# Gas Laws and Pressure

by bluegirlbalance
Tags: laws, pressure
 P: 9 As a hot air balloon rises to increasingly higher altitudes describe what happens to the volume of the balloon if at constant pressure? My teacher had us do a PTV cards activity, where we took an index card and wrote the letters P(for pressure), T(for temperature), and V(for volume). This activity was supposed to help us learn about pressure. For example, if the pressure increases and the temperature remains constant, what happens to the volume? It increases. For the question above, if the pressure remains constant as the hot air balloon rises, the volume increases, right?
 Mentor P: 39,720 Correct. PV = nRT.
 P: 16 as the hot air balloon rises, temperature gets lower, right? as temperature drops but all other variables stay the same, volume must drop berkeman why do you say the volume increases? what am i missing?
P: 263

## Gas Laws and Pressure

PV = nRT
If R is a constant, and temperature remains the same. Moles aren't changing. Therefore, if the pressure goes up, the volume must go down. If the volume went up too, then that would completely change the answer! Do you see why?
 P: 16 But we are supposed to assume a constant pressure according to the question if the pressure is constant, the only variable changing as the hot air balloon rises, would be it is getting colder, which means less volume
 Mentor P: 39,720 The pressure inside the balloon is constant, but the pressure outside is dropping. So the net force on the balloon walls would increase toward the outside, and the balloon would grow. Of course, that assumes that the temperature is not going down substantially (the problem did not address this explicitly), and that the balloon walls are elastic.
Mentor
P: 39,720
 Quote by berkeman The pressure inside the balloon is constant, but the pressure outside is dropping. So the net force on the balloon walls would increase toward the outside, and the balloon would grow. Of course, that assumes that the temperature is not going down substantially (the problem did not address this explicitly), and that the balloon walls are elastic.
Actually, the more I think about it, I am not sure of the answer. I don't understand what the problem statement means by "what happens to the volume of the balloon if at constant pressure?". What's at a constant pressure? The absolute pressure of the gas inside the balloon?
 Sci Advisor HW Helper PF Gold P: 3,725 Hot air balloons are open on the bottom. Volume remains constant. Density of hot air at constant P is less so the balloon is buoyant and rises. At lower external pressures (higher altitude) hot air is forced out the bottom of the balloon and therefore, the number of moles is less. Pressure inside the balloon must be the same as the outside at equilibrium regardless of altitude. If this condition is not met, air will flow from high pressure to low pressure (out the bottom). This is not the case for Helium balloons where the number of moles of gas remains constant.
Emeritus