Unit 12: Thermodynamics Think of a steam engine:

Unit 12: Thermodynamics Changes of state of water:

Unit 12: Thermodynamics Heat of fusion: H f is the amount of thermal energy needed to melt 1 kg of a substance. Heat required to melt a solid: Q = mH f

Unit 12: Thermodynamics Heat of vaporization: H v is the amount of thermal energy needed to vaporize 1 kg of a liquid. Q = mH v

Unit 12: Thermodynamics Example: Suppose you are camping in the mountains. You need to melt 1.50 kg of snow at 0.0 ˚C and heat it to 70.0 ˚C to make hot cocoa. How much heat will be needed? H f = 3.34x10 5 J/kg, H v = 2.26x10 6 J/kg (9.40x10 5 J)

Unit 12: Thermodynamics Thermodynamics involves the study of the changes in thermal properties of matter.

Unit 12: Thermodynamics First Law of Thermodynamics: the change in thermal energy (ΔU ) of an object is equal to the heat added (Q) to the object minus the work done (W) by the object. ΔU = Q – W This is a restatement of the Law of Conservation of Energy.

Unit 12: Thermodynamics

A device that is able to continuously convert thermal energy to mechanical energy is a heat engine.

Unit 12: Thermodynamics A heat engine transforms heat at high temperatures into mechanical energy and low-temperature waste heat Hot reservoir at T H Cold reservoir at T L

Unit 12: Thermodynamics

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Unit 12: Thermodynamics If heat engines completely converted thermal energy into mechanical energy with no waste heat, then the first law of thermodynamics would be obeyed. But waste heat is ALWAYS generated and even the ideal engine would generate some waste heat. Randomly distributed particles of gas are not observed to spontaneously arrange themselves in specific ordered patterns.

Unit 12: Thermodynamics The second law of thermodynamics states that nothing is 100% efficient – there are always inefficiencies – there are always losses -- there is always waste heat/energy.

Unit 12: Thermodynamics Entropy: the measure of the disorder in a system ◦ A dropped baseball falls due to gravity and possesses potential and kinetic energies that can be recovered to do work. ◦ The falling baseball collides with many air molecules that absorb some of its energy ◦ The air molecules move in random directions at random speeds ◦ The energy absorbed from the baseball causes disorder among the molecules. ◦ The greater the molecular speeds, the greater the disorder, increasing entropy.

Unit 12: Thermodynamics Entropy: the change in entropy of an object is equal to the heat added to the object divided by the temperature of the object in kelvins. ΔS = Q/T

Unit 12: Thermodynamics The third law of thermodynamics states that natural processes go in a direction that maintains or increases the total entropy of the universe! ◦ All things will become more and more disordered unless some action is taken to keep them ordered. ◦ Increase in entropy and the second law are like statements of probability of events happening.

Unit 12: Thermodynamics Third law predicts heat flows spontaneously from a hot object to a cold object. We know daily events occur spontaneously, or naturally, in one direction.

Unit 12: Thermodynamics A spoon lying on a table will not, on its own, become red hot on one end and icy cold at the other.

Unit 12: Thermodynamics If you dive into a swimming pool, you know you will push the water molecules away as you enter the water. The molecules CANNOT throw you back up to the diving board.

Unit 12: Thermodynamics Neither of these imagined processes violate the first law of thermodynamics. They do not occur because they would violate the second law of thermodynamics.