1
$\begingroup$

Can one build up pressure in system for example kettle with hole sufficent of releasing water vapor ?

Kettle starts with 21c 1atm inside and outside and will be heated as fast as possible, can this system build up pressure inside The kettle even If it has hole that can let the steam escape? And can anyone explain why it won’t or will?

1st scenario: pressure cannot build up because Steam can escape from hole with as much kinetic energy as it has been heated. Hence pressure cannot build up and pressure stays somewhat consistent, and steam escapes only because it requires more room and has kinetic energy to an velocity to expand and because of hole it only expands in one direction.

2nd scenario: If hole is small enough it creates some sort of “resistance” and unknown force to me, enables the pressure difference increase inside kettle and this is the reason why steam escapes?

Can one explain which scenario is more correct or explain correctly the phenomena.

$\endgroup$
2
  • 1
    $\begingroup$ The phenomenon of the steam rushing out already shows that pressure is being built up. Without a pressure gradient steam inside the kettle will only diffuse out and won't ejects out. $\endgroup$ – Philip.P Apr 7 '20 at 12:16
  • $\begingroup$ Not necessarily, if I’m not mistaken if you have floating water ball and you would heat it with same vigor it would expand with same vigor in every direction, notion that element has expand velocity isn’t prove of pressure difference. $\endgroup$ – Johannes Ihanus Apr 7 '20 at 12:35
1
$\begingroup$

If heating fast enough, one can build a pressure, since it takes finite time for the pressure to equilibrate through a small hole. Let us take, for example, a punched air baloon or a tire - initially the pressure inside is high and the extra air escapes through the hole; it happens quite fast, but not instantly.

An example even closer to the question is the behavior of a pressure cooker - in principle, the valve is never super-tight - it is just a metallic ball on the top of the hole, so there are definitely micro-holes through which the steam can escape. Yet, these holes are too small to slow the pressure equilibration, so that the inside pressure can build.

$\endgroup$
12
  • $\begingroup$ But if I understood correctly there is a maximum speed of heating water by convection that is quality of substance, and that speed cannot be faster than the speed of expansion/ kinetic energy, what would be the force then tahat resist the movement enough to enable the pressure build up? $\endgroup$ – Johannes Ihanus Apr 7 '20 at 12:20
  • 1
    $\begingroup$ @JohannesIhanus Wouldn't that only matter directly if the vapor was able to escape in every direction? It heats up and gains kinetic energy, but why would you expect all the gas with increased kinetic energy to go directly for the hole (especially if it's very small compared to the vessel)? It's not like heating it is only going to provide kinetic energy to the water near the opening; and in the direction towards the opening. Instead, consider that a lot of it will start hitting against the sides of the container, and other vapours; this is pressure. $\endgroup$ – JMac Apr 7 '20 at 12:26
  • 1
    $\begingroup$ I'm not sure why the time it would take to escape is negligible. The smaller the hole relative to the size of the vessel (and molecules), the less likely it is to go out of the hole on any subsequent bounce. This means that any given molecule could bounce around quite awhile before escaping. $\endgroup$ – JMac Apr 7 '20 at 12:51
  • 1
    $\begingroup$ @JohannesIhanus It would depend on a lot of things like the size of the hole and how much heat you're supplying, and how you supply it. $\endgroup$ – JMac Apr 7 '20 at 13:03
  • 1
    $\begingroup$ Thx I think I’m able to form adequate formula now.( I forgot the probability of escape of the one given molecule ) $\endgroup$ – Johannes Ihanus Apr 7 '20 at 13:16

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.