4
$\begingroup$

I've been in a few classes now that teach about energy, and I feel they're always so bad. There is always such a rush to start using equations to get problems solved, and I often feel I don't have an intuitive grasp on energy at all. I guess this question is also about pressure.

Questions:

  1. Does pressure decrease with time? If pressure is caused by the force of gas molecules hitting the inside surface of a container, don't they transfer energy and eventually slow down enough to totally reduce pressure?
  2. Along the same lines, energy can be released or absorbed by a reaction as work or heat. If the only change that happens is through work (in something that is totally insulated - where gas is formed and pressure is created), after the pressure is created, doesn't it require continuous energy to maintain pressure against a constant force of 1 atm (or whatever it is)? Heat dissipates to all things, and that makes sense as everything gets colder in time. The same does not seem true for pressure.

Thanks in advance.

Noobsauce

$\endgroup$
2
$\begingroup$

Does pressure decrease with time? If pressure is caused by the force of gas molecules hitting the inside surface of a container, don't they transfer energy and eventually slow down enough to totally reduce pressure?

No. As you know, an aerosol can remains pressurised indefinitely until it is used. The molecules do not slow down if no energy transfer occurs, because there are no smaller bodies to transfer their energy to.

At the start of a game of snooker/pool/billiards (whatever you play in your part of the world) the balls are arranged in a triangle at the bottom of the table. You place the white ball at the top of the table and hit it into the other balls. When the collision occurs, the white ball shares its energy (normally unevenly) with the other balls. What we see is an increase in disorder. Even if we neglect friction, once this has occured it is very unlikely (impossible even) that all the other balls would stop and the white ball would have all the energy again. The way the energy of the white ball is spread between the other balls is a macroscopic display of how kinetic energy is dissipated by friction.

You will learn later on that while energy must be conserved, disorder in a closed system can only stay the same or increase; it can never decrease. Disorder is quantified by entropy but the quantification is beyond the scope of your question.

The balls on the table will soon stop moving, due to friction. What happen is that the kinetic energy is shared around in a more disordered way amongst the molecules of the balls and table. The energy is still there (the balls and table are very very slightly warmer) but it is now distributed in random movements of the molecules so that the balls are no longer moving.

Once it gets to this level, the molecules just keep vibrating (or in the case of a gas, colliding.) There is no friction in these interactions, because there is nothing smaller than a molecule for energy to be dissipated into.

(well actually there is one thing: the molecules could shake themselves free of the solid they are in and become a liquid, shake themselves free of the liquid they are in and become a gas, or even shake themselves apart and become smaller molecules or atoms. But breaking these bonds or interactions can only occur with additional heat absorption, or by cooling of the material.)

after the pressure is created, doesn't it require continuous energy to maintain pressure against a constant force of 1 atm (or whatever it is)?

"work" (which is a transfer of mechanical energy) only occurs when something is moved against a force. For example, when you put your finger over the end of a bicycle pump and squeeze the plunger, your squeezing arm does work. If you release the plunger, it moves outward again, releasing the stored energy.

The fact that you find it difficult to maintain the force on the plunger indefinitely does not mean that any work is being done. Your body is not designed to maintain a force with perfect efficiency; energy is expended within your body to maintain that muscle contracted. But if you had a latch on the bicycle pump, you could close that and maintain the pressure indefinitely.

Similarly, when you lie down on an airbed, you increase the pressure inside it. You have no expenditure of energy while you are lying on the bed, yet you maintain the pressure. Similarly the aerosol can maintains its internal pressure with no expenditure of energy.

| improve this answer | |
$\endgroup$
3
$\begingroup$

1.Does pressure decrease with time?

The ideal gas law tells us that $$PV=nRT$$ or $$\mathrm{P=\frac{[nRT]}{[V]}}$$

If the temperature, number of moles or volume does not change, then the pressure will not change.

If pressure is caused by the force of gas molecules hitting the inside surface of a container, don't they transfer energy and eventually slow down enough to totally reduce pressure?

Yes, that's correct. If energy is transferred out of the system, then the temperature will fall and the pressure will decrease. But if everything is already at the same temperature, there will be no energy flow in or out and the pressure will remain constant.

after the pressure is created, doesn't it require continuous energy to maintain pressure against a constant force

No, there is no requirement that energy must be supplied to maintain the pressure. Again, if the gas, the vessel and the surroundings are all at the same temperature the pressure will not change. However, if the entire system is not at thermal equilibrium, then heat will flow so as to put things at equilibrium and the temperature will change.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.