why does air expand when it rises
Why does air get colder as it expands? The frequency of atomic collisions decrease as air expands, therefore the air gets
cooler. Temperature is just the average heat of a substance. That is, if you take the kinetic energy (heat) of all the particles in a given volume, and divide by the volume you get the energy density, which we call temperature. If you have a certain amount of air, the particles have a fixed amount of heat (unless you let the heat pass to some other substance or use some to do work) and so if you let the air expand you decrease the temperature (mathematically you are dividing the heat by a larger number). Thus the blast of a CO fire extinguisher can be used to cool a can of your favorite beverage, or freeze an attacking Blob! (for you monster movie buffs out there! ) Answered by: Rob Landolfi, Science Teacher, Washington, DC The Ideal Gas Law states pV = nRT, where P = The pressure of the gas in Pa, V = Volume of gas in m, n = Number of moles of gas, R = A constant of about 8. 314 and T = Temperature in K. As a gas (like air) expands, the value of V increases and this has the effect of increasing T (The temperature). As the energy needed to increase it's temperature must be supplied from somewhere, the gas takes the energy from the surrounding system giving the effect of cooling.
This is a principle used in refrigeration. Answered by: Peter Talman, B. S. , Post Grad Student, Portsmouth University, UK 1. While PRESSURE is related to the number of collisions, temperature is not. Temperature is defined only by the average K. E. of all the gas molecules. 2. Temperature is also not directly based on an energy 'density'. It is an average over the total number of molecules, which does NOT change when an enclosed gas is expanded. 3. This answer doesn't address the fact that as V increases, P decreases, which would allow T to remain constant and still satisfy the relationship. The third answer does come closest, however. Imagine an enclosed gas pushing against a piston surrounded by a vaccuum. Pressure will cause the piston to move, expending energy. The energy has to come from somewhere, and that is the K. E. of the gas molecules. It's simply this loss of K. E. that lowers the gas temperature. Michael Onstad's answer to this question perpetuates a misconception. The question is, 'Why does air cool as it expands? ' Temperature is related to neither energy density nor frequency of molecular collisions. Temperature *is* related to molecular KE. Air molecules lose KE only by doing work on other molecules or by fighting Earth's gravity (i. e. , rising).
So, expanding air does NOT always cool; it depends on the nature of the expansion. Did you know that not just metals but air expands on heating as well? But how? What makes air expand on heating? What is a land breeze? And what is a sea breeze? Let us know more! Air expands on heating. Why does this happen? It is because the particles of air on becoming warm, move away from each other and take up more space. So we can say that air on becoming hot occupies more space. Since the particles move away from each other, the air becomes less dense and lighter in weight. As a result, warm air rises up. On the contrary, on cooling the particles of air come closer to each other. Due to this, the volume of air becomes less, the air becomes more dense and heavier. This can be demonstrated by a simple activity. Activity I Materials Required: balloons, trough, ice cold water, hot water, bottle Method: Take a bottle. Tie the balloon tightly on the neck of the bottle. Place the bottle in a container having hot water. Record your observation. Now place the bottle in cold water. Again observe.
Observation: When the bottle is placed in hot water, the balloon expands. When the bottle is placed in cold water, it contracts. Inference: This happens because when the bottle is placed in hot water, the air in it becomes warm and expands due to which balloon also expands. When the bottle is placed in cold water, the air in the bottle contracts due to which balloon also contracts. Daily life examples where you can see the air expands on heating Hot air balloon rises up:P This is because the air in the balloon expands on heating. It also becomes lighter hence rises up. Monsoon Winds : Monsoon winds are caused due to more heating of land at the equator during summer months. Due to this, the air above the land at equator rises up. The moisture-laden winds from oceans towards southern hemisphere blow towards the equator. These are the monsoon winds which cause rainfall. Land Breeze and Sea Breeze: They are caused due to uneven heating of the air. The land always gets heated and gets cooled faster as compared to water. In coastal areas, during the daytime, the air above the land gets heated very quickly as compared to water in the sea. The air above the land is hotter and lighter rises up. The cool air above the sea rushes towards land to take its place.
This causes sea breeze. During night land gets cooled faster whereas sea is still hot. The air above the sea being comparatively hotter rises up. The air above the land rushes towards the sea. This causes the land breeze. What Causes Winds? Wind is caused due to uneven heating of the earth. Different parts of the earth receive the different amount of heat. The Equator receives direct sun rays due to which it gets more heated up as compared to poles where the sun rays are slanting. The air from the equator rises up and cold air from poles rushes towards the equator to take its place. This causes the convection current in air called winds. Questions For You Q1. A fire alarm usually detects smoke in case of fire. Where should such an alarm be placed in a room? Ans: (d) on the ceiling. This is because the hot air of fire will rise up towards the ceiling and will be easily detected. Q2. A flag mounted on a flag post near the sea coast flutters in the direction of the sea. At what time of the day does this happen at midnight or in the afternoon? Ans: P This happens during midnight. This is because land breeze blows during midnight which flutters the flag in the direction of the sea.
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