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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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