Another brain teaser (sort of on-topic for some :-) )

[Clarke Echols]

OK, we had fun with hoses that freeze above the freezing temperature of water.  Now here is another one
from college-level heat and thermodynamics.  My cousin, a mechanical engineering graduate was interviewed
for a job at Hewlett-Packard about 30 years ago, and got hit with this one.  I found it perplexing when I
encountered it too.


You want a hot cup of coffee, so you pour it into the mug, but before you get any farther on your
enjoyment, an important matter comes up that will require you to leave the coffee for five minutes.
You want it as hot/warm as possible when you return.  The question is:  Do you add the  cream now,
or wait until you get back?  Why?

Now this *has* to be on-topic for coffe-drinking bus nuts, right? :-)   

Clarke

[gumpy]

It really don't matter if you add it now or later cause that 5 minute job will turn into an hour, and you'll forget where you put the cup down anyway. For cost saving measures, wait to add the cream, then you're not wasting cream in coffee that's gonna be cold when you get back.

Or, you could leave the cold cream out so it warms up closer to the temp of the coffee while you're gone.

Actually, ther better plan is to just pour the coffee it back in the pot and go do your job. It'll be hot when you get back.

But I'm wondering why you can't take it with you?  That's what insulated cups with lids are for.

[Ace]

This is probably a trick question but my simple mind says to put the cream in when you return because it's cold and adding cold to hot will cool down the hot somewhat!

Like I said, My simple common sense mind says that but I'm probably wrong! 

Ace

[H3Jim]

And mine says to add it now.  If it cools some now, the temperature differential between the coffee and the air will be less during the time period you are busy.  The lower temperature differential will cause it to loose less btu's during the allotted time than if the temp differential was greater during that time.

[FloridaCliff]

I say add the cream now!

The cream will always be the same temp, the coffee will be cooler in 5 minutes, so adding the cream later will cause the coffee to cool down faster due to the temperature difference of the coffee.


Cliff

[Nick Badame Refrig/ACC]

You want a hot cup of coffee, so you pour it into the mug, but before you get any farther on your
enjoyment, an important matter comes up that will require you to leave the coffee for five minutes.
You want it as hot/warm as possible when you return.  The question is:  Do you add the  cream now,
or wait until you get back?  Why?


The hell with all that scientific stuff, Drink the coffee first, wash it down with the cream, then attend to the "important" whatever...

Thats my answer, I'm sticking to it!

Nick-

[kyle4501]

Assumptions:
coffee = 200F
cream = 40F
room = 78F

The liquid with the greater difference will transfer heat faster than the liquid with the lesser temp difference.

Adding cream will reduce the coffee temp when it is added.

Since cream is 38 deg cooler than room & coffee is 122 deg hotter, I'd immagine that adding the cream NOW will minimize heat lost to the room & would be the theoretical way to go.


But I live in the real world, & life is short. Enjoy your coffee now. Who knows, that important matter may take care of itself while you are drinking your coffee 

OR you could just use the microwave to re-heat it when you return.

OR, if you're old school, you could leave the cup on a little hot plate while you save the world.   

[John Z]

Seems to me that more heat is lost being transferred to the mug (assuming a ceramic type of mug) than is lost to the air. Add the cream when you get back.

[Dreamscape]

I AGREE WITH NICK......

You want a hot cup of coffee, so you pour it into the mug, but before you get any farther on your
enjoyment, an important matter comes up that will require you to leave the coffee for five minutes.
You want it as hot/warm as possible when you return. The question is: Do you add the cream now,
or wait until you get back? Why?


The hell with all that scientific stuff, Drink the coffee first, wash it down with the cream, then attend to the "important" whatever...

Thats my answer, I'm sticking to it!

Nick-

Paul

[skipn]

  Personally I would never ruin good coffee or cream by mixing them together.

  So:
   I am with Kyle and would add to the pour parameter of pouring over a spoon onto the coffee so as to float the cream on top of the coffee in the
attempt to create a barrier.

