Help with a physics question about heat transfer

12

Could anyone tell me the answer to:

DOES A CUP OF HOT COFFEE COOL QUICKER IF MILK IS ADDED BEFORE OR AFTER STIRRING?

Please excuse the caps, it was a copy n paste

I mean I was thinking that if u add it after, a small amount of the coffee would cool quickly and the rest woiuld cool slower. Whilst if u add it before, all the coffee would cool at the same rate, somewhere in between.

Any ideas?

Reply 1

F1 fanatic

16

joker_900

Could anyone tell me the answer to:

DOES A CUP OF HOT COFFEE COOL QUICKER IF MILK IS ADDED BEFORE OR AFTER STIRRING?

Please excuse the caps, it was a copy n paste

I mean I was thinking that if u add it after, a small amount of the coffee would cool quickly and the rest woiuld cool slower. Whilst if u add it before, all the coffee would cool at the same rate, somewhere in between.

Any ideas?

I think it's to do with temperature gradients. The important thing is that heat depends on the temperature gradient, ie how rapidly the coffee changes with temp. The higher the gradient, the steeper the gradient and the faster the loss. If you stir it after then you lower the gradient by making it more uniform and so it will cool quicker.

I *think* that is the right way of thinking about it.

Reply 2

Morbo

17

joker_900

Could anyone tell me the answer to:

DOES A CUP OF HOT COFFEE COOL QUICKER IF MILK IS ADDED BEFORE OR AFTER STIRRING?

Please excuse the caps, it was a copy n paste

I mean I was thinking that if u add it after, a small amount of the coffee would cool quickly and the rest woiuld cool slower. Whilst if u add it before, all the coffee would cool at the same rate, somewhere in between.

Any ideas?

That's a silly question.

The better interview question is which cup of freshly poured coffee is hotter: the cup that has been left for a minute and then had milk added, or the cup that has had milk added and then left for a minute?

Reply 3

F1 fanatic

16

Worzo

That's a silly question.

The better interview question is which cup of freshly poured coffee is hotter: the cup that has been left for a minute and then had milk added, or the cup that has had milk added and then left for a minute?

yeah, I thought it was that for a few minutes... then actually re-read it. It would make much more sense

Reply 4

F1 fanatic

16

Sentooran

What's the answer? :smile:

If you add the milk first and leave it then the temperature difference between the air and the coffee will be smaller, as the coffee is cooled by the milk and so the rate of cooling is lower. If you add the milk after then the temperature difference is higher between coffee and air and it cools a lot in the time before you add the milk. It's a variation on the OP question, but is somewhat more intuitive.

Reply 5

Mehh

8

F1 fanatic

If you add the milk first and leave it then the temperature difference between the air and the coffee will be smaller, as the coffee is cooled by the milk and so the rate of cooling is lower. If you add the milk after then the temperature difference is higher between coffee and air and it cools a lot in the time before you add the milk. It's a variation on the OP question, but is somewhat more intuitive.