There is a lowest possible T V0

26 Absolute Temperature There is a coldest possible temperature Absolute zero. All objects will transfer heat to an object at absolute 0. Experiment show (e.g. V0 ) that the coldest possible T is o C. Kelvin scale measures T from Absolute 0 in units of 1 o C T K =T c +273 o 26

27 27 Putting all of these statements together gives the ideal gas law (microscopic form): k = J/K is Boltzmanns constant The ideal gas law can also be written as (macroscopic form): R = N A k = 8.31 J/K/mole is the universal gas constant and n is the number of moles.

28 28 Example (text problem 13.41): A cylinder in a car engine takes V i = m 3 of air into the chamber at 30 C and at atmospheric pressure. The piston then compresses the air to one-ninth of the original volume and to 20.0 times the original pressure. What is the new temperature of the air? Here, V f = V i /9, P f = 20.0P i, and T i = 30 C = 303 K. The ideal gas law holds for each set of parameters (before compression and after compression).

29 29 Example continued: Take the ratio: The final temperature is The final temperature is 673 K = 400 C.

30 30 Putting all of these statements together gives the ideal gas law (microscopic form): k = J/K is Boltzmanns constant The ideal gas law can also be written as (macroscopic form): R = N A k = 8.31 J/K/mole is the universal gas constant and n is the number of moles.

31 Question When the temperature of a quantity of gas is increased A)the pressure must increase. B)the volume must increase. C)the pressure and/or the volume must increase. D)none of the above

32 Question A pot of water on the stove is heated from 25 o C to 100 o C. By what factor does the temperature in Kelvin change? A)T 2 = 4T 1 B)T 2 = 1.25T 1 C)T 2 = 0.80T 1 D)T 2 = 0.20T 1

33 Question Inside your air-conditioned apartment, you blow up a balloon as large as possible and then take it outside on a hot summer day. The balloon is most likely to then A)shrink. B)remain the same size. C)expand and pop.

34 Question The Kelvin temperature of an ideal gas is doubled and the volume is halved. How is the pressure affected? A) increases by a factor of 2 B) increases by a factor of 4 C) stays the same D) decreases by a factor of 2 E) decreases by a factor of 4

35 Kinetic Theory of the Ideal Gas 35 An ideal gas is a dilute gas where the particles act as point particles with no interactions except for elastic collisions. Point particles can have only KE, no internal PE Add heat (energy) to gas, energy increases KE increases. But if we add heat, temperature also increases. T depends on KE

36 Kinetic Theory of Ideal Gas 36 T~ KE av /molecule if T =absolute 0, molecules dont move. T T T More total energy, but same T, same average KE

37 Temperature is related to average KE This is true even for liquids and solids 37

38 Pressure is caused by collisions 38 Gas particles have random motions. Each time a particle collides with the walls of its container there is a force exerted on the wall. The force per unit area on the wall is equal to the pressure in the gas.

39 39 The pressure will depend on: The number of gas particles ~ N Frequency of collisions with the walls ~ v Amount of momentum transferred during each collision ~ mv P~ Nmv 2 ~N KE mol

40 40 The pressure in the gas is Where is the average translational kinetic energy of the gas particles; it depends on the temperature of the gas.

41 Typical air molecule is moving more than 1,000 Miles/hr. Some move faster some slower

42 42 The average kinetic energy also depends on the rms speed of the gas where the rms speed is

43 What is rms? root mean square v 2 rms is the average of the square of the velocities. It is not the square of the average 43 Average of squares rms

44 What is rms? root mean square v rms is the average of the square of the velocities. It is not the square of the average 44

45 rms Example v 1 = 4 v 2 = 8 v av = 6 v 1 2 = 16 v 2 2 = 64 45

46 46 The distribution of speeds in a gas is given by the Maxwell- Boltzmann Distribution.

47 47 Example (text problem 13.60): What is the temperature of an ideal gas whose molecules have an average translational kinetic energy of J?

48 48 Example (text problem 13.70): What are the rms speeds of helium atoms, and nitrogen, hydrogen, and oxygen molecules at 25 C? ElementMass (kg)rms speed (m/s) He H2H N2N O2O On the Kelvin scale T = 25 C = 298 K.

49 Question At a given temperature, a hydrogen molecule has a speed of 800 m/s. At the same temperature, an oxygen molecule has a speed of A) 800 m/s. B) 400 m/s. C) 200 m/s. D) 100 m/s.

50 50

51 Question When will a real gas behave most like an ideal gas? A)at high temperatures and high pressures B)at low temperatures and high pressures C)at low temperatures and low pressures D)at high temperatures and low pressures

52 Question The rms speed of a box of molecules which are moving at non uniform speeds is greater than the average speed. A) always B) sometimes C) never 52

53 Thermal expansion 53 Most objects including liquids and solids expand when their Temperature increases

54 54 An objects length after its temperature has changed is is the coefficient of linear expansion where T = T T 0 and L 0 is the length of the object at a temperature T 0.

55 55 Example (text problem 13.84): An iron bridge girder (Y = N/m 2 ) is constrained between two rock faces whose spacing doesnt change. At 20.0 C the girder is relaxed. How large a stress develops in the iron if the sun heats the girder to 40.0 C? Using Hookes Law: = K 1 (from Table 13.2)

56 56 How does the area of an object change when its temperature changes? The blue square has an area of L 0 2. With a temperature change T each side of the square will have a length change of L = TL 0. L0L0 L 0 + L

57 57 The fractional change in area is:

58 58 The fractional change in volume due to a temperature change is: For solids = 3

59 Question A metal plate with a hole cut in it is heated. As the plate expands, A)the hole expands. B)the hole shrinks. C)the hole stays the same size.

60 Question You can loosen the metal lid on a glass jar by running it under hot water. Given that the lid and the jar have roughly the same diameter, compare the expansion of the diameter of the lid to that of the jar. ( steel = 12 x K -1, glass = 3.25 x K -1 ) A) L lid = 3.7 L jar B) L lid = 0.27 L jar C) L lid = 7.4 L jar D) L lid = 1.9 L jar


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