Thermodynamics

In a liquid the particles are still joined but can move around Solid Liquid Gas In a solid the particles are packed side by side and cannot move. They vibrate when they are heated In a gas the particles are not joined and can move around in straight lines and do not react with each other In a liquid the particles are still joined but can move around

Solid Liquid Gas 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 Solids have a shape and a volume Solids cannot 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

Pressure x volume is a constant (always the same figure) Gas laws Mathematically Pressure x volume is a constant (always the same figure) P X V = K P1 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 Laws When 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 Laws When the gas particles are heated to a higher temperature they have more kinetic energy and move about faster occupying a larger volume

When the temperature increases the volume increases 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

Combining the three laws we get Gas Laws Combining the three laws we get P x V = K T

Gas Laws 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 P1 x V1 = T1 P2 x V2 T2

Always use the Kelvin tEmperature scale in your calculations oC KELVIN 273 20 293 40 313 60 333 80 353 100 373 -273

Remember to change temperature to Kelvin 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 1.2 T2 2 = 295

Gas Laws Example 2 = 295K 1.2 T2 295K x 1.2 2 T2 = 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.

This time temperature stays the same 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 P2 x 1 573 1 x 5 293 =

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