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L 20 Thermodynamics [5] Heat, Work, and Internal Energy the First Law Of

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L 20 Thermodynamics [5] Heat, Work, and Internal Energy the First Law Of L 20 Thermodynamics [5] Heat, work, and internal energy • The gas has internal energy, as indicated by its temperature • if heat is added its internal energy increases heat, work, and internal energy • if the gas expands and does gas the 1st law of thermodynamics work on the atmosphere, its internal energy decreases the 2nd law of thermodynamics • the 1st law of thermodynamics heat order to disorder Æ entropy keeps track of the balance between the heat, work and internal energy of the gas the first law of thermodynamics work done by or on a gas • the change in internal energy of the gas • if a gas does work (expansion) its internal energy goes down and so does its temp. = the heat absorbed by the gas • if work is done on a gas (compression) its minus the work done by the gas internal energy goes up and so does its temperature system • the internal energy of a gas can be HEAT internal WORK energy changed by adding or taking away heat or by having the gas do work or doing work • this is a simple energy accounting on the gas principle (law of conservation of energy) Change in EXAMPLE internal energy HEAT WORK • What is the change in the internal energy increase in 0 of a gas if 3000 J of heat are added while increase 0 on gas the gas does 1000 J of work? decrease out 0 • change in internal energy decrease 0 by gas = Heat in - work done increase in on gas = 3000 J - 1000 J = 2000 J decrease out by gas the internal energy increases by 2000 J all quantities measured in Joules or Calories 1 Heat engines The 2nd Law of Thermodynamics • Not all of the heat can be converted into work. • A heat engine is a device that uses heat • try to understand the difference between work (input, which you must pay for in some energy and heat energy form) to do work (output which is useful). • give the block a push– it will stop due to friction • A central issue is how much of the heat • the kinetic energy is converted to HEAT taken in can be converted into work • but, I cannot make the block move by heating it! • The outcome is first of all limited by the 1st law (you can’t get more out than goes in) Heat – disordered energy order to disorder • When an object is heated, the energy of all • All naturally occurring processes go in the of its molecules is increased. direction from order to disorder • however, the molecules do not all move in • for example: ice always melts the same direction Æ they move about in all directions Æ this is what we mean by • ice, the solid state of H2O is more ordered disordered (or thermal) energy than water, the liquid state • on the other hand, if we want to get the • in a solid all the molecules are lined up in system to do some useful work, we want it a regular (ordered) array to move in some particular direction • There is far less order in the liquid state Work is ordered energy, heat is Heat Engines disordered energy • an engine operates in a cycle • It is possible to convert some of the HOT • fuel is burned to make heat random energy to do useful work • some of the heat is converted into Qin work • when a gas is allowed to expand, some of • the heat that is not converted to its random thermal energy is converted WORK work is removed to bring the system into work back to the beginning state W • since the system is always returned • the 2nd law explicitly prohibits all of the out heat from being converted into work to the original state the change in Qout internal energy is ZERO • this is just a fact of natureÎ the way • energy accounting: Qin = Wout + Qout things work! COLD W efficiency = out Qin 2 st nd heat engine 1 and 2 Laws of Thermodynamics • the 1st law requires that work out = heat in – heat out • the 2nd law says that it is impossible to make the heat out = 0, not all the heat energy can be converted into work, some must be discarded – thermal waste • engine efficiency = work out / heat in work can be used to run • no engine can be 100% efficient Î this is an electric generator or turn the shaft of a a law of nature! propeller Heat engine example Order/disorder statement A heat engine, operating in a cycle, of the 2nd Law of Thermodynamics absorbs 10,000 J of energy from a heat source, performs work, and discards 6,000 • the total disorder of an object is quantified J of heat to a cold reservoir. (a) how much in a parameter called ENTROPY work is performed? (b) what is this engine’s efficiency? (c) what is the change in • in terms of entropy the 2nd law states that internal energy of this system? the entropy of an isolated object never decreases – entropy either stays the same (a) W = Q -Q = 10,000 J – 6,000 J = 4,000 J out in out or increases (b) efficiency= Wout/Qin = 4,000/10,000 = 0.4 or 40 % (c) the change in internal energy for a system operating in a cycle is ZERO internal combustion engine refrigerators and air conditioners HOT • Heat engines in reverse • You can make heat flow backward (cold to hot) only if there is an input of work WORK • in an air conditioner or refrigerator, this work must be COLD supplied by electricity. at cruising speeds this cycle happens at 3000 times/min (50 /s) 3.
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