# Thermodynamics - pressure and temp.

• Niles
In summary, the pressure and temperature of a gas are closely related and affect each other. When the volume of a gas decreases, the pressure increases and when the volume increases, the pressure decreases. In the first scenario, the volume of the balloon decreases when it is put in the freezer, causing the pressure to decrease as well. In the second scenario, the volume of the gas in the hose is the same on both sides because the buckets are the same size, but the pressure and temperature are different on each side. The pressure and temperature of a gas can also be affected by the amount of gas present and the ideal gas law can be used to calculate these relationships. Ultimately, the temperature of a gas is a measure of its average kinetic energy
Niles

## Homework Statement

Ok, I'm a little confused about the connection between pressure and temperature. Let's take two scenarios:

1) I have a balloon filled with helium at 30 degrees, and then I put it in the freezer. Then the volume changes, but the pressure stays constant, right?

2) I have the following setup:

The two buckets have different temperature - so the gas inside the hose has different temperatures at the sides 1 and 2. But why isn't the volume in part 1 bigger than the volume in part 2? Is that because the pressure is not constant?

I can't quite figure these things out.

sincerely Niles.

1) I have a balloon filled with helium at 30 degrees, and then I put it in the freezer. Then the volume changes, but the pressure stays constant, right?

Well why does the volume decrease?

More fundamentally, why is the balloon "inflated"? Because the air inside exerts a pressure that causes the fabric to expand and voila, inflated balloon. If you shove in too much air the pressure is too great and the balloon ruptures. Reduce the pressure and the balloon shrinks a bit. Normally you reduce the pressure by letting air out. Cooling it however will slow the molecules in the air, reducing their average kinetic energy, so they're not going to spread out and cover as much space, and the pressure is reduced, which is why the volume decreases

For part B, it's as simple as the two buckets are the same size. You always assume the gas expands to fill its container, so there you have it.

EDIT: So you can infer everything you'd need to know from the ideal gas law. PV=nRT, even if it's not an ideal gas the basic relationships are the same

If you increase pressure while holding volume constant, temperature has to increase(so that the equality holds, you made the left side bigger, n and R are constants, got to make the right side bigger) and similarly for all the relationships. Basically remember that in a gas the temperature is a measurement of average kinetic energy. If there's a high temperature the molecules are bouncing a lot harder and are going to spread out and hit walls harder, meaning increased pressure, unless you allow the walls to expand, then increase volume. If you have gas with a set temperature and you shrink the volume, you have all those molecules with whatever kinetic energy now confined to a smaller space. If you're bouncing off the walls already, and you make the walls closer, you're going to be bouncing harder, and so on

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## What is the relationship between pressure and temperature in thermodynamics?

In thermodynamics, pressure and temperature are directly proportional to each other. This means that as temperature increases, pressure also increases and vice versa. This relationship is described by the ideal gas law, which states that pressure is equal to the product of the gas constant, temperature, and the number of moles of gas, divided by the volume.

## How does pressure affect the behavior of a gas?

Pressure is a measure of the force exerted by gas molecules on the walls of its container. As pressure increases, the gas molecules are forced closer together and move faster, resulting in an increase in temperature. This increase in temperature can cause the gas to change state, such as from a liquid to a gas, or from a gas to a solid.

## What is the difference between absolute and gauge pressure?

Absolute pressure is a measurement of the total pressure exerted by a gas, including atmospheric pressure. Gauge pressure, on the other hand, is a measurement of the pressure above atmospheric pressure. This means that gauge pressure can be either positive or negative, while absolute pressure is always positive.

## How does pressure change with altitude?

As altitude increases, the pressure in the atmosphere decreases. This is because the air molecules at higher altitudes have less weight and exert less force on the Earth's surface. This decrease in pressure can have significant effects on living organisms, such as difficulty breathing at high altitudes due to the lower oxygen levels.

## How is temperature related to the kinetic energy of gas molecules?

Temperature is a measure of the average kinetic energy of gas molecules. As temperature increases, the kinetic energy of gas molecules also increases, causing them to move faster and collide more frequently with each other and the walls of their container. This results in an increase in pressure and a larger volume for the gas.

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