Thermodynamics

Chapter 24

Topics

• Thermodynamics– First law– Second law

• Adiabatic Processes

• Heat Engines

• Carnot Efficiency

• Entropy

Thermodynamics

• Is the study of heat and its transformation into mechanical energy

• It is the study of heat and work

Absolute Zero

• Is the lowest possible temperature that a substance may have– 0 Kelvin

• Absolute zero is where molecules of a substance have minimum kinetic energy

Question

• Which is larger, a Celsius degree or a Kelvin?

• Neither. They are equal

First Law of Thermodynamics

• The heat added to a system equals the sum of the increase in internal energy plus the external work done by the system

• This is a restatement of the energy conservation applied to heat– Energy can neither be created nor destroyed

• Whenever heat is added to a system, it transforms to an equal amount of some other form of energy

• ∆Heat =∆internal energy + work– Adding heat, increases internal energy – If heat is zero, changes in internal energy =

the work done on or by the system– If work is done on system, the internal energy

increases– If work is done by the system, the internal

energy decreases

Questions

• If 10 J of energy is added to a system that does no external work, by how much will the internal energy of that system be raised?

• 10 J

• If 10 J of energy is added to a system that does 4 J of external work, by how much will the internal energy of that system be raised?

• 6J. 10J-4J=6J

Adiabatic Processes

• Is one usually of expansion or compression, wherein no heat enters or leaves a system

• Example: compression and expansion of gases in the cylinders of an engine

• When work is done on a gas by adiabatically compressing it, the gas gains internal energy and becomes warmer

• When a gas adiabatically expands, it does work on its surroundings and gives up internal energy and becomes cooler

Second Law of Thermodynamics

• Heat does not spontaneously flow from a cold object to a hot object

• No machine can be completely efficient in converting energy to work; some input energy is dissipated as heat

• All systems tend to become more and more disordered as time goes by

Heat Engines and the Second Law

• A heat engine is any device that changes internal energy into mechanical work

• When work is done by a heat engine running between two temperatures, Thot and Tcold, only some of the input heat at Thot can be converted to work, and the rest

is expelled as heat at Tcold

Carnot Efficiency

• Ideal efficiency = Thot – Tcold

Thot

• Only in perfect conditions will the ideal efficiency by 100% for a machine– friction is always present and efficiency is

always less than ideal

Questions

• What is the ideal efficiency of an engine if both its hot reservoir and exhaust are the same temperature, 400K?

• 0; (400-400)/400=0

• What is the ideal efficiency of a machine having a hot reservoir at 400 K and a cold reservoir at absolute zero, 0K?

• 1; (400-0)/400=1

Order Tends to Disorder

• Natural systems tend to proceed toward a state of greater disorder

• Example:– Consider a stack of pennies on a table, all

heads up. Suppose somebody knocks them off and they topple to the floor. The pennies will not land all heads up. Order becomes disordered.

Entropy

• Is a measure of the disorder of a system

• Whenever energy freely transforms from one form to another, the direction of transformation is toward a state of greater disorder (greater entropy)

• Disorder increases; entropy increases; available energy of the system doing work decreases

Example

• A new deck of cards comes out of its box in ordered suits. Shuffle the deck once and you have disorder. Shuffle is again and you have more disorder. Think of the probability of shuffling the deck enough times to get some degree of order from the disorder