# Homework Help: Temp/Pressure Relationship Open Parcel of Air

1. Jul 6, 2017

### firefuze

I know that in a closed parcel of air, if we increase temperature then pressure rises and density remains constant

If we warm a parcel of air that is open i.e not enclosed, such as the atmosphere.
What happens to pressure? do we say it increases or decreases?

The book I have states; If the heated air is not contained, its molecules-given greater kinetic energy will move further apart and will expand and its density will decrease. The parcel of heated air will continue to expand until its pressure is equal to that of the surrounding air

2. Jul 6, 2017

### BvU

Hello ff,

I agree with the book statement. If you expand it a little: before heating the pressure was also that of the surrounding air. So imho the pressure remains equal.

If you go into this further: the change in density and volume does mean that the force balance on the heated volume of air changes and as a consequence it may want to move upwards.

Last edited: Jul 6, 2017
3. Jul 6, 2017

### Staff: Mentor

Suppose you have a closed parcel of air in a cylinder with a massless frictionless piston above, and atmospheric air outside. What do you think will happen if you add heat to the parcel of air in the cylinder, in terms of its volume, its pressure, and its density?

4. Jul 6, 2017

### CWatters

Book answer is good. It's how a hot air balloon works. They are open at the bottom.