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isobaric expansion



Hello,

I have a question on isobaric expansion (this is taken from the paper published by Rozier and Viennot):
”An ideal gas is heated at constant pressure. Its volume and temperature both increase. Why?”

The question is quite easy to answer from the point of view of energy. Heat transferred to the system is partially used to mechanical work (= P*deltaV) and partially to increase kinetic energy of the molecules and hence temperature.

But what happens in a molecular level? The molecules bounce from a piston and exert a force on it. When temperature increases the molecules have greater momentum which results to greater force in collisions. Increase in volume decreases number of collisions per time unit. The overall effect is that average force per area, pressure, remains constant.

Is there momentarily greater pressure exerted on the piston for instance in the beginning of the process? A student of mine asked this and I couldn’t convince him that pressure is constant all the time (that’s why it is called isobaric!). Have I given the correct argument or am I missing something?

Regards,

Antti Savinainen
Kuopio Lyseo High School/IB
Finland