PV=nRT question

[Meredith92]

Hey guys,

I always somehow get tripped up on this kind of thinking...
so i know that when air is heated it becomes less dense
But I always get confused about whether we should have figured this out based on the effects of V or P

so PV=nRT if T increases, PV should also increase. If air is heated, is it that the increase in T causes V to increase? and then P decreases as a result? I can never think of this in a clear/ systematic way! I'd appreciate any help!

Thanks

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[asf503]

Hey guys,

I always somehow get tripped up on this kind of thinking...
so i know that when air is heated it becomes less dense
But I always get confused about whether we should have figured this out based on the effects of V or P

so PV=nRT if T increases, PV should also increase. If air is heated, is it that the increase in T causes V to increase? and then P decreases as a result? I can never think of this in a clear/ systematic way! I'd appreciate any help!

Thanks

When air is heated, it becomes less dense if it is able to expand. If air is in a non expandable box, the same amount of air in grams will occupy the same volume; thus, density=amount/volume would stay the same. What would change in a situation where you increase T then, according to PV=nRT, must be pressure.

If the air is in a piston system (ie it can expand), things change. Since volume can now be changed, the volume should now increase. The heat you're putting into the system by increasing T should go into expanding the volume. So V goes up but P stays the same.

Does that help?

 

[Meredith92]

That makes sense! Where I get confused is if they talk about air in a balloon or just an area filled w
Air that can expand... Do I treat this as the system where p is constant and v changes?

 

[mehc012]

That makes sense! Where I get confused is if they talk about air in a balloon or just an area filled w
Air that can expand... Do I treat this as the system where p is constant and v changes?

Pressure is basically the gas molecules bouncing off the walls and pushing them outward. If the pressure between two areas is inequal, then the push from one side will be stronger than the push from the other side. If those areas are separated by a flexible/movable wall (as in the balloon or a piston), then the wall will be shoved away from the stronger push (higher pressure), changing the volumes, and eventually equalizing the pressures (though clearly that's the ideal...in the nonideal world you need to account for the elastic force of the balloon or the friction of the piston). So, yes, to simplify, in those situations where V is not fixed, P should equalize (requiring a ΔV).

 

[MedPR]

Hey guys,

I always somehow get tripped up on this kind of thinking...
so i know that when air is heated it becomes less dense
But I always get confused about whether we should have figured this out based on the effects of V or P

so PV=nRT if T increases, PV should also increase. If air is heated, is it that the increase in T causes V to increase? and then P decreases as a result? I can never think of this in a clear/ systematic way! I'd appreciate any help!

Thanks

Idk why people go into these lengthy explanations.

T up = gas molecules gain kinetic energy = move faster = more collisions with container = higher pressure assuming constant volume.

If pressure is constant then volume will increase (like blowing hot air into a balloon).

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[pacer]

Hey guys,

I always somehow get tripped up on this kind of thinking...
so i know that when air is heated it becomes less dense
But I always get confused about whether we should have figured this out based on the effects of V or P

so PV=nRT if T increases, PV should also increase. If air is heated, is it that the increase in T causes V to increase? and then P decreases as a result? I can never think of this in a clear/ systematic way! I'd appreciate any help!

Thanks

If you are considering the density of air, its better to use the

Density = mass/volume

When air is heated, it expands, hence its volume increases.

Density and volume are inversely proportional and thus, increase in volume leads t a decrease is density. That's why heated air has lower density.

You can also think of the PV = nRT formula.

To heat air, you increase T, which is directly proportional to V and increases volume. This once again, leads to a decrease in density.