# Hot Air Balloon Equation

1. Apr 6, 2013

### Poseidonho

I had meet a problem when I study the concept of the hot air balloon. The equation was giving

Then I reverse to

When the volume is less then 1m^3, the temperature calculated giving negative temperature (In kelvin and degree also) but suppose had a greater in temperature with my understanding for the theory.

So is that any wrong in the equation or my understanding?

2. Apr 6, 2013

### SteamKing

Staff Emeritus
In order for the units of your formula to balance correctly, the following quantities must be specified:

Temperatures are in degrees Kelvin

Pressure in Pascals

Volume in cubic meters

R for air is 287.058 J/kg/K

3. Apr 7, 2013

### jbriggs444

As I understand it, you derived an equation that gives the required temperature to give a specified lift, holding everything else (ambient temperature, ambient pressure, volume of the balloon) constant. Now you wonder -- can the equation be correct even though it sometimes gives a negative temperature as a result?

Yes, it can be correct. If the load you are trying to lift is heavier than the mass of the air displaced by the balloon then no matter how hot you heat the air in your balloon, it can never be light enough to have a net upward buoyancy greater than the weight of the load.

The ideal gas law says that a fixed volume with a negative temperature has a negative mass. This is not physically reasonable, but does account for the solution you are getting when the load is too high.

4. Apr 7, 2013

### Poseidonho

Thank You very much, I understand d