Thermodynamics - Phys.hawaii.edu

Thermodynamics

Thermodynamic Systems, States,and ProcessesA thermodynamicsystem is described byan equation of state,such as the ideal gaslaw. The “location” ofthe state can beplotted on a p–Vdiagram, as at left.

Thermodynamic Systems, States,and ProcessesA process changesthe state of asystem. A processmay be:reversible (3–4) orirreversible (1–2);if it proceedsthrough a sequenceof equilibrium statesit is reversible.

The First Law of ThermodynamicsThe first law is a statement of conservation of energyin thermodynamic processes:Here, Qis the heat transferred, ΔUis the change ininternal energy, and Wis the work.

The First Law of ThermodynamicsIt is important to get the signs of Q, ΔU, and W correct.The diagram below illustrates the sign conventions.

The First Law of ThermodynamicsIt can be shownthat the workdone inexpanding a gasis given by:

The First Law of ThermodynamicsGeneralizing,we see that:The work done by asystem is equal tothe area under theprocess curve on ap–V diagram.

The First Law of ThermodynamicsThe heat transferredand the work doneboth depend on thepath taken from state1 to state 2; thechange in internalenergy depends onlyon the end points.

Thermodynamic Processes for anIdeal GasAn isothermalprocess is one inwhich thetemperature doesnot change.

Thermodynamic Processes for anIdeal GasThe first law of thermodynamics gives:The work done is:

Thermodynamic Processes for anIdeal GasAn isobaricprocess is onein which thepressure doesnot change.

Thermodynamic Processes for anIdeal GasThe first law of thermodynamics gives:The work done is:

Thermodynamic Processes for anIdeal GasAn isometricprocess is one inwhich the volumeis constant.In this case, nowork is done, so

Thermodynamic Processes for anIdeal GasAn adiabaticprocess is onewhere there is noheat transfer.

Thermodynamic Processes for anIdeal GasIn this case,Andwhere

Thermodynamic Processes for anIdeal Gas

The Second Law ofThermodynamics and EntropyThe second law of thermodynamics:1. In a thermal cycle, heat energy cannot be completelytransformed into mechanical work.2. It is impossible to construct an operational perpetualmotion machine.3. It’s impossible for any process to have as its sole result thetransfer of heat from a cooler to a hotter body

The Second Law ofThermodynamics and EntropyAnother statement of the second law of thermodynamics:All naturally occurring processes move toward a state of greaterdisorder or disarray.It is a lot easier to break things than it is to put themback together again!

The Second Law ofThermodynamics and EntropyWhat is entropy?We cannot measure it directly, only changes in it.Entropy:-Is a measure of disorder-Is increasing in the universe-Is a measure of a system’s ability to do useful workDetermines the direction of time

The Second Law ofThermodynamics and EntropyThe entropy of an isolated system never decreases.The universe is the ultimate isolated system;therefore, the total energy of the universeincreases during every natural process.There are processes during which the entropy doesnot change; however, during any process theentropy of the universe can only increase orremain constant.

Heat Engines and Thermal PumpsA heat engine converts heat energy into work.According to the second law of thermodynamics,however, it cannot convert *all* of the heatenergy supplied to it into work.Basic heat engine: hot reservoir, cold reservoir,and a machine to convert heat energy into work.

Heat Engines and Thermal PumpsThis is a simplified diagram of a heat engine,along with its thermal cycle.

Heat Engines and Thermal PumpsAn important quantity characterizing a heat engine isthe net work it does when going through an entirecycle.

Heat Engines and Thermal PumpsThermal efficiency of a heat engine:For an ideal gas heat engine,

Heat Engines and Thermal PumpsYet another restatement of the second law ofthermodynamics:No cyclic heat engine can convert its heat input completelyto work.

Heat Engines and Thermal PumpsA thermal pump is the opposite of a heat engine:it transfers heat energy from a cold reservoir to ahot one.As this will not happen spontaneously, work mustbe done on the pump as well.Examples of thermal pumps: refrigerator, airconditioner

Heat Engines and Thermal PumpsAn air conditioner removes heat from the house andexhausts it outside, where it is hotter.

Heat Engines and Thermal PumpsA refrigerator doesthe same thing,except the warm airis exhausted intothe kitchen.

Heat Engines and Thermal PumpsThe purpose of a refrigerator or air conditioner is tokeep a cool place cool; we describe such a device byits coefficient of performance (COP).Typical COPs are in the range of 3–5; this is the ratioof the heat removed to the work done to remove it.

Heat Engines and Thermal PumpsHeat pumps can also be used to warm cool places,such as a house in winter. The fundamental operationis the same—heat is taken from a cool place and sentto a warm one—but now the point is to keep thehouse warm, not to keep the outside cold. There is adifferent COP that describes how well a heat engine isdoing this:

The Carnot Cycle andIdeal Heat EnginesThe Carnot cycle consists of two isotherms and twoadiabats.

The Carnot Cycle and Ideal HeatEnginesAn ideal Carnot engine—with perfect isothermsand adiabats—has the maximum efficiency a heatengine can have.This leads to the third law of thermodynamics:It is impossible to reach absolute zero in a finite number ofthermal processes.

SummaryFirst law of thermodynamics is conservation ofenergy:Thermodynamic processes: isothermal, isobaric,isometric, adiabaticWork done:

SummarySecond law of thermodynamics specifies thedirection a spontaneous interaction can take.Entropy is a measure of disorder:During any kind of process, the entropy of theuniverse can only stay the same or increase.

SummaryA heat engine converts heat energy into work.Efficiency:A thermal pump extracts heat from a cold reservoirand transfers it to a hot one.A Carnot engine has the highest efficiency possible:

reversible (3-4) or irreversible (1-2); if it proceeds through a sequence of equilibrium states it is reversible. The First Law of Thermodynamics . Basic heat engine: hot reservoir, cold reservoir, and a machine to convert heat energy into work. Heat Engines and Thermal Pumps