Laws of Thermodynamics

LAWS OF

THERMODYNAMICS

Presented by Raja Wajahat

• The four laws of thermodynamics define

fundamental physical quantities (temperature,

energy, and entropy) that characterize

thermodynamic systems.

• The laws describe how these quantities behave

under various circumstances, and forbid certain

phenomena.

2Presented by Raja Wajahat

ZEROTH LAW OF THERMODYNAMICS


Zeroth law of thermodynamics

• If two systems are both in thermal equilibrium with a

third then they are in thermal equilibrium with each

other


Zeroth law of thermodynamics

• The importance of the law as a foundation to the

earlier laws is that it allows the definition of

temperature in a non-circular way without

reference to entropy, its conjugate variable.

• Such a temperature definition is said to be

'empirical'.


LAW OF CONSERVATION OF

ENERGY

FIRST LAW OF

THERMODYNAMICS


FIRST LAW OF

THERMODYNAMICS• The increase in internal energy of a closed system is

equal to the heat supplied to the system minus work

done by it.


First Law encompasses several

principles1. The law of conservation of energy.

2. The concept of internal energy and its

relationship to temperature.

3. The flow of heat is a form of energy transfer.

4. Work is a process of transferring energy to or

from a system.


• Combining these principles leads to one traditional

statement of the first law of thermodynamics:

• it is not possible to construct a machine which will

perpetually output work without an equal amount

of energy input to that machine.

• Or more briefly, a perpetual motion machine is

impossible.


ENTROPY OF AN ISOLATED

SYSTEM

SECOND LAW OF

THERMODYNAMICS


ENTROPY

• Entropy is an extensive property. It has the

dimension of energy divided by temperature, which

has a unit of joules per kelvin (J K-1) in the

International System of Units (or kg m2 s-2 K-1 in

basic units).

• But the entropy of a pure substance is usually given

as an intensive property — either entropy per unit

mass (SI unit: J K-1 kg-1) or entropy per unit amount

of substance (SI unit: J K-1 mol-1).


SECOND LAW OF

THERMODYNAMICS• The entropy of an isolated system never decreases;

such a system will spontaneously evolve toward

thermodynamic equilibrium, the configuration with

maximum entropy.

• Systems that are not isolated may decrease in

entropy, provided they increase the entropy of their

environment by at least that same amount.


SECOND LAW OF

THERMODYNAMICS• Since entropy is a state function, the change in the

entropy of a system is the same for any process that

goes from a given initial state to a given final state,

whether the process is reversible or irreversible.

• However, irreversible processes increase the

combined entropy of the system and its

environment.


SECOND LAW OF

THERMODYNAMICS• According to the second law of thermodynamics, in

a theoretical and fictional reversible heat transfer,

an element of heat transferred, δQ, is the product

of the temperature (T), both of the system and of

the sources or destination of the heat, with the

increment (dS) of the system's conjugate variable,

its entropy (S).


More on Entropy

• Entropy may also be viewed as a physical measure

of the lack of physical information about the

microscopic details of the motion and configuration

of a system, when only the macroscopic states are

known.

• The law asserts that for two given macroscopically

specified states of a system, there is a quantity

called the difference of information entropy

between them.


More on Entropy

• This information entropy difference defines how much

additional microscopic physical information is needed to

specify one of the macroscopically specified states,

given the macroscopic specification of the other - often

a conveniently chosen reference state which may be

presupposed to exist rather than explicitly stated.

• A final condition of a natural process always contains

microscopically specifiable effects which are not fully

and exactly predictable from the macroscopic

specification of the initial condition of the process.


More on Entropy

• This is why entropy increases in natural processes -

the increase tells how much extra microscopic

information is needed to distinguish the final

macroscopically specified state from the initial

macroscopically specified state.


ENTROPY AT ABSOLUTE

ZERO

THIRD LAW OF

THERMODYNAMICS


THIRD LAW OF

THERMODYNAMICS• The entropy of a perfect crystal of any pure

substance approaches zero as the temperature

approaches absolute zero.


THIRD LAW OF

THERMODYNAMICS• At zero temperature the system must be in a state

with the minimum thermal energy. This statement

holds true if the perfect crystal has only one state

with minimum energy. Entropy is related to the

number of possible microstates according to:


THIRD LAW OF

THERMODYNAMICS• Where S is the entropy of the system, kB Boltzmann's

constant, and Ω the number of microstates (e.g.

possible configurations of atoms).

• At absolute zero there is only 1 microstate possible

(Ω=1 as all the atoms are identical for a pure

substance and as a result all orders are identical as

there is only one combination) and ln(1) = 0.


THIRD LAW OF

THERMODYNAMICS• A more general form of the third law that applies to

a systems such as a glass that may have more than

one minimum microscopically distinct energy state,

or may have a microscopically distinct state that is

"frozen in" though not a strictly minimum energy

state and not strictly speaking a state of

thermodynamic equilibrium, at absolute zero

temperature:


THIRD LAW OF

THERMODYNAMICS• The entropy of a system approaches a constant

value as the temperature approaches zero.

• The constant value (not necessarily zero) is called

the residual entropy of the system.


Thank You!

Presented by Raja Wajahat

Laws of Thermodynamics

Engineering

raja wajahatthe

raja wajahatentropyentropy

raja wajahatcombining

maximum entropy

combined entropy

closed system

energy input

dimension of energy