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[5] Heat, Work, and Internal Energy the First Law of Thermodynamics Work Done by Or on a Gas EXAMPLE Meteoro

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[5] Heat, Work, and Internal Energy the First Law of Thermodynamics Work Done by Or on a Gas EXAMPLE Meteoro L 20 Thermodynamics [5] Heat, work, and internal energy • The gas has internal energy, the temperature is the average KE • if heat is added its internal energy • heat, work, and internal energy increases st • if the gas expands and does work • the 1 law of thermodynamics on the atmosphere, its internal • the 2nd law of thermodynamics energy decreases • Heat and work are forms of energy gas • Heat engines which can change the internal • order to disorder Æ entropy energy heat •the 1st law of thermodynamics • Electric cars and Hybrid cars 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) st EXAMPLE Meteorology and the 1 Law • What is the change in the internal energy of a gas if 3000 J of heat are added while the gas does 1000 J of • Air temperature rises as heat is added work? • Air temperature rises (or falls) as pressure increases (decreases) • change in internal energy • In processes called adiabatic the amount of heat ∆IE = Qin − Wout added or lost is very small = 3000 J - 1000 J = 2000 J • As a parcel of air rises it expands adiabatically and its temperature decreases by about 10° C the internal energy increases by 2000 J for each kilometer of elevation 1 Heat engines The 2nd Law of Thermodynamics • A heat engine is a device that uses heat • Not all of the heat can be converted into work. (input, which you must pay for) to do work • try to understand the difference between work (output, which is useful). energy and heat energy • The central issue is how much of the heat • give the block a push– it will stop due to friction taken in can be converted into work • the kinetic energy is converted to HEAT • This is quantified by the engine efficiency • but, I cannot make the block move by heating it! • The amount of heat that can be converted to work is limited by the 1st law Îyou can’t get more out than goes in Heat – disordered energy order to disorder • All naturally occurring processes go in the • When an object is heated, the energy of all direction from order to disorder of its molecules is increased. • ice melts when placed in water; it never gets colder and the water gets warmer • however, the molecules do not all move in • ice, the solid state of H2O is more ordered than the same direction Æ they move about in water, the liquid state all directions Æ this is what we mean by • in a solid all the molecules are lined up in a disordered (or thermal) energy regular (ordered) array; there is less order in the • on the other hand, if we want to get the liquid state, and even less in the gaseous state system to do some useful work, we want it • when salt is put in water it dissociates; crystals to move in some particular direction of salt never spontaneously form in a salt water solution Work is ordered energy, heat is Heat Engines disordered energy • an engine operates in a cycle • It is possible to convert some of the random HOT • fuel is burned to make heat 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 into WORK work is removed to bring the system work back to the beginning state (cycle) Wout • since the system is always returned • the 2nd law explicitly prohibits all of the to the original state the change in Q internal energy is ZERO heat from being converted into work out • First Law Î Q = W + Q • this is a fact of natureÎ the way things are! in out out COLD W efficiency = out Qin 2 steam engines 1st and 2nd Laws of Thermodynamics • For an engine operating in a cycle the 1st law requires that: work out = heat in – heat out • the 2nd law says that it is impossible to make the heat out (Qout) = 0 Î not all the heat energy can be converted into work, some must be discarded – thermal waste work can be used to run • engine efficiency = work out / heat in an electric generator or • no engine can be 100% efficient Î this is turn the shaft of a propeller a law of nature! Heat engine example Second law of thermodynamics A heat engine, operating in a cycle, absorbs 10,000 J of energy from a heat source, performs work, and I. (Kelvin) It is impossible to have a heat discards 6,000 J of heat to a cold reservoir. engine that is 100 % efficient (a) how much work is performed? Î Not all of the heat taken in by the (b) what is this engine’s efficiency? engine can be converted to work (c) what is the change in internal energy of this engine? II. (Clausius) In a spontaneous process, heat (a) Wout = Qin-Qout = 10,000 J – 6,000 J = 4,000 J flows from a hot to a cold substance (b) efficiency= Wout/Qin = 4,000/10,000 = 0.4 or 40 % Î Work must by done to move heat from (c) ∆IE = 0, the change in internal energy for an a cold to a hot substance. engine operating in a cycle is ZERO refrigerators and air conditioners internal combustion engine HOT • Heat pump (heat valves engine in reverse) • You can make heat flow backward (cold cylinder to hot) only if there is piston connecting an input of work rod WORK • in an air conditioner crankshaft or refrigerator, this work must be COLD supplied by electricity. at cruising speeds this cycle happens at 3000 times/min (50 /s) 3 Electric cars Hybrid Automobiles • Propelled by a battery-powered electric Combination of 2 different systems motor – does not need gasoline • A hybrid car propels itself on fuel or batteries • No tailpipe emissions – reduces urban pollution and greenhouse gases • It combines electrical and mechanical power • It switches easily between fuel, batteries, or both • Batteries must be charged, but electric • It can recharge its batteries during braking. In a power is provided by domestic fuel sources conventional auto, much of the kinetic energy is • Chevy Volt – charged on $1.50/day dissipated as heat during braking. • Range limited by battery charge • It can turn its engine off when not needed • Poor acceleration • It can restart its engine quickly and easily • Excellent fuel economy up to 50 mpg (Toyota • expensive Prius) Hybrid: gas engine + electric motor Disadvantages of hybrid cars • Expensive gasoline • Limited top speed engine • Î Poor acceleration (0 – 60 mph in 10 s) • Mechanically complicated • Î Batteries must be replaced; discarded batteries pose an environmental issue But, hybrid cars are a new technology that is constantly being improved 4.
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