What's cooler? The coffee at the bottom of the cup or the coffee at the top of the cup?

[WobbleWobble]

My girlfriend and I are having a little argument about which would be cooler, the coffee at the bottom of the cup or the coffee at the top of the cup.

Let's assume two different cases, a regular Starbucks cup covered with a lid (with the little drink holes) and an open cup. Or does it even matter much?

I've always remembered that drinking something like hot chocolate using a straw will lessen the chances of burning your tongue because the hotter parts of the drink rises to the top.

Her argument is that the air will cool the coffee on top, like in a case where people drink soup they skim the top because it's cooler.

We've been bickering about this for the past couple days and neither of us will budge on our answers. What does ATOT think?

 

[Pastore]

It's physics, look up why the top of water freezes first, there in you'll have your answer.

 

[iotone]

hmmm... heated air rises, so the stuff at the bottom is cooler??

not that this has anything to do with heating air (since it's liquid), but i figure heat might dissipate that way as well??

 

[Pastore]

Originally posted by: iotone
hmmm... heated air rises, so the stuff at the bottom is cooler??

not that this has anything to do with heating air (since it's liquid), but i figure heat might dissipate that way as well??

Using that logic water would freeze from the bottom up.

 

[destrekor]

doesn't it have something to do with density? Even with a cup, the liquid might be more dense at the bottom.
or i could be wrong

 

[spidey07]

there will be convection currents moving and the temp will equalize. there shouldn't be any noticble difference except the top layer cooling quicker because of contact with the air.

 

[imported_Tick]

The top is exposed more to the air, even in a lidded cup, and hence the heat is moved to the air from the coffee.

 

[darkxshade]

convection... it's cooler @ the top. Just think about why water boils.

 

[Howard]

What's the temperature of the cup and how much longer after the pouring of the coffee is the temperature measured?

 

[TitanDiddly]

I will take a break and reconsider my statement with google.

 

[KingofCamelot]

Cooler at the top, but it really shouldn't be a noticeable difference, as I can't see it being more than a degree or two cooler, unless its like 32 F out or something.

 

[theNEOone]

Originally posted by: Pastore
It's physics, look up why the top of water freezes first, there in you'll have your answer.

first of all einstein, it's chemistry not physics. second, water remains frozen because (a) it is exposed to cold air, (b) has a high specific heat and (c) because it is less dense as a solid than as a liquid.

none of those help when considering this particular case.

also, the cup vs. soup argument. think about a soup bowl and how it has a large surface area on the top that is exposed to much cooler air. it reasons to believe that liquid in a bowl with a large area exposed to colder air will cool much faster than liquid in a cup with a smaller surface area through which to dissipate heat. this probably has to do with several factors however, including cup material, liquid properties, etc.


=|

 

[WobbleWobble]

Originally posted by: Howard
What's the temperature of the cup and how much longer after the pouring of the coffee is the temperature measured?

Let's say a cup of hot coffee (~95C) and it's been sitting for a minute.

 

[LeiZaK]

Originally posted by: darkxshade
convection... it's cooler @ the top. Just think about why water boils.

Because it gets hot?

 

[Kalvin00]

with a lid the bottom has to be cooler...heat rises...turn on a lava lamp to demonstrate this. Which end will become hot first? The top will be warm and the bottom will still be cool.

Lid off, probably the opposite.

 

[Pastore]

Originally posted by: theNEOone

Originally posted by: Pastore
It's physics, look up why the top of water freezes first, there in you'll have your answer.

first of all einstein, it's chemistry not physics. second, water remains frozen because (a) it is exposed to cold air, (b) has a high specific heat and (c) because it is less dense as a solid than as a liquid.

none of those help when considering this particular case.

also, the cup vs. soup argument. think about a soup bowl and how it has a large surface area on the top that is exposed to much cooler air. it reasons to believe that liquid in a bowl with a large area exposed to colder air will cool much faster than liquid in a cup with a smaller surface area through which to dissipate heat. this probably has to do with several factors however, including cup material, liquid properties, etc.


=|

Pretty sure I learned about the laws of thermodynamics in physics, but whatever.

 

[Indolent]

Originally posted by: LeiZaK

Originally posted by: darkxshade
convection... it's cooler @ the top. Just think about why water boils.

Because it gets hot?

You can boil water at room temperature, not that this has anything to do with the original question.

 

[dullard]

Originally posted by: spidey07
there will be convection currents moving and the temp will equalize. there shouldn't be any noticble difference except the top layer cooling quicker because of contact with the air.

The convection currents are interesting. The outside cools and the cooler outer liquid sinks. The water can't accumulate in the bottom, so something has to go up. Water goes up in the core. the result is the center of the cup has rising water and the outside of the cup has sinking water.

If you watch a glass of beer closely, you'll see that the small gas bubbles sink. Sinking bubbles sounds like a contradiction. Convection is the reason. You cannot see the inner bubbles rise, but you see the outer bubbles sink with the sinking beer on the outside.

I've got 20 minutes to kill, a hot mug of water (no lid), and a thermocouple. Let me post some experimental data.

 

[intogamer]

Top it

 

[spidey07]

Originally posted by: dullard

Originally posted by: spidey07
there will be convection currents moving and the temp will equalize. there shouldn't be any noticble difference except the top layer cooling quicker because of contact with the air.

The convection currents are interesting. The outside cools and the cooler outer liquid sinks. The water can't accumulate in the bottom, so something has to go up. Water goes up in the core. the result is the center of the cup has rising water and the outside of the cup has sinking water.

If you watch a glass of beer closely, you'll see that the small gas bubbles sink. Sinking bubbles sounds like a contradiction. Convection is the reason. You cannot see the inner bubbles rise, but you see the outer bubbles sink with the sinking beer on the outside.

I've got 20 minutes to kill, a hot mug of water (no lid), and a thermocouple. Let me post some experimental data.

sounds good.

I can't imagine any more than a few degrees. Because of course the coffee was all the sae temp when it was put in the cup.

this is an interesting question however because of the insulation of a coffee cup.

 

[arcenite]

When coffee steams, it steams from the top... not the bottom.

 

[Kalvin00]

Originally posted by: arcenite
When coffee steams, it steams from the top... not the bottom.

..irrelevant and a pointless statement?

 

[Lonyo]

Originally posted by: arcenite
When coffee steams, it steams from the top... not the bottom.

What the hell.

 

[dullard]

Standard mug, 2/3rds full with boiling water, measured about 5 mm from top and 5 mm from bottom as close to the center of the mug as I can get.

I'm rounding off decimal points, since my type E thermocouple is only accurate to +- 0.5°C.

Yes, my measurements do cause some disturbance, but this is just a fun quick exercise.

Time: Top Temperature in °C, Bottom Temperature in °C
~10 sec: 95, 94
1 min: 89, 89
2 min: 86, 86
3 min: 82, 82
4 min: 79, 79
5 min: 76, 76
6 min: 74, 74 (close to being off by a degree, but rounding minimizes differences)
7 min: 72, 71 (a clear 1°C colder at the bottom)
8 min: 70, 69
9 min: 67, 67
10 min: 66, 65

Looks like the difference is negligible. Experiment ending at 10 min mark.

 

[UlricT]

Originally posted by: Indolent

Originally posted by: LeiZaK

Originally posted by: darkxshade
convection... it's cooler @ the top. Just think about why water boils.

Because it gets hot?

You can boil water at room temperature, not that this has anything to do with the original question.

lowering pressure would work yeah...

but as to why water boils, its because the flame is at the bottom!!!