Its really confusing me...................if warm air expands then it will be cooled as it expands thus there is warming. PLz help and WHY does warm air expand by the way??
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There are MULTIPLE types of expansion processes. Some expansion processes have a gas expand as it warms, some expansion processes have a gas reduce in temperature as it expands.
It all depends on the boundary conditions of the expansion.
The processes can be generalized by the polytropic condition.
P*V^n = constant.
n can be any value, the constant can be any value for the particular process.
n is called the polytropic index, and I will give it a number as I discuss each SPECIAL CASE.
For the case of the isobaric expansion (n = 0), a constant pressure process, the gas increases in volume (hence expands) as it is heated, and simultaneously increases in temperature Some of the heat that goes in to it stays in it as internal energy, and a slightly smaller amount exits as work done by the gas.
For the case of isothermal expansion (n=1), a constant temperature process, the gas increases in volume (hence expands) as heat is added to it, but it DOESN'T change its temperature. All heat that goes in to the gas exits as work done on the surroundings.
For the case of adiabatic expansion (n = k, a special flavor dependent constant called adiabatic index, k=1.4 for air), a process of NO HEAT TRANSFER, the gas increases in volume (hence it expands), but you cannot say it is heated. In fact, what actually happens is that the gas REDUCES IN TEMPERATURE. The loss of internal energy goes directly to work done on the surroundings.
Notice that I do not like to use the word "cool" in this explanation? That is because the gas doesn't reject heat. It reduces in temperature, but it doesn't reject heat. The verb "cool" sounds as if it implies a rejection of heat as well as a reduction in temperature, which makes it a very confusing word for any process involving gasses.
For the case of an isochoric process (n=infinity), constant volume (aka, NOT an expansion), heat might be added to the gas, but it CAN'T expand nor contract. It just "attempts" to expand as it builds up greater and greater pressure.
It all depends on the boundary conditions of the expansion.
The processes can be generalized by the polytropic condition.
P*V^n = constant.
n can be any value, the constant can be any value for the particular process.
n is called the polytropic index, and I will give it a number as I discuss each SPECIAL CASE.
For the case of the isobaric expansion (n = 0), a constant pressure process, the gas increases in volume (hence expands) as it is heated, and simultaneously increases in temperature Some of the heat that goes in to it stays in it as internal energy, and a slightly smaller amount exits as work done by the gas.
For the case of isothermal expansion (n=1), a constant temperature process, the gas increases in volume (hence expands) as heat is added to it, but it DOESN'T change its temperature. All heat that goes in to the gas exits as work done on the surroundings.
For the case of adiabatic expansion (n = k, a special flavor dependent constant called adiabatic index, k=1.4 for air), a process of NO HEAT TRANSFER, the gas increases in volume (hence it expands), but you cannot say it is heated. In fact, what actually happens is that the gas REDUCES IN TEMPERATURE. The loss of internal energy goes directly to work done on the surroundings.
Notice that I do not like to use the word "cool" in this explanation? That is because the gas doesn't reject heat. It reduces in temperature, but it doesn't reject heat. The verb "cool" sounds as if it implies a rejection of heat as well as a reduction in temperature, which makes it a very confusing word for any process involving gasses.
For the case of an isochoric process (n=infinity), constant volume (aka, NOT an expansion), heat might be added to the gas, but it CAN'T expand nor contract. It just "attempts" to expand as it builds up greater and greater pressure.
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You are ignoring pressure.
A parcel of warm air in a body of cooler air at the same pressure, will expand.
But a body of air that is reduced in pressure will cool down relative to the air at its former pressure.
By the same token raise the relative pressure, and the temperature will increase.
Warm air expands because of the increased motion of the molecules. They "make themselves more room to move".
A parcel of warm air in a body of cooler air at the same pressure, will expand.
But a body of air that is reduced in pressure will cool down relative to the air at its former pressure.
By the same token raise the relative pressure, and the temperature will increase.
Warm air expands because of the increased motion of the molecules. They "make themselves more room to move".