thermal physics hl
TRANSCRIPT
Thermal Physics HL
Assumptions of the kinetic theory
1. Molecules behave as if they were hard, smooth, elastic spheres. (i.e. the collisions are perfectly elastic)
2. Molecules are in continuous rapid, random motion.3. The average kinetic energy of the molecules is
proportional to the absolute temperature of the gas.4. The molecules do not exert any appreciable
attraction on each other.5. The volume of the molecules is infinitesimal when
compared with the volume of the gas.6. The time spent in collisions is small compared with
the time between collisions.
Pressure and temperatureGay-Lussac’s Law
As the temperature is reduced, the pressure decreases (Why?)
If the graph is extrapolated, then the intercept gives the value of absolute zero These results do not depend on the type of gas.
P1 / T1 = P2 / T2
Pressure and volume
Take measurements of pressure and volume
Plot a suitable graph to determine the relationship between these 2 variables.
Boyles’ Law
Boyle’s Law
The pressure of a fixed mass of gas at constant temperature is inversely proportional to the volume.
p1V1 = p2V2
Boyle’s Law animation
Charles’ Law
The pressure of a fixed mass of gas at constant volume is directly proportional to the absolute temperature.
Hyperlink
Charles’ Law animation
Equation of state for an ideal gas
Combining the previous gas laws gives
Therefore, for 1 mole of an ideal gas;
PV = (a constant)×T
PV = nRTP = Pressure V = Volume
n = number of moles
R = Universal gas constant
T = Temperature (KELVIN)
Gas equation animation
Click on image to activate pV = nRT
Gas properties simulation
Describe the concept of the absolutezero of temperature and the Kelvin
scale of temperature.
Questions1. A diver exhales a bubble of volume
2cm3 at a depth of 30m (Pressure = 4 Atmos). What is the volume of the bubble when it reaches the surface? (Isothermal). (8)
2. A fixed volume of gas is heated from 100kPa at 270C to 350kPa. What is it’s new temperature? (777)
3. A sample of Neon gas occupies a volume of 45 litres at 100kPa and 200C. How many moles of gas are there? (1.85)
A-level Q’s 1a,2,3,5a(i),6a,6b(i),7(a),8(a)(b)(c),9,10(a),11(a),13
Tsokos
Page 181 Q’s 1- 9.
Ideal Gases and Real Gases
• Real gases behave as ideal gases at room temp and pressure.
• The gas molecules become “interacting” at high temperatures and high pressures. Therefore they lose there ideal properties.
• Ideal gases cannot be liquefied.
• Ideal gases obey the gas equation.
Work done in compressing a gas
The work done by this force is w = Fs = PAs, since F=PA
but As is the change in the volume occupied by the gas, ΔV. therefore;
W = PV
Deduce an expression for the work involved in a volume change of a gas at constant pressure.
Conservation of energy
If we add energy to a fixed mass of gas, the gas will increase in temperature (internal energy ΔU).
ΔU =
1st Law of Thermodynamics
We can add energy by heating (temperature gradient) = Q
Or by working (no temperature difference) = W
Q = ΔU + WQ = Heat energy added to the gas
ΔU = Temperature increase of the gas
W = Work done by the gas.
State the first law of thermodynamics.
Students should be familiar with the terms system and surroundings. They should also appreciate that if a system and its surroundings are at different temperatures and the system undergoes a process, the energy transferred by non-mechanical means to or from the system is referred to as thermal energy (heat).
1st Law questions
• 1b
• 10(a),(c),(dii)
• Tsokos page 193 Q’s 1
Q = ΔU +W
W
Q
ΔU
P-V changes
1. Change of p (and T) at constant volume; an isovolumetric change.2. Change of V (and T) at constant pressure; an isobaric change.3. Change in p and V at constant temperature; an isothermal change.4. Change in p and V in an insulated container (no heating of the gas); an adiabatic change.
Isothermal gas processes
For a fixed mass and temperature of gas, state the values of Q, ΔU and W as the gas expands.
Isothermal P-V changes
Q =
ΔU =
W =
Isochoric changes (Volume)
Isochoric changes
Q =
ΔU =
W =
Isobaric changes (Pressure)
Isobaric changes Q =
ΔU =
W =
Adiabatic changes
Q =
ΔU =
W =
Adiabatic changes
These are defined as processes where no heat can flow in or out of the system. This occurs when the change happens too rapidly for the heat to be exchanged. Therefore they result in a change in temperature of the gas
Ideal gas processes
Isobaric process: pressure constant V/T constant
Isochoric process: volume constant p/T constant
Isothermal process: temperature constant pV constant
Hyperlinkhttp://www.walter-fendt.de/ph14e/gaslaw.htm
Conservation of energy for Carnot cycle
Curve AIsothermal
expansion at TH
Work done by the
gas
Curve B Adiabatic expansion
Work done by the
gas
Curve CIsothermal
compression at TC
Work done on the
gas
Curve DAdiabatic
compression
Work done on the
gas
For each part of the cycle, findQ = ?ΔU = ?W = ?
Otto cycle
Click to play
Carnot cycle
How does energy enter and leave the gas in a Carnot cycle?
Picture has a hyperlink
Work done in a thermodynamic process
The product of pressure and volume represents a quantity of work. This is represented by the area below a p-V curve.Therefore, the area enclosed by the four curves represents the net work done by the engine during one cycle.
Tsokos
Page 193 Q’s 2-5
Second Law: Entropy a measure of the amount of energy which is unavailable to do work
a measure of the disorder (of the energy) of a system
How “useful” is the energy? Which can do the most work for us? 100j of energy in petrol or 100j of energy as heat? The heat is “disordered” or “higher entropy”
Every time we change energy from one form to another, we increase the entropy of the Universe.
Second Law of Thermodynamics
The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by the first law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy.
Second Law: Heat Engines Second Law of Thermodynamics: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.
Second Law: Refrigerator Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.
Tsokos
Page 194 Q’s 9-13.