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THERMODYNAMICS
Solid
Liquid Gas
In a solid the particles are
packed side by side and cannot
move. They vibrate when they
are heated
In a liquid the particles are
still joined but can move around
In a gas the particles are not joined and
can move around in straight lines and do not react with each
other
Solid
Liquid Gas
Solids have a shape and a
volumeSolids cannot be
compressed
Liquids do not have a shape
but have a volume
Liquids cannot be compressed
Gases do not have shape or volume, they can spread out and fill
the shape they are contained in
Gases can be compressed
GAS LAWS
Compression (increasing pressure) is caused by exerting a force on an area
Pressure = Force N/m2 (Pascals)
Area
Increasing force on the same area means increasing pressure
GAS LAWS
Force
Applying a force on the are of this piston will apply a pressure on the
gas inside
GAS LAWS Increasing the pressure results in
a decrease in volume
The volume decreases at the same rate as the
pressure increases
Force
GAS LAWS MathematicallyPressure x volume is
a constant (always the same figure)
P X V = KP1 x V1 = P2 x V2 = K
Force
This is called Boyle’s Law
EXAMPLE
If the gas in a cylinder has a volume of 4m3 at a pressure of 4 N/m2 What will the volume be if the
pressure is increased to 8 N/m2 ?
P1 x V1 = P2 x V2 = K
4 x 4 = 8 x V2 = 16
V2 = 16 ÷ 8 = 2m3
GAS LAWSWhen the gas
particles are at a low temperature they have a low
amount of kinetic energy and are not moving about very much and do not
occupy much volume
GAS LAWSWhen the gas particles are
heated to a higher temperature they have more kinetic energy and move
about faster occupying a larger
volume
GAS LAWS
When the temperature increases the volume increases
V= K T
GAS LAWS
If the gas is heated and the volume cannot increase (in a closed
container) the pressure increases P = K
T
GAS LAWS
Combining the three laws we get
P x V = K T
GAS LAWS
P1 x V1 = T1
P2 x V2 T2
A gas has a constant pressure, temperature and volume, if one of the quantities
changes and another stays constant the third will change to compensate
ALWAYS USE THE KELVIN TEMPERATURE SCALE IN YOUR CALCULATIONS
oC KELVIN
0 273
20 293
40 313
60 333
80 353
100 373
-273 0
GAS LAWS EXAMPLE
The temperature of 2m3 of air is 22oC. It is compressed to a volume of 1.2m3 whilst being kept at a constant pressure of 1 bar. Calculate
the final temperature of the air.
Remember to change temperature to Kelvin
22oC = 295K
GAS LAWS EXAMPLE (PRESSURE STAY CONSTANT)
V1 = T1
V2 T2
2 = 295
1.2 T2
GAS LAWS EXAMPLE
2 = 295K
1.2 T2
T2 =295K x 1.2
2
T2 = 177K ( -96oC)
GAS LAWS EXAMPLE
Dry steam is compressed isothermally from a pressure of 1 bar to a pressure
of 10 bar. The initial volume of the steam is 2m3. Calculate the volume of
the steam after compression.
GAS LAWS EXAMPLE This time temperature stays the same
So P1V1 = P2V2
1 x 2 = 10 x V2
V2 = 1 x 2 = 0.2m3
10
GAS LAWS EXAMPLE
An air compressor operates with a compression ratio of 5:1. If the air is at a pressure of 1 bar and a
temperature of 20oC before compression and the temperature after compression is 300oC. What will
the final pressure be?
V1 = 5 V2 = 1
GAS LAWS EXAMPLE (PRESSURE STAY CONSTANT)
P1 x V1 = T1
P2 x V2 T2
1 x 5 293
P2 x 1 573
=
GAS LAWS EXAMPLE (PRESSURE STAY CONSTANT)
P1 x V1 = T1
P2 x V2 T2
1 x 5 x 573 293 x1
P2= = 9.8 bar