Physics Question

Dave · 3 Oct 2011 at 13:43

Deadbeat said:
Wait... isn't delta T the relative difference in temperature? In this case, both being exactly 15K?

This is why I do coding in the morning and not the theory

I'll now plead that the heat lost via radiation will be stronger for the higher-temp cup as the heat loss is proportional to the absolute temperature (to the fourth power).

Better thing to do is to go and make a cup of tea!

Deadbeat · 3 Oct 2011 at 13:52

Dave said:
This is why I do coding in the morning and not the theory I'll now plead that the heat lost via radiation will be stronger for the higher-temp cup as the heat loss is proportional to the absolute temperature (to the fourth power).

I don't recognise this theory off the bat, but I'm pretty sure the rate of heat exchange, assuming all other things to be equal, would be directly related to the temperature difference, therefore the cooler glass would absorb heat energy at the same rate as the warmer glass would release it. Still, it's a long time since I studied physics at any level, so grab yourself a bagel and a cup of tea, then come back and explain it to me - I'm actually interested, not being disparaging

spp · 3 Oct 2011 at 14:03

They will arrive at equilibrium at the same time. Heat transfer is dependant on temperature difference. Since the difference is the same, it does not matter if one temperature is greater and one lower, the heat transfer is the same.

DRZ · 3 Oct 2011 at 14:58

spp said:
They will arrive at equilibrium at the same time. Heat transfer is dependant on temperature difference. Since the difference is the same, it does not matter if one temperature is greater and one lower, the heat transfer is the same.

This can be shown to not be true or at the very least there are some results that are challenging to explain. Seriously, take my earlier suggestion and read up on the Mpemba Effect.

I don't really see how the fact it was being discussed in the pub makes any difference either. 2 + 2 doesn't equal 5 just because I'm in the pub - I don't accept that the location of the discussion should result in a lack of precision. Given that it is fairly clear that there won't be a vast difference in times between the two changes in temperature it is safe to assume that you are going to need to look at the finer mechanics of the situation.

bagsjoni · 3 Oct 2011 at 15:59

i Hate Physics because of the assumptions there!. One time as i was calculating the area of a drop of a fluid and i assumed it to be in form of a sphere. I was market wrong because i was supposed to assume it was in form of a cylinder!.

I see you already have assumptions there!

Glaucus · 3 Oct 2011 at 16:01

I would assume hot water would cool faster than cold water warming. As there are more processes for heat transfer in hot water. Such as steam.

Deadbeat · 3 Oct 2011 at 16:21

DRZ said:
This can be shown to not be true or at the very least there are some results that are challenging to explain. Seriously, take my earlier suggestion and read up on the Mpemba Effect.

I don't really see how the fact it was being discussed in the pub makes any difference either. 2 + 2 doesn't equal 5 just because I'm in the pub - I don't accept that the location of the discussion should result in a lack of precision. Given that it is fairly clear that there won't be a vast difference in times between the two changes in temperature it is safe to assume that you are going to need to look at the finer mechanics of the situation.

One of the wonderful things about physics is that you can make the most simple concept infinitely complex, but I disagree with you. Yes, assuming simplistic conditions is going to throw off the accuracy and no, in a real-world application it wouldn't be correct to assume a perfect system, but this is not a situation that requires deeper levels of complexity - hell, it's not like the old airplane on a treadmill question. You're taking pedantism to a totally unnecesssary level by including heat transfer via evaporation or condensation into consideration - you might as well go the whole hog and include air pressure, humidity and light intensity of the environment in your consideration, as well as the composition material of the glass and the surface it rests on, the method of observation, the relative purity of the water, the nature and balance of those impurities and whether anybody has farted near the setup in the last week - all of which can have an equally negligible impact on the process.

If you're going to be obsessively anal about the fine points, at least do it properly. If you tell me that you'd get this involved were this question to come up amongst your friends at the pub, I'd dispute that you'd ever be sat in a pub with people you could call friends.

AcidHell2 said:
I would assume hot water would cool faster than cold water warming. As there are more processes for heat transfer in hot water. Such as steam.

Steam? At 40C? Hehe, maybe the air pressure really does have a lot more to do with the rate of energy transfer than the effect of evaporation or condensation in this experiment

Glaucus · 3 Oct 2011 at 16:23

Yes steam, steam is water vapour and the hotter something is the more evaporation you have, at 40 degrees you have far more than at 20 degrees and would actually help to slow the warming process down, isn't this the process used in hot countries and keeping water in clay/terracotta pots, wick effect combined with evaporation, keeping the water inside cool.. Although I suppose common terminology is condensed water vapour in the air.

So perhaps replace steam with evporation.

Deadbeat · 3 Oct 2011 at 16:29

AcidHell2 said:
Yes steam, steam is water vapour and the hotter something is the more evaporation you have, at 40 degrees you have far more than at 20 degrees. Although I suppose common terminology is condensed water vapour in the air.

So perhaps replace steam with evporation.

Aah, you've caught mme by the short & curlies there - I've always understood steam to be a direct result of boiling water and thus wouldn't be applied to evaporated water vapour, but again it's a while since I diddled with the specifics. Then again, the process of evaporation is little different than localised boiling - best stick with evaporation, you don't want to go overheating my work-addled brain

The Dread Pirate · 3 Oct 2011 at 16:31

I need more information on said glasses of water:

1. What shape are they

2. what height

3. are they half-full or half empty?

