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Thermodynamics

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**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

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**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

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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

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Gas laws Force Applying a force on the are of this piston will apply a pressure on the gas inside

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**Gas laws Increasing the pressure results in a decrease in volume**

The volume decreases at the same rate as the pressure increases Force

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**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

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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

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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

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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

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**When the temperature increases the volume increases**

Gas Laws When the temperature increases the volume increases V= K T

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Gas Laws If the gas is heated and the volume cannot increase (in a closed container) the pressure increases P = K T

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**Combining the three laws we get**

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

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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 V = T1 P2 x V2 T2

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**Always use the Kelvin tEmperature scale in your calculations**

oC KELVIN 273 20 293 40 313 60 333 80 353 100 373 -273

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**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

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**Gas Laws Example (Pressure stay constant)**

V = T1 V2 T2 1.2 T2 = 295

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Gas Laws Example = 295K 1.2 T2 295K x 1.2 2 T = T = 177K ( -96oC)

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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.

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**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

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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

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**Gas Laws Example (Pressure stay constant)**

P1 x V = T1 P2 x V2 T2 P2 x 1 573 1 x 5 293 =

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**Gas Laws Example (Pressure stay constant)**

P1 x V = T1 P2 x V2 T2 1 x 5 x 573 293 x1 = P2 = bar

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