Physics 213: Lecture 3, Pg 1 Packet 3.4 Thermodynamics Thermodynamics l Internal Energy l W = PΔV l 1 st Law of Thermodynamics: ΔU = Q – W l Define: Adiabatic, isothermal, isobaric & isochoric and show these on a P-V diagram l Irreversibility & disorder l Entropy is a measure of disorder l State 2 nd Law of Thermodynamics l Heat engine efficiency, η = W/Q h l Carnot Engine l Energy Degradation

Physics 213: Lecture 3, Pg 2 Internal Energy l Is the total potential and kinetic energy of the molecules in a substance. Potential energy is associated with intermolecular forces. Kinetic energy includes both translational and rotational motion. l When we consider an ideal gas, the intermolecular forces are assumed to be zero! l Internal energy of a gas comes only from the random kinetic energy of the atom of the gas.

Physics 213: Lecture 3, Pg 3 Ideal Gas & Internal Energy l E k = 1/2mv 2 = 3/2kT so, U = 3/2 NkT l U =3/2nRT l Δ U = 3/2 nR Δ T

Physics 213: Lecture 3, Pg 4 Ideal Gas & Internal Energy l What is the internal energy of 30 moles of oxygen gas at room temperature? l U =3/2nRT

Physics 213: Lecture 3, Pg 5 Ideal Gas & Internal Energy l If the room were moving at a high velocity would that mean the internal energy of the gas would be greater?

Physics 213: Lecture 3, Pg 6 Work done on or by a gas Work done on or by a gas l Imagine compressing a gas by exerting a force on the piston from the outside. l Consider heating the piston and it expands to perform work. l W = F x D & F = PA l W = P (A x D) l W = P Δ V

Physics 213: Lecture 3, Pg 7 Example l A gas is compressed at constant pressure 2.00 x 10 5 Pa from a volume of 2.00 m 3 to a volume of m 3. What is the work done on the gas. If the temperature initially was 40˚ C what is the final temperature of the gas?

Physics 213: Lecture 3, Pg 8 Thermodynamic Processes Thermodynamic Processes l CLICK HERE – TO GO OVER EACH PROCESS!! CLICK HERE Isochoric Isobaric Isothermal Const. Volume Const. Pressure Const. Temp l Remember! Area under curve is Work Done!

Physics 213: Lecture 3, Pg 9 Adiabatic Expansion Adiabatic Expansion l Rapid expansion or compression of a gas. l No Heat (Q) can flow in or out of the system. ΔQ = 0 l Any work done equals a direct change in internal energy. ΔU = ΔW l Bottle Rockets Bottle Rockets l Diesel Engines Diesel Engines

Physics 213: Lecture 3, Pg 10 Figure Adiabatic Heating

Physics 213: Lecture 3, Pg Heat and work are forms of energy transfer and energy is conserved. The First Law of Thermodynamics (FLT)  U = Q - W by work done by the system change in total internal energy heat added to system or  U = Q + W on State Function Process Functions

Physics 213: Lecture 3, Pg 12 "Process Problems" For which process is W the largest? smallest? For which process is Q the largest? smallest?

Physics 213: Lecture 3, Pg 13 Physics Joke l Once all the scientists die and go to heaven. They decide to play hide-n-seek and Einstein goes first. Einstein counts up to 100 and then start searching. Everyone starts hiding except Newton. Newton just draws a square of 1 meter and stands in it, right in front of Einstein. Einsteins counting....97,98,99,100, opens his eyes and finds Newton standing in front. Einstein says "Newtons out, Newtons out." Newton denies and says I am not out. He claims that he is not Newton. All the scientists come out and he proves that he is not Newton. How??

Physics 213: Lecture 3, Pg 14 His proof: l Newton says: I am standing in a square of area 1m square.. That means I am Newton per meter square.. Hence I am Pascal. Since newton per meter square = Pascal

Physics 213: Lecture 3, Pg 15 Conceptual Checkpoint 18-2 Which is the adiabatic curve?

Physics 213: Lecture 3, Pg 16 The second law of thermodynamics When objects of different temperatures are brought into thermal contact, the spontaneous flow of heat that results is always from the high temperature object to the low temperature object.

Physics 213: Lecture 3, Pg 17 Heat Engines l Energy goes in l Useful Work taken out l Some gets wasted l Max Efficiency: T H – T c / T h= x 100

Physics 213: Lecture 3, Pg 18 The 2 nd Law of Thermodynamics l The second law of thermodynamics deals with the limitations imposed on heat engines: that is on devices whose aim is to covert heat (disordered energy) into mechanical energy (ordered energy).

Physics 213: Lecture 3, Pg 19 The 2 nd Law of Thermodynamics l The Entropy of an isolated system never decreases. l It is impossible for heat to (spontaneously) flow from a cold to a hot object. l It is impossible for a heat engine working in a cycle to absorb heat and perform an equal amount of work. l The most efficient heat engine operating between two given temperatures is the Carnot Engine.

Physics 213: Lecture 3, Pg 20 Entropy l Entropy like Internal energy is a State Function! l Entropy Is a measure of the disorder of a system. l ΔS = ΔQ/T If ΔQ > 0 entropy increases If ΔQ < 0 entropy decreases.

Physics 213: Lecture 3, Pg 21 Philosophy l Three Laws of Thermodynamics (paraphrased): First Law: You can't get anything without working for it. Second Law: The most you can accomplish by work is to break even. Third Law: You can't break even.