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1 In one episode of the The Simpsons, after Lisa constructs a perpetual motion machine whose energy increases with time, Homer scolds her with: In this house, we obey the laws of thermodynamics! ********************************************************************************************* In the late 1940s, German theoretical physicist Arnold Sommerfeld, having previously written a series of books in physics: mechanics (1943), electrodynamics (1948), optics (1950), etc., was asked why he had never written a book on thermodynamics? The following is his humorous (but fairly accurate) and frequently quoted answer: Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you anymore. 361-Lec 3 30aug12 1

2 State the First Law in simple words and the main equation ΔE = q + w Energy change of system during process Energy absorbed by system because of temperature difference between system and surroundings. Something in surrroundings must get colder or hotter, or else q = 0 Energy absorbed by system because of a force acting in surroundings. Something in surroundings must move (or some mechanical happening that could cause something to move, e.g., charging a battery.) 2

3 System: whatever you want it to be Surroundings: everything else Types of systems Isolated: Exchanges no energy and no matter Closed: sealed, insulated, no mechanical connection Exchanges energy but no matter sealed ldbll balloon of gas Open: Exchanges energy and matter hen s egg, a cell 3

4 Calculating Work work = w = ernal force x distance (f (if the force is constant) between the system and surroundings displacement in the system Sign depends on whether system expands or contracts in response to the force. 4

5 What if the force is NOT constant? a very small amount of work = dw = f ( x ) dx where dw, dx mean infinitesimally small (but we can think of this as just meaning really, really, really, small); f (x) means the force is NOT CONSTANT, but depends on x. Now just add up all the small bits: x 2 w = f ( x) dx x 1 Constant Force Case w x = f dx = f x x ) 2 x1 ( 2 1 = f Δx 5

6 Earth s gravity is a nearly constant force. force = mass x acceleration = mg, where g= 9.8 ms 2 at the earth s surface If you hold ld1 kg (2.22 lbs) you feel a force = 1 kg x ms 2 2 = 9.8 kg m s 2 = 9.8 Newtons = 9.8 N Work to raise this 1 meter = 98kg 9.8 m s 2 x 1 m = 98kgm s 2 = 9.8 Joule = 9.8 J Work for you to climb a 1000 m mountain (3300 ft): = 100 kg x 9.8 ms 2 x 1000 m 100 x 10 x 1000 = 10 6 J = 10 3 kj = 250 kcal = ~2.5 slices of bread. Doing nothing, your body requires ~2500 kcal/day. that is the equivalent of climbing 10,000 * m, or 25 slices of bread! 6 * Climbing Mt. Everest starting at sea level

7 Expansion/Compression of System with Constant External Pressure consider a cylinder filled with gas and capped with a piston force Δx work = w = state 1 state 2 f Δx multiply top and bottom by Area of w = f A AΔx = pressure x volume = cylinder P ΔV = A but, what is the sign of w?? 7

8 but, what is the sign??? ΔV is obviously negative and pressure is positive, but work is done on the system. Therefore, w is positive. The correct equation for this wpδvpvkind of work is : = ALWAYS TRUE, for constant ernal pressure. SI unit of Pressure 1 Nm 2 = 1 Pascal = 1 Pa Note: 1 Nm m 3 = 1 Jm 3 = 1 Pa also 1 bar = 1 x 10 5 Pa (the modern standard state) 1 atm = x 10 5 Pa 1 liter = 1 L = 1x10 3 m 3 so: 1 L atm = 10 3 m 3 x x 10 5 J m 3 = J 8

9 Other common units of pressure 1 atm = 14.7 lbs/square inch 1 atm = 760 mm of mercury = 760 torr = 29.9 inches Hg This comes from the density of Hg, 13.6 g/cm3 or 13.6 x 10 3 kg m 3 and pressure = acceleration of gravity x density x height 98ms x 13.6 x 10 3 kg m 3 x m = x 10 5 kg m 1 s 2 = x 10 5 (kg m s 2 )m 2 = x 10 5 Pa = 1 atm Problems you can do now (partially): 15a,c, 16a,f, 19 9

10 Now we talk about heat: Zeroth Law of Thermodynamics: (inserted after the 1 st 3 Laws, and often not mentioned) If two objects are in thermal equilibrium with a third object, they are in thermal equilibrium with each other. Thermal equilibrium i means that t the two objects are at the same temperature. Energy always flows from hotter object to a colder object when they are in contact. (which is so obvious to every human that it goes without saying) 10

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