Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: microwave heat

Learn more physics!

Most recent answer: 08/28/2008
Q:
It seems to me that when I reheat my coffee in the microwave it cools off faster than the coffee from the coffee maker. I don’t know if it’s the same temperature as the coffeemaker makes the brew, but approximately the same temperature according to ’feel’. Is there a difference in the quality of heat from a microwave vs other forms of cooking? Thanks.
- Meredith (age 53)
Texas
A:
That's a really interesting observation. If you mean that the coffee in the coffee maker takes longer to cool down than the hot coffee in the cup, that makes a lot of sense. If you mean that the coffee in the cup cools down slower if it's from the coffee maker, I'm surprised.

The coffee maker has a big carafe (or something like that) which also heats up as the coffee is made, and it holds more than a cup. There isn't as much surface per volume of all that hot stuff as there is for a single cup. Since heat leaves via the surface, you expect the cup to cool more quickly.


There shouldn't be any difference in the 'quality of heat' of the coffee from different sources. There can be special cases of materials which have hidden 'degrees of freedom' (internal chemical properties, structures of little crystals. etc) which can be different even though the ordinary temperature is fixed. Then as those materials equilibrate, energy is traded with those internal modes, so the initial state of those modes affects how quickly the material cools. However, there isn't anything like that in a cup of coffee.

Mike W.

You should try a carefully controlled experiment, using a thermometer. It's important to use the same kinds of cups at the same temperature. If it's impractical to heat up the cups to the temperature of the coffee (say, pouring hot coffee into a cup from a coffee maker doesn't involve heating up the cup the way sticking the cup in the microwave will), then you should pour both kinds of heated, stirred coffee into two cold cups (using styrofoam cups may be the easiest way to reduce the effect of the cup temperature -- a styrofoam cup has a very low heat capacity). Stirring helps even out nonuniformities in the temperature distribution in the coffee. There really shouldn't be any nonuniformities in the temperature distribution in the coffee from the coffee maker, as these heat from the bottom and convection currents stir the coffee already. But the coffee int he microwave may be heated on top more than on the bottom (or maybe the other way around, depending on the pattern of standing waves in your microwave and the electrical conductivity of your coffee). If the microwaved coffee is hot on top and colder on the bottom, convection won't even it out (like the water in a lake or the ocean on a sunny day -- it's much warmer near the surface).

Tom

(published on 10/22/2007)

Follow-Up #1: microwaved coffee

Q:
It could be that a microwave will not heat up the liquid evenly. This is especially noticeable with some microwave ovens without a turn table. Another thing to notice is that when liquids are heated in a sauce pan and then transferred into a bowl, the outermost edge becomes the colder (probably due to convection of the container (bowl). The opposite is true of microwaved liquids which are always hotter around the edge. So while they seem to be the same temperature initially, the microwaved liquid will cool faster than the coffee pot because the centre of the microwaved liquid being colder; will help cool the edge which is what is felt. Many instructions for microwaved food and liquids will always advise for "standing time". This is to allow for equilibrium.
- Chris Wilson
W. Yorkshire, England
A:
Thanks, your idea makes a lot of sense.

Mike W.

(published on 08/28/2008)

Follow-up on this answer.