I Compressing gas with piston: reasons for temperature increase?

[ikihi]

So when volume decreases, pressure increases according to Boyle's Gas Law and the ideal Gas Law. In other words, compressing gas into a smaller volume increases the vapor pressure. And also, According to Gay-Lussac's Law and ideal Gas Law, when pressure increases on a gas, temperature also increases.

So am I correct in thinking that when a piston compresses gas ,the volume decreases; which increases pressure. Then this same higher pressure should make the temperature increase?

What are all of the sources behind higher pressure equaling higher temperature?

If temperature is proportional to the average kinetic energy of gas molecules then would just the fact of having more molecules in a smaller space increase the temperature due more kinetic collisions between the molecules and closer walls? Does it depend on how fast the volume is reduced by compression?

When you reduce a container volume by rapid compression, the temperature increases due to work on on the gas? This work transfers kinetic energy to gas molecules and extra energy should increase the temperature?

 

[mjc123]

Boyle's law relates pressure and volume of a gas at constant temperature. Gay-Lussac's law relates the pressure and temperature at constant volume. You can't apply both of them at the same time. The ideal gas law generalises these laws (and Charles's law) to relate pressure, volume and temperature. Depending on the process, you can change any two of these properties while maintaining the third, or you can change all three at once. Thus it is not true to say, as a general rule, that reducing the volume increases the pressure and increases the temperature. It may do, it may not.

 

[jbriggs444]

When you reduce a container volume by rapid compression, the temperature increases due to work on on the gas? This work transfers kinetic energy to gas molecules and extra energy should increase the temperature?

Yes. Rapid compression or slow compression, both do work. Both result in gas molecules moving more energetically.

A compression rapid enough to produce pressure waves or turbulence in the gas would result in more work being done compared to a slow compression. Normally one would avoid such complications when trying to grasp the basics.

 

[ikihi]

Boyle's law relates pressure and volume of a gas at constant temperature. Gay-Lussac's law relates the pressure and temperature at constant volume. You can't apply both of them at the same time. The ideal gas law generalises these laws (and Charles's law) to relate pressure, volume and temperature. Depending on the process, you can change any two of these properties while maintaining the third, or you can change all three at once. Thus it is not true to say, as a general rule, that reducing the volume increases the pressure and increases the temperature. It may do, it may not.

When volume is reduced, pressure should increase and temperature should decrease according to these laws. So the reason for the temperature increase is only because of the work done on the gas molecules by the piston?

 

[jbriggs444]

When volume is reduced, pressure should increase and temperature should decrease according to these laws.

No. Those laws do not require that.

Normally in an adiabatic compression, volume is reduced, pressure is increased and temperature is also increased while amount of substance remains unchanged.

The air in the cylinder gets hot and pressurized. You can inject the kerosene to make the Diesel cycle go.

If you are doing things adiabatically then the relevant ideal gas law is PV=nRT. The others are not applicable because you are not holding the required things constant.

 

[Anand Sivaram]

You can compress the gas typically in two processes - adiabatic compression or isothermal compression. Boyle's law speak only about isothermal compression.
For Adiabatic process the equation is PV^(gamma) = Constant.

For example, you compress air at 1 bar and 300K temperature to a volume of 1/10. If it is isothermal compression you could get a pressure of 10 bar after compression with the same 300K temperature.

But, if it is Adiabatic compression using the Adiabatic equation, you would see that the pressure would be rather 25 bars.
Now, using the Universal gas law with volume 1/10, pressure 25 bars the new temperature would be around 750K. That is the difference.