lecture_1 thermo

8/11/2019 lecture_1 thermo

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Thermodynamics 1

MENG 1001

Mr. R. Ramkissoon


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Thermodynamics is the sciencedealing with the relations between

the properties of a substance and

the quantities of 'work' and 'heat'which cause a change of state.


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System-A thermodynamicsystem

is defined as a quantity of matter

or a region in space chosen for

study.

Boundary- Physical such as the wall ofa vessel.

Surrounding those portions of matter

external to the system which areaffected by changes occurring withinthe system.


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Types of Systems

There are three main types of systems

Closed systems Open systems

Isolated systems


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CLOSED AND OPEN

SYSTEMS

Closed systems(also known as control

masssystems) incorporate a fixed amount

of mass. Energy (work and heat) can cross

the boundary, but mass cannot cross the

boundary. The boundary can move, expand,

and contract as necessary to enclose the

fixed system mass.


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Open System

Open systems(also known as controlvolumesystems) allow both mass and

energy (heat and work) to cross the

boundary. The boundary can still move and

it can contain different amounts of mass at

different times. E.g. an expander


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An isolated systemis a special case of a

closed system in which neither mass nor

energy can cross the boundary. E.g. cooler,thermos


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WORK AND HEAT

Work is usually defined as a forceFacting

through a displacementx,the displacement

being in the direction of the force. W=fFdx

Work is done by a system if the sole effect

on the surroundings (everything external tothe system) could be the raising of a weight.


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The sign convention for work is that work

done by a system is positive; work done on a

system is negative.


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Heat is defined as the form of energy

that is transferred across the boundaryof a system at a given temperature to

another system (or the surroundings)

at a lower temperature by virtue of thetemperature difference between the two

systems.

Heat is a transient phenomenon


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Work is 'something'which appears at theboundary when a system changes its state

due to the movement of a part of theboundary under the action of a force

Heat is 'something'which appears at theboundary when a system changes its statedue to a difference in temperature betweenthe system and its surroundings

Heat, like work, is a transient quantity

and

2

1 21WdW 21

2

1

QdQ


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TEMPERATURE AND THE ZEROTH

LAW OF THERMODYNAMICS

Thezerothlaw of thermodynamicsstates

that if two bodies are in thermal equilibriumwith a third body, they are also in thermal

equilibrium with each other.


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Temperature Scales

Celsius scale

Fahrenheit scale ((C *1.8) + 32)

Kelvin scale(C + 273.15)

Rankine scale(C 1.8 + 491.67)

(F + 459.67)


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Properties, States and Processes

A thermodynamic propertyis a quantity thatdescribes the state of a system and is

independent of the path to the state, i.e. the

history, e.g. mass, volume, energy, pressureand temperature.

Thermodynamic properties are either

intensive

extensive.


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Extensiveproperties varies with mass and

depend on the size or extent of the system, eg

mass, total volume . The value of an extensive property for an overall

system is the sum of its value for the parts into

which the system is divided, i.e. it is additive.

I ntensiveproperties are independent of mass, and

are not additive, e.g. temperature, pressure,

specific volume. The system has the same value

for any part of a homogenous system as it doesfor the whole system.


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If the value of any extensive property is

divided by the mass of the system, the

resulting property is intensive and is calleda specificproperty. A specific property is

normally depicted by use of common letters

e.g. Specific Volume,v .


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State may be identified or described

by certain observable, macroscopic

properties (temp, pressure).

Each of the properties of a substance ina given state has only one definite

value.


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Equilibrium

A system is in thermal equi l ibr iumwhen the

temperature is the same throughout the system,i.e. thermal equilibrium is directly related to

temperature.

A system is in thermodynamicequilibriumif no

changes can occur in the state of the system

without the aid of an external stimulus. We can

test if a system is in equilibrium by isolating it

and observing whether any changes in it occur.


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A closed system is said to undergo a

cyclic process, or cycle,when it passes

through a series of states in such a way

that its final state is equal in all

respects to its initial state.

This implies that all its properties haveregained their initial values.

The cycle


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WQJ

Q WJis a proportionality factor that depends on

the units used for work and heat.

In SI units the joule (J) is used as the unit

for both work and heat and any energy unit.

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