4. Do either of the drinks belong to me?

DRZ · 3 Oct 2011 at 16:33

Deadbeat said:
If you tell me that you'd get this involved were this question to come up amongst your friends at the pub, I'd dispute that you'd ever be sat in a pub with people you could call friends.

I have a wonderful circle of friends thanks

I'm sure the OP can confirm that I really would go into that level of detail and I am sure that over a few beers we have done exactly that. 5p says that this discussion was likely with a guy both of us know (and who occasionally posts on here as TrX).

Deadbeat · 3 Oct 2011 at 16:38

Capn' $qu!ff said:
I need more information on said glasses of water:

1. What shape are they

2. what height

3. are they half-full or half empty?

4. Do either of the drinks belong to me?

Assuming perfect conditions, they'd be exactly the same shape - height, width, even thickness and the same material composition for both, as well as exactly the same volume of water. But if we're taking near-negligible factors into consideration, we'd need to know exactly what manufacturing process was used to apply a C.V. to the final rate based on likelihood of non-uniform density, manufacturing impurities impairing or aiding conductivity, contamination from storage conditions and where that dirty lab tech's been sticking his hands before setting up the experiment, he's never been the cleanest fellow and I happen to know he's been quite chatty with that Melissa from reception, you know, the one with the lazy eye who always smells like she's just come off a treadmill and who knows what that'll do to the scientific impartiality...

And no, neither of the drinks belongs to you. They belong to science.

DRZ said:
I have a wonderful circle of friends thanks I'm sure the OP can confirm that I really would go into that level of detail and I am sure that over a few beers we have done exactly that. 5p says that this discussion was likely with a guy both of us know (and who occasionally posts on here as TrX).

I'll take that bet! Actually, I don't think I can afford it on my last month's payslip, but I still think you're taking things too far.

The Dread Pirate · 3 Oct 2011 at 16:42

Deadbeat said:
Assuming perfect conditions, they'd be exactly the same shape - height, width, even thickness and the same material composition for both, as well as exactly the same volume of water. But if we're taking near-negligible factors into consideration, we'd need to know exactly what manufacturing process was used to apply a C.V. to the final rate based on likelihood of non-uniform density, manufacturing impurities impairing or aiding conductivity, contamination from storage conditions and where that dirty lab tech's been sticking his hands before setting up the experiment, he's never been the cleanest fellow and I happen to know he's been quite chatty with that Melissa from reception, you know, the one with the lazy eye who always smells like she's just come off a treadmill and who knows what that'll do to the scientific impartiality...

And no, neither of the drinks belongs to you. They belong to science.

Oh I see... can I have a drink anyhoo I'm parched?

p.s. leave Melissa out of this she's a saint!

Vonhelmet · 3 Oct 2011 at 17:30

Deadbeat said:
Aah, you've caught mme by the short & curlies there - I've always understood steam to be a direct result of boiling water and thus wouldn't be applied to evaporated water vapour, but again it's a while since I diddled with the specifics. Then again, the process of evaporation is little different than localised boiling - best stick with evaporation, you don't want to go overheating my work-addled brain

Steam is water vapour at a temperature in excess of 100 degrees C.

Glaucus · 3 Oct 2011 at 17:38

Vonhelmet said:
Steam is water vapour at a temperature in excess of 100 degrees C.

So a mug of coffee or tea doesn't produce steam as it isn't at 100c. Also steam is usually condensed water vapour in the air, so that isn't at a high temperature either,

Amleto · 3 Oct 2011 at 17:45

Dave said:
This is why I do coding in the morning and not the theory I'll now plead that the heat lost via radiation will be stronger for the higher-temp cup as the heat loss is proportional to the absolute temperature (to the fourth power).

Better thing to do is to go and make a cup of tea!

you can hardly quote stefan/boltzmann for the "well known" black-body that is water...

spp · 3 Oct 2011 at 18:14

DRZ said:
This can be shown to not be true or at the very least there are some results that are challenging to explain. Seriously, take my earlier suggestion and read up on the Mpemba Effect.

I don't really see how the fact it was being discussed in the pub makes any difference either. 2 + 2 doesn't equal 5 just because I'm in the pub - I don't accept that the location of the discussion should result in a lack of precision. Given that it is fairly clear that there won't be a vast difference in times between the two changes in temperature it is safe to assume that you are going to need to look at the finer mechanics of the situation.

I understand what you are saying, and agree, if we were going to measure the difference at an accurate enough level, then yes, there would be a difference due to microphysics processes, of which things like evaporation and the Mpemba effect would fall under. I had considered the question in a perfect world physics exam scenario, where the water can be treated as a single entity with one variable parameter, temperature. I don't know by how much the evaporation would increase the time difference on both liquids reaching 25 degrees. Does anyone have an idea?

DAnDan · 3 Oct 2011 at 18:28

spp said:
I understand what you are saying, and agree, if we were going to measure the difference at an accurate enough level, then yes, there would be a difference due to microphysics processes, of which things like evaporation and the Mpemba effect would fall under. I had considered the question in a perfect world physics exam scenario, where the water can be treated as a single entity with one variable parameter, temperature. I don't know by how much the evaporation would increase the time difference on both liquids reaching 25 degrees. Does anyone have an idea?

The difference in rate of evaporation of a the glasses of water in this question would be fairly trivial, definitely assumed to be equal in a question like this.

But anyway, the rate of heat transfer would be based on the temperature difference between the two things, and as the temperature difference is the same, they'll cool at the same rate.