Does pressure alone create heat?

Question

I'm having an argument with someone. If there are no other factors involved (friction/combustion,etc.) will pressurizing air make it heat up?

Answer 1

As long as a fixed amount is held in a fixed volume, yes.

Assuming you have an ideal gas:

PV = nRT proves this is so.

P = pressure
V = volume
n = number of moles (proportionate to number of atoms or molecules) of gas
R = is the Universal Gas Constant (a constant)
T = temperature

So, if P gets larger and V stays the same...

only n or T may change...and assuming you are neither adding or taking away gas, T must get proportionally larger.

Answer 2

The answer to this is more complex than you might think.

PV = const.T does not help a lot here. It says you could certainly compress a given mass of gas without its temperature changing, provided you reduced its volume. Equally it says that the only way to increase the pressure while keeping the volume the same is to increase the temperature (note that there is no mechanical way to pressurise a gas while keeping its volume the same).

Of course, a comressor does increase the pressure of a given volume of gas (a cyclinder of air for instance) but it does this by adding more gas - so the mass does not stay the same and PV = RT does not apply.

However, if you carry out an adiabatic compression of a fixed mass of gas (adiabatic means without letting energy in or out of the gas) then the gas will be heated because you have to do work on the gas to compress it (push a force through a distance) and this adds energy to the system. As temperature is a measure of the energy of the molecules, this must rise.

Answer 3

Yes, compressing air will cause it to heat up because it forces the molecules closer together, resulting in an increased rate of molecular collision, and the resulting molecular friction releases heat. When a high pressure tank, like a SCUBA tank, is being filled with air, it is usually cooled by immersion in water. This is necessary because the generated heat increases the pressure, and if the tank were allowed to be filled warm, the internal pressure would later decrease as the tank cooled.

Answer 4

Yes, pressurizing air will make it heat up. You can then cool it off again while it is pressurized and you will have cool, pressurized gas. For example, if you used an air compressor to put compressed air into a gas cylinder, you would find that the cylinder became hot. If you then close the valve on the cylinder and let it sit at room temperature, it will cool off and the pressure will drop a little. You will then have room-temperature compressed air. If you then opened the valve, the air coming out would be colder than room temperature.

Answer 5

Yes it will. One way to look at this is to consider that compressing air takes energy, and energy has to be conserved. So the pressurized air stores the energy by increasing the average velocity of its molecules. This average velocity is what determines temperature. Also, because it affects how hard the molecules bang against the sides of the container, it increases the pressure.

Answer 6

yes, The specific heat capacities of substances comprised of molecules (distinct from the monatomic gases) are not fixed constants and vary somewhat depending on temperature. The pressure at which specific heat capacity is measured is especially important for gases and liquids. The standard pressure was once virtually always “one standard atmosphere” which is defined as the sea level–equivalent value of precisely 101.325 kPa (760 torr).

Answer 7

yes air does heat up in the compression process and cools when you decompress it. Is why air from a compressor feels cold. but compressed air in a tank will not hold a higher temp than the air surrounding the tank.

Answer 8

Yes

Answer 9

Yes

Answer 10

Yes