# Thermodynamics (piston)

By the formula PV/T = PV/T,
If there is a piston, and it compresses, so the volume gets smaller, the temperature will go down too..

But, I think if the volume gets smaller, the space is smaller, so the gas particle will collide more frequently, which makes the temperature goes up.
And, my logic breaks what formula says.
I really have no idea what actually V/T = V/T says.

And, I want to know what pressure really is on gas particle.
Is it the force given by a gas particle to another per the gas particle area? Or, is it the pressure given by gas particles to the piston ??
If the piston compresses, will the pressure also get bigger?

Last edited:

The actual formula is PV = nRT, don't mind nR it's just a constant, so PV/T doesn't change too and you already know that, but for just re-arrange the Eq P/T = nR/V, as the volume gets smaller the P/T ratio does up, this know depend on the kind of the piston and the process (isothermal process thus no temperature change or isobaric which means to pressure change), but I'll assume that you have an everyday piston and an everyday use, then It's temperature goes up and so does the pressure, Pressure is force per unit m*m, if you take an element of the surface of your piston and calculate the force acting (Impulses) you'll end up getting the pressure, as the volume compress collision happens quite frequent thus pressure goes up, the kinetic energy of the gas particles rises too, Good lucl !

• 0pt618 and terryds
PAllen
By the formula PV/T = PV/T,
If there is a piston, and it compresses, so the volume gets smaller, the temperature will go down too..
Focusing first on this: really? Suppose T is held constant via equilibrium with a large body of fluid, while the piston is slowly pressed. What does that imply if V is cut in half?

The actual formula is PV = nRT, don't mind nR it's just a constant, so PV/T doesn't change too and you already know that, but for just re-arrange the Eq P/T = nR/V, as the volume gets smaller the P/T ratio does up, this know depend on the kind of the piston and the process (isothermal process thus no temperature change or isobaric which means to pressure change), but I'll assume that you have an everyday piston and an everyday use, then It's temperature goes up and so does the pressure, Pressure is force per unit m*m, if you take an element of the surface of your piston and calculate the force acting (Impulses) you'll end up getting the pressure, as the volume compress collision happens quite frequent thus pressure goes up, the kinetic energy of the gas particles rises too, Good lucl !

Thanks,
But, I'm a bit confused now.
PV = nRT
The piston is pushed down -> volume of gas gets smaller
So, of course that the pressure will get bigger.
But, the temperature??? It'll stay the same, right ?
Because volume gets smaller, but the pressure gets bigger, and I think the moles stay the same too.

Focusing first on this: really? Suppose T is held constant via equilibrium with a large body of fluid, while the piston is slowly pressed. What does that imply if V is cut in half?

Of course the pressure will be doubled.

PAllen
• 0pt618 and terryds
Just wanted to add this is known as an adiabatic process (no heat transfer).

• terryds
Just wanted to add this is known as an adiabatic process (no heat transfer).
Isotermic = the temperature stays the same

What's the difference ?

PAllen
Isotermic = the temperature stays the same

What's the difference ?
They are very different. To maintain temperature while a piston is pushed, heat must be transferred out of the system. With an adiabetic process, there is no heat transfer, and temperature increases as the piston is pushed (due to work done on the gas by whatever is pushing the piston). The link I gave before shows how to compute the temperature increase (under idealized assumptions) for a given volume reduction for an adiabetic=isentropic process.

• terryds
OldEngr63
Gold Member
Better not drive a car, motorcycle, or use a gasoline powered mower until you figure this out. It might fool you and refuse to run until you know what you want to have happen inside the engine.

• DeldotB