describe saturated vapor pressure and also explain how refrigerator works_ _ education for all

7/29/2019 Describe Saturated Vapor Pressure and Also Explain How Refrigerator Works_ _ EDUCATION for ALL

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Describe saturated vapor pressure and

also explain how refrigerator works?

Posted on April 30, 2012 by adnan

SATURATED VAPOR PRESSURE

The process of evaporation in a closed container will proceed until there are as many

molecules returning to the liquid as there are escaping. At this point the vapor is said to be

saturated, and the pressure of that vapor (usually expressed in mmHg) is called the saturated

vapor pressure.

Since the molecular kinetic energy is greater at higher temperature, more molecules can

escape the surface and the saturated vapor pressure is correspondingly higher. If the liquid is

open to the air, then the vapor pressure is seen as a partial pressure along with the other

constituents of the air. The temperature at which the vapor pressure is equal to the

atmospheric is called the boiling point.

EVAPORATION

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Ordinary evaporation is a surface phenomenon some molecules have enough kinetic energy

to escape. If the container is closed, equilibrium is reached where an equal number of

molecules return to the surface. The pressure of this equilibrium is called the saturation vapor

pressure.

In order to evaporate, a mass of water must collect the large heat of vaporization, so

evaporation is a potent cooling mechanism. Evaporation heat loss is a major climatic factor

and is crucial in the cooling of the human body.

Refrigerator:The operation of a refrigerator is based on two physical principles. First, when a

substance changes from liquid state to gaseous state, its temperature would remain at its

boiling point until the liquid is all evaporated. The substance has to absorb certain amount of

energy, called latent heat, during the change. On the other hand, the boiling point of a

substance rises under high pressure, thus the gasified substance may return to its liquid

state and releases the latent heat.

There is a pipe partly placed inside a refrigerator and partly outside it. A special substance

inside the pipe has a very low boiling point. It is continuously circulated within the pipe by a

pump at the bottom of the pipe (Fig. 1). There is a tiny hole at the upper part of the pipe. It

slows down the speed of the substance inside the outer pipe, and thus the substance there

would be under high pressure. As a result, the boiling point of the substance in this area

would rise, and hence the substance would change from gaseous state to liquid state,

releasing the latent heat. When the substance enters into the refrigerator through the tiny

hole, its pressure would be reduced and its boiling point would return to its previous low level.

The substance would then change from liquid state to gaseous state; when the heat (latentheat) inside the refrigerator is absorbed during the change, the temperature inside the

refrigerator would be lowered, resulting in a freezing effect.

In addition, since the substance outside the refrigerator has to release its heat, it is painted

black a substance in black emits more radiation to facilitate the release. Besides, many

large metal pieces are placed around the pipe for the same purpose.
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Cooling by evaporation:

Because of the large heat of vaporization of water, the evaporation from a liquid surface is a

very effective cooling mechanism. The human body makes use of evaporative cooling

by perspiration to give off energy even when surrounded by a temperature higher than body

temperature. The cooling process is an example of the approach to thermal equilibrium.

Heat transfer by vaporization:

If part of a liquid evaporates, it cools the liquid remaining behind because it must extract the

necessary heat of vaporization from that liquid in order to make the phase to the gaseous

state. It is therefore an important means of heat transfer in certain circumstances, such as

the cooling of the human body when it is subjected to ambient temperatures above the normal

body temperature.

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