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