5.18

Cloud formation

·         Place a little water in the bottom of a 1½ litre plastic bottle

·         Squeeze a few times

·         Introduce a small amount of smoke

·         Squeeze and release several times

·         When you squeeze, the cloud disappears; when you release, the cloud reforms

 

 

Explanation

·         When the pressure increases the temperature increases and vica versa

·         The smoke particles are nucleating sites on which the water can condense

 

5.18 use the relationship between the pressure and Kelvin temperature of a fixed mass of gas at constant volume:

                p₁ / T₁ = p₂ / T₂

p₁ = Pressure at the beginning [kPa, bar or atm ]

T₁ = Absolute temperature at the beginning [K]

p₂ = Pressure at the end [kPa, bar or atm]

T₂ = Absolute temperature at the end [K]

(Note: the units of temperature must be Kelvin, not ^oC!  The units of pressure can be any, as long as the same at the beginning and the end)

 

 

5.18 Ideal graph and conclusion

09 November 2011

15:15

 

 

A motor car tyre is filled to a pressure of 3 bar at 20°C. After a long journey, the tyre reaches a temperature of 55°C. What is the pressure now?

p₁ / T₁ = p₂ / T₂

3/(20+273) = p₂ / (55+273)

Pressure = 3.36 bars

If we cool the gas in a rigid, sealed tin can, what happens to the pressure inside the can?

The pressure will drop.

Explain your answer to part a by using the Kinetic Theory.

-          Temperature decreases

-          Less frequent collisions and slower collisions

-          Particles hit the walls with less force

-          Same area

-          P = F/A so pressure drops