Unit 12: ThermodynamicsThink of a steam engine:

Unit 12: ThermodynamicsChanges of state of water:

Unit 12: ThermodynamicsHeat of fusion: Hf is the amount of

thermal energy needed to melt 1 kg of a substance.

Heat required to melt a solid:

Q = mHf

Unit 12: ThermodynamicsHeat of vaporization: Hv is the amount

of thermal energy needed to vaporize 1 kg of a liquid.

Q = mHv

Unit 12: ThermodynamicsExample: 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? Hf = 3.34x105 J/kg, Hv = 2.26x106 J/kg

(9.40x105 J)

Unit 12: ThermodynamicsThermodynamics involves the study of

the changes in thermal properties of matter.

Unit 12: ThermodynamicsFirst 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.

U = Q – WΔ

This is a restatement of the Law of Conservation of Energy.

Unit 12: Thermodynamics

Unit 12: ThermodynamicsA 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 TH

Cold reservoir at TL

Unit 12: Thermodynamics

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: ThermodynamicsThe 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: ThermodynamicsEntropy: 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: ThermodynamicsEntropy: 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.

S = Q/TΔ

Unit 12: ThermodynamicsThe 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: ThermodynamicsThird 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: ThermodynamicsA spoon lying on a table will not, on its

own, become red hot on one end and icy cold at the other.

Unit 12: ThermodynamicsIf 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: ThermodynamicsNeither of these imagined processes

violate the first law of thermodynamics.They do not occur because they would

violate the second law of thermodynamics.