1-3 物態（中）

A liquid can turn into gas at its boiling point as well as below its boiling point. The process of turning into gas below the boiling point is called "evaporation", which is what we will discuss in this video. 0:00 Opening 0:43 Evaporation 2:25 Cooling effect of evaporation 4:40 Evaporation and condensation 8:01 Factors affecting rate of evaporation Show all topics https://bit.ly/physics_concepts_eng ------------------------------ Supplementary information: [Relative humidity] In the video, I did not properly define relative humidity, but only described how it is like when the relative humidity reaches 100%. This is because defining and measuring relative humidity is not a simple issue. Understanding its definition and the related calculations would require deeper knowledge of gases and liquids. Relative humidity is more than a number that you see in the weather report. It is also an important thing to understand when it comes to the construction of steam turbines. [Vapour vs gas] In this video we have mentioned "vaporization" and "water vapour", so it would be natural to ask, what is "vapour"? What is the difference between "vapour" and "gas"? Well, for our level, we may consider the two as the same thing. Every substance has a "critical temperature". If the temperature of a gas is lower than the critical temperature, then the gas is a vapour. Therefore, a vapour is a kind of gas. However, at our level, we won't come across a gas above its critical temperature, so there is no need for us to distinguish the two. Some people say that a vapour is a group of tiny liquid droplets floating in the air and is not actually a gas. This is a misconception. Liquid droplets floating in the air should be called "aerosol" instead of "vapour". [Water vapour vs steam] What is the difference between "water vapour" and "steam"? For our level, again, we may consider the two as the same thing. Both of them refer to the gas state of water. One may guess that "water vapour" refers strictly to the "vapour" form of water, while "steam" refers to the more general "gas" form of water. Well... this is not the case. Indeed "water vapour" quite simply refers to the vapour form of water, while "steam" is a rather complicated thing. Steam can be classified into "wet" and "dry", or it can be classified into "saturated" and "unsaturated". So you may have heard about "unsaturated wet steam" and so on. "Wet steam" is a mixture of gaseous water and water aerosol, while "dry steam" is pure gaseous water. "Saturated steam" is a steam which has reached the state of "100% relative humidity" as described in the video, while "unsaturated steam" is a steam with a density lower than that of a saturated steam. ------------------------------ Acknowledgements: Royalty Free Music from https://audiohub.com #DSE #physics #thermodynamics #物理 #熱和氣體 #熱力學