Thermodynamics

THERMODYNAMICSThermodynamics is the study of energy relationships that involve heat, mechanical work, and other aspects of energy and heat transfer.

Central Heating

A THERMODYNAMIC SYSTEM• A system is a closed environment in which

heat transfer can take place. (For example, the gas, walls, and cylinder of an automobile engine.)

Work done on gas or work done by gas

INTERNAL ENERGY OF SYSTEM

• The internal energy U of a system is the total of all kinds of energy possessed by the particles that make up the system.

Usually the internal energy consists of the sum of the potential and kinetic energies of the working gas molecules.

TWO WAYS TO INCREASE THE INTERNAL ENERGY, U.

HEAT PUT INTO A SYSTEM

(Positive)

+U

WORK DONE ON A GAS (Positive)

WORK DONE BY EXPANDING

GAS: W is positive

WORK DONE BY EXPANDING

GAS: W is positive

-UDecreas

e

-UDecreas

e

TWO WAYS TO DECREASE THE INTERNAL ENERGY, U.

HEAT LEAVES A SYSTEM Q is

negative

Qout

hot

WoutWout

hot

THERMODYNAMIC STATE

The STATE of a thermodynamic system is determined by four factors:

• Absolute Pressure P in Pascals

• Temperature T in Kelvins• Volume V in cubic

meters• Number of moles, n, of working gas

THE FIRST LAW OF THERMODYAMICS:

• The net heat put into a system is equal to the change in internal energy of the system plus the work done BY the system.

Q = U + W final - initial)

• Conversely, the work done ON a system is equal to the change in internal energy plus the heat lost in the process.

SIGN CONVENTIONS FOR FIRST LAW

• Heat Q input is positive

Q = U + W final - initial)

• Heat OUT is negative

• Work BY a gas is positive• Work ON a gas is negative

+Qin

+Wout

U

-Win

-Qout

U

APPLICATION OF FIRST LAW OF THERMODYNAMICS

Example 1: In the figure, the gas absorbs 400 J of heat and at the same time does 120 J of work on the piston. What is the change in internal energy of the system?

Q = U + W

Apply First Law:

Qin

400 J

Wout =120 J

Example 1 (Cont.): Apply First Law

U = +280 J

Qin

400 J

Wout =120 J

U = Q - W

= (+400 J) - (+120 J)

= +280 J

W is positive: +120 J (Work OUT)

Q = U + W

U = Q - W

Q is positive: +400 J (Heat IN)

FOUR THERMODYNAMIC PROCESSES:

• Isochoric Process: V = 0, W = 0 • Isobaric Process: P = 0 • Isothermal Process: T = 0, U = 0 • Adiabatic Process: Q = 0

• Isochoric Process: V = 0, W = 0 • Isobaric Process: P = 0 • Isothermal Process: T = 0, U = 0 • Adiabatic Process: Q = 0

Q = U + W

HEAT ENGINES

• Absorbs heat Qhot

• Performs work Wout

• Rejects heat Qcold

A heat engine is any device which through a cyclic process:

Cold Res. TC

Engine

Hot Res. TH

Qhot Wout

Qcold

THE SECOND LAW OF THERMODYNAMICS

It is impossible to construct an engine that, operating in a cycle, produces no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work.

Not only can you not win (1st law); you can’t even break even (2nd law)!

Wout

Cold Res. TC

Engine

Hot Res. TH

Qhot

Qcold

THE SECOND LAW OF THERMODYNAMICS

Cold Res. TC

Engine

Hot Res. TH

400 J

300 J

100 J

• A possible engine. • An IMPOSSIBLE engine.

Cold Res. TC

Engine

Hot Res. TH

400 J 400 J

EFFICIENCY OF AN ENGINE

Cold Res. TC

Engine

Hot Res. TH

QH W

QC

The efficiency of a heat engine is the ratio of the net work done W to the heat input QH.

e = 1 - QC

QH

e = = W

QH

QH- QC

QH

EFFICIENCY EXAMPLE

Cold Res. TC

Engine

Hot Res. TH

800 J W

600 J

An engine absorbs 800 J and wastes 600 J every cycle. What is the efficiency?

e = 1 - 600 J

800 J

e = 1 - QC

QH

e = 25%

Question: How many joules of work is done?

REFRIGERATORSA refrigerator is an engine operating in reverse: Work is done on gas extracting heat from cold reservoir and depositing heat into hot reservoir.Win + Qcold = Qhot

WIN = Qhot - Qcold

Cold Res. TC

Engine

Hot Res. TH

Qhot

Qcold

Win

THE SECOND LAW FOR REFRIGERATORS

It is impossible to construct a refrigerator that absorbs heat from a cold reservoir and deposits equal heat to a hot reservoir with W = 0.If this were possible, we could establish perpetual motion!

Cold Res. TC

Engine

Hot Res. TH

Qhot

Qcold