Skip

[WEC4104]

I'll vote for the "add the cream now, before you leave" answer.  When the coffee is at it's hotest it is losing heat to it's surroundings at the fastest rate.  Leaving it at the accelerated heat transfer rate by waiting to add the cream will yield a greater overall heat loss.

One variable which also comes into play involves what is going on with the cream while it is waiting to be added. If the cream stays in the fridge @ 40 degrees, then the above answer definitely holds true.  But if it is left out while you run the errand, it will begin to warm, thereby incurring the rath of your coworkers.  If the cream is in the form of those little preportioned sealed plastic cups, you could slip one or two into an inside pocket while you were away.  They might be 80 degrees before you added them to the coffee, which might be enough of a reason to wait.

In any event, be sure to cover the coffee mug before you leave it to reduce heat loss. Also, after adding the cream, stir only minimally to reduce the fluid motion and the transfer of heat to the mug walls.

WEC4104  

[edvanland]

I also agree with Nick, why waste a good cup of coffee on something that some one else thinks is so important?  Now if there is a fire in your back pants pocket then by all means leave the coffee, no on 2nd thought take it with you as you run and if all else fails use the coffee to put out the fire.
ED
MCI 7

[Clarke Echols]

One problem with hot drinks when poured into a mug is that much heat is given up to the mug --
especially if it is made of ceramic.  If plastic, it's much less.  Either way, it doesn't change the outcome of
the question.

Heat lost by the coffee in the drinker's absence is determined by the difference between the coffee
temperature and air temperature.  Thus, lowering the temperature up front reduces the heat loss to
the air in comparison with leaving it undiluted, assuming the coffee temperature isn't reduced to the
air temperature by mixing [which would be very unlikely if the coffee is close to 200 degrees and the
room air is at 70].  It also varies, depending on the temperature of the cream.

If we have 8 ounces of coffee at 200 degrees and 2 ounces of cream at 40 degrees, mixing them
immediately drops the liquid temperature by close to 20% of the difference; i.e., 32 degrees.  The
coffee is now at about 168.

Assuming the coffee would drop by 50 degrees in 5 minutes (depends on the kind of cup, whether
insulated, color (black body radiation into the air as discussed in the freezing hose question earlier)
and various other factors such as air movement around the cup, etc.).  Adding 2 ounces of 40-degree
cream then would reduce the temperature of the 150-degree coffee by 20% of 110 degrees, or
22.  150 mnus 22 is 128 degrees, a 40-degree difference due to when the cream is added.

This assumes the specific heat (the amount of heat required to heat or cool a substance per unit
mass, measured in calories (metric units) or BTUs (English units of measure)) of the coffee and
cream are close to identical.  Otherwise, the exact answer will vary somewhat, but the principle
remains the same.

Admittedly, the scenario is a bit off-the-wall because few occasions would be encountered that
call for such a decision, and if there is a microwave oven nearby, it becomes an entirely trivial
matter to deal with. :-)

As for me, I never drink the stuff.  But I do like an occasional cup of hot chocolate as long as it
is well above lukewarm, but not hot enough to burn the tongue.  Postum is another treat I
enjoy -- especially either of them with grilled cheese sandwiches.

And speaking of grilled cheese sandwiches, if you don't want to put the oven on broil, pop the
bread in the toaster, smear with butter when done, sprinkle shredded cheese on the bread and
pop into the microwave for 40 seconds or so to melt it until it gets a bit bubbly.  Yumm!

So much for brain teasers until I come up with something else to make physics fun and relevant to
ordinary (or not so ordinary) life occurances. :-)

And when you get that bus on the road, don't put it on cruise control, thinking it's auto-pilot, and
go back to the galley to fix yourself a snack.  [The story is told of a rich Arab who did that and created
a bit of a problem in traffic.  I don't know if that actually happened or not...  I heard it from an RV
salesman in Denver many years ago.]

Clarke

[Dallas]

Clarke,

I want to thank you for your threads delving into physics.

Being an avid learner and reader, I always enjoy challenges that require my brain.

I have learned a lot from you over the years and hope to continue doing so into the future.

I guess, with my less than highschool education, I need all the help I can get! LOL.

Dallas