# Relation between pressure and volume when heating a gas

Hi all,

A happy 2018 for all PF members.

I have a question. Im trying to understand some basic principle of thermodynamics. My book tells me tge following:

"Heating of a gas at a constant pressure uses more heat as when heating the same gas a constant volume"

How do i interpret this? I find this paradoxical. If I visualize this for myself I come at the following:

I have a gas this gas is e.g. in a box which I will heat up to e certain temperature. Let say it gets 10 deg C higher. If the pressure is maintained constant (which i dont understand first of all, i thought that the pressure would increase as I heat the gas) I need more energie (joules) as when I heat up the gas to the same 10 deg C high but with a constant volume.

This all confuses me a lot. Can anybody help me understand this phenomena?

Raymond

jbriggs444
Homework Helper
If the pressure is maintained constant (which i dont understand first of all, i thought that the pressure would increase as I heat the gas)
If pressure is maintained constant then volume must be increasing. For instance, you are heating a gas in a cylinder with a piston at one end. The heated gas did mechanical work to move the piston against a [nearly] equal resistance. That takes energy.

• ElectricRay
phyzguy
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