Random question about heat

[Simbu]

So, lets say you have two items (cube / sphere / whatever) of some substance (the substance might influence the answer to my question). You suspend each item in their own room, with other influences such as light, wind and other influential variables absent.

The ambient temperature is say... 30 degree C. You have one item at 50 degrees C and the other at 10 degrees C, then suspend them in the rooms. Presumably each will normalise to the room temperature, but which will do it first?

Do items cool or rise in temperature to an ambient temperature faster?

[moreflaps]

In the absence of convection they cool/heat at the same rate. With convection present the problem becomes shape dependent and quite difficult to solve (need CFD)

HTH

Cheers

[driverrob]

The nature of the surface governs the rate of loss of heat by radiation.
An ideal black body radiates heat in proportion to the 4th power of its absolute temperature (Stephan's law). That may confuse more than it helps.

Basically, the hotter one loses heat faster. Whether it drops in temperature faster depends on any differences in the two objects' mass, surface area and specific heat capacity. The darker and duller the surface, the closer it is to being that ideal "black body".

The objects also absorb heat from their surroundings so the rate of loss of heat (and therefore rate of drop in temperature will depend on that temperature difference.

If one starts below room temp then a similar argument applies for heat energy and temp. rise.

[Simbu]

Thanks for the replies chaps.

As I suspected, the answer is both complicated and as is frequently the case 'it depends'.

[tapkaJohnD]

A far more interesting Q is, "Is the Mpemba Effect true"

Erasto Mpemba, as school student, noticed that hot water froze more quickly than cold. This counter-intuitive finding was confirmed by local University researchers. Since then the Mpemba Effect has be considered a contraversial but real phenomenon, whose cause is not understood.

See: http://en.wikipedia.org/wiki/Mpemba_effect

John

[annodomini2]

tapkaJohnD said:

A far more interesting Q is, "Is the Mpemba Effect true"

Erasto Mpemba, as school student, noticed that hot water froze more quickly than cold. This counter-intuitive finding was confirmed by local University researchers. Since then the Mpemba Effect has be considered a contraversial but real phenomenon, whose cause is not understood.

See: http://en.wikipedia.org/wiki/Mpemba_effect

John

Supposedly solved it recently:

https://medium.com/the-physics-arxiv-blog/d8a2f611...

[moreflaps]

annodomini2 said:

tapkaJohnD said:

A far more interesting Q is, "Is the Mpemba Effect true"

Erasto Mpemba, as school student, noticed that hot water froze more quickly than cold. This counter-intuitive finding was confirmed by local University researchers. Since then the Mpemba Effect has be considered a contraversial but real phenomenon, whose cause is not understood.

See: http://en.wikipedia.org/wiki/Mpemba_effect

John

Supposedly solved it recently:

https://medium.com/the-physics-arxiv-blog/d8a2f611...

And that 'solution' is IMHO rubbish. The timescale of bond changes is far faster than that of freezing so here should be no difference in bond structure when the water reaches zero regardless of the starting temperature -the bonds have no memory...

Cheers

[Asterix]

This might a good place to ask my question.

I've noticed when cooking that water that is rapidly boiling seems to not release much steam (relatively). When you remove the pan from the heat and the boiling ceases, there is a much greater release of steam.

Is that just poor observational skills from me or a known phenomena?

[Simpo Two]

Asterix said:

I've noticed when cooking that water that is rapidly boiling seems to not release much steam (relatively). When you remove the pan from the heat and the boiling ceases, there is a much greater release of steam.

Is that just poor observational skills from me or a known phenomena?

Steam is invisible. What you see is water vapour, ie steam condensing as the pan cools down

[deckster]

Asterix said:

This might a good place to ask my question.

I've noticed when cooking that water that is rapidly boiling seems to not release much steam (relatively). When you remove the pan from the heat and the boiling ceases, there is a much greater release of steam.

Is that just poor observational skills from me or a known phenomena?

Largely because what we commonly call steam isn't in fact gaseous water - it's condensed water vapour that has cooled in the air. Water in its gaseous form is colourless (ie to all intents and purposes, invisible) and so although a vigorously boiling pan of water will actually release more of it, it will not be visible until it has cooled enough to condense. This will typically happen far enough away from the pan itself that the vapour has significantly dispersed and therefore is far less noticeable than the vapour that is released by a cooling pan of water, which condenses much closer to the pan.

[Asterix]

Thanks chaps - makes perfect sense.

[Jinx]

Asterix said:

This might a good place to ask my question.

I've noticed when cooking that water that is rapidly boiling seems to not release much steam (relatively). When you remove the pan from the heat and the boiling ceases, there is a much greater release of steam.

Is that just poor observational skills from me or a known phenomena?

Probably due to condensation of the water vapour - above the pan whilst on the hob the air column is hotter than the air surrounding the hob. Removing the pan means the water vapour is now hitting cold air causing it to condense into larger droplets and therefore more visible.

[MiseryStreak]

Jinx said:

Asterix said:

This might a good place to ask my question.

I've noticed when cooking that water that is rapidly boiling seems to not release much steam (relatively). When you remove the pan from the heat and the boiling ceases, there is a much greater release of steam.

Is that just poor observational skills from me or a known phenomena?

Probably due to condensation of the water vapour - above the pan whilst on the hob the air column is hotter than the air surrounding the hob. Removing the pan means the water vapour is now hitting cold air causing it to condense into larger droplets and therefore more visible.

This is precisely the reason, and exactly the same reason as to why your breath condenses outside but not inside. The same amount of invisible steam or water vapour (same thing) is coming from the boiling pan but it won't condense to water droplets suspended in air and produce a visible fog over the hot cooker, as soon as you move it away from the heat source it will.

It's also how clouds are formed, air saturated with water vapour is cooled to the dew point and condenses becoming visible water droplets.