# Gay lussac law

1. Dec 8, 2014

### gracy

why the bottle shrinks?(1)I know according to Gay lussac law Temperature ∝ pressure, as temperature decreases pressure also decreases .But why bottle would shrink if pressure decreases,because decreases in pressure just mean molecules are not hitting that fast.
(2)And if bottle shrinks it's volume no longer remain constant so how can gay lussac law explain this.Because according to gay lussac law volume should remain constant.

2. Dec 8, 2014

### Staff: Mentor

Pressure is a force exerted on the surface. When the pressure inside goes down external force crushes the bottle.

3. Dec 8, 2014

Which one?

4. Dec 8, 2014

### Staff: Mentor

That of the atmosphere. You are forgetting we are being squeezed by the 1 atm of the atmospheric pressure all the time. It doesn't matter when the bottle contains the air at the same pressure, but when you remove the air from the bottle, situation changes.

5. Dec 8, 2014

### gracy

You mean external pressure>internal pressure that's why bottle shrinks.

6. Dec 8, 2014

Yes.

7. Dec 8, 2014

### gracy

So in this video before the bottle or can is put in cold water ,the pressure inside was 1 atm?

8. Dec 8, 2014

### Staff: Mentor

Yes. It quickly drops down when the can is cooled down.

9. Dec 8, 2014

### gracy

But why?
When can cooled down ,pressure dropped down from 1 atm,so when can was heated at 0:40-0:50 seconds in this video pressure should have increased from 1 atm,right?So why didn't the can expand?

10. Dec 8, 2014

### Staff: Mentor

Can is opened, so when it is heated gases have no problems evacuating, and the pressure inside is the same as a pressure around. Then they put the can to water with the opening directed down, so when the pressure inside the can drops down rapidly, it starts to suck water. But the water is a dense liquid, and it can't be sucked up as fast as the gas could be - so before the pressure equilibrates, forces acting on the can have enough time to do the damage.

To be honest, I don't like this experiment, as the explanation of what is happening is far from obvious.

11. Dec 8, 2014

### gracy

Thanks.Very nice explanation.

12. Dec 9, 2014

### Staff: Mentor

Also note that it is not about Gay Lussac law (which is why I don't like the experiment). It may look like that's what is going on, but they added some water to the can. That means after heating boiling water purged most of the air from the can (the water vapor pressure was still making the inside pressure up to 1 atm). When they put the can into water what is happening is not the cooling of the gas, but condensation of the water. Net effect - inside pressure dropping down fast and external pressure crushing the can - is the same, but the physics behind the pressure drop is different from what they want you to think it is.