Humidity & Vapour Pressure — A/L Physics

What must I remember about Humidity & Vapour Pressure?

• Vapour Pressure Concepts: Saturated Vapour Pressure (SVP) is the equilibrium vapor pressure exerted when liquid and vapor phases are in dynamic equilibrium; Unsaturated Vapour Pressure (UVP) is exerted when rate of evaporation exceeds condensation.
• Properties of SVP vs UVP: SVP depends SOLELY on temperature (exponentially increases with temperature, completely independent of volume); UVP obeys ordinary ideal gas laws (Boyle's and Charles's laws).
• Boiling Point Definition: The temperature at which the Saturated Vapour Pressure of a liquid equals the external atmospheric pressure.
• Relative Humidity (RH): Ratio of actual mass of water vapor present in a given volume of air to the mass of water vapor required to saturate the same volume at the same temperature: RH = (P_actual / SVP_{atcurrenttemp}) * 100% = (SVP_atdewpoint / SVP_{atcurrenttemp}) * 100%.

Test yourself (1 minute)

1. Dew Point

   The temperature to which unsaturated air must be cooled at constant pressure to become saturated with water vapor (at dew point, actual vapor pressure becomes the SVP).

2. Wet and Dry Bulb Hygrometer

   Evaporation from wet muslin cools the wet bulb; larger temperature difference (depression) indicates drier air and lower relative humidity.

3. Pressure Cooker Principle

   Increasing pressure above liquid raises its boiling point (water boils at ~120 °C at 2 atm), cooking food much faster.

Practise Humidity & Vapour Pressure questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 4: Thermal Physics

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Where does Humidity & Vapour Pressure fit in the syllabus?

Unit 4.7 · Year 1 · Term 2