# Gas in light bulb

1. Jan 8, 2014

### MathewsMD

If you have a light bulb that weighs 10 g when empty, how does adding an ideal gas inside of it change the mass?

If I'm not mistaken, the force the gas exerts on the light bulb (well, a perfectly symmetrical object is a better example) is the pressure, and it would be pointing in all directions. Overall, all the upward and downward vectors (as well as others) of the force of the individual gas molecules would cancel, right? So then, how exactly would the force of gravity of this system be different from 10 g if the Fup-gas = Fdown-gas, just in opposite directions? Wouldn't this cancel the effects of the light bulb being pushed down since the net force from the gas (the weight from the gas) is 0.

2. Jan 8, 2014

### SteamKing

Staff Emeritus
A helium filled balloon rises not because the helium pressure vectors inside the envelope cancel, but because the balloon with the helium inside is lighter than the air it displaces.

In a balloon, the pressure of the inflating gas gives the envelope its inflated shape. In a light bulb, which has a glass envelope that is stiffer than that of a balloon, the pressure of the gas inside pushes against the glass, which in turn, pushes back against the gas pressure.

3. Jan 8, 2014

### phinds

Do you understand the difference between weight and mass? You seem to be using one to ask about the other.

4. Jan 9, 2014

### Staff: Mentor

Forces acting inside of the bulb and forces acting outside of the bulb are not the same forces.

5. Jan 9, 2014

### Staff: Mentor

The OP is actually correct on the logic, just wrong on the conclusion: the pressure on the bottom surface of the lamp is higher than on the top surface, due to the weight of the gas.

The problem is just a usually useful simplifying assumption (equal pressure everywhere) that isn't 100% correct and in this case leads you to the wrong answer.

Last edited: Jan 9, 2014