A practical question about thermodynamics

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Discussion Overview

The discussion revolves around practical thermodynamics related to preserving the heat of coffee. Participants explore the effects of adding cream and sugar, the influence of cup color on heat retention, and the mechanisms of heat loss such as conduction, convection, and radiation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that adding cream to coffee may initially cool it by transferring heat to the cream, potentially slowing convection.
  • Another participant argues that adding cream will cool the coffee, emphasizing that the rate of cooling is proportional to the temperature difference between the coffee and its surroundings.
  • A participant questions whether using a dark-colored cup would be better for heat retention, while also considering the color of the cream added.
  • One participant proposes that adding sugar should be done early, similar to cream, while another counters that sugar may absorb too much heat to dissolve, potentially lowering the temperature excessively.
  • A later reply provides a specific heat absorption value for sugar, suggesting that its effect on cooling is minimal.
  • Another participant states that dark colors are worse for heat retention because they radiate heat more efficiently than light colors, referencing the design of good coffee thermoses.

Areas of Agreement / Disagreement

Participants express differing views on the effects of adding sugar and the implications of cup color on heat retention. There is no consensus on whether adding sugar is beneficial or detrimental, and opinions vary on the impact of cup color.

Contextual Notes

Participants