sign in sign up
<<
Home , Heat, Internal energy, Work (physics), Work (thermodynamics)

, , and internal L 20 [5] • The has , as measured by its • 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 which can change the internal energy „ order to disorder Æ heat order to disorder Æ entropy •the 1st law of thermodynamics „ 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 )

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 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 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 taken in can be converted into work • the is converted to HEAT • This is quantified by the • 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. • for example: always melts

• however, the molecules do not all move in • ice, the solid state of H2O is more ordered the same direction Æ they move about in than , the state all directions Æ this is what we mean by • in a solid all the molecules are lined up in disordered (or thermal) energy a regular (ordered) array; there is less • on the other hand, if we want to get the order in the liquid state system to do some useful work, we want it • When salt is put in water it dissociates; to move in some particular direction crystals 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 • is burned to make heat • some of the heat is converted into energy to do useful work Qin work • when a gas is allowed to expand, some of • the heat that is not converted to its random is converted into WORK work is removed to bring the system back to the beginning state (cycle) work 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 • energy accounting: Q = W + Q • this is a fact of natureÎ the way things are! in out out COLD W efficiency = out Qin

2 engines 1st and 2nd

• 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, 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 law of thermodynamics A , 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 engine

HOT • Heat (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 .

at cruising this cycle happens at 3000 /min (50 /s)

3 Hybrid Automobiles Hybrid: gas engine + electric motor

Combination of 2 different systems

• A hybrid car propels itself on fuel or batteries gasoline • It combines electrical and mechanical engine • It switches easily between fuel, batteries, or both • It can recharge its batteries during braking. In a conventional auto, much of the kinetic energy is dissipated as heat during braking. • It can turn its engine off when not needed • It can restart its engine quickly and easily

Combining Mechanical Power Disadvantages of hybrid cars

• A hybrid automobile uses a that • Expensive blends • Limited top – rotary mechanical power from its fuel-burning engine • Î Poor (0 – 60 mph in 10 s) – rotary mechanical power from its electric • Mechanically complicated motors • Î Batteries must be replaced; discarded – rotary mechanical power from its batteries pose an environmental issue • That “transaxle” can transfer mechanical power between any of those devices in either direction!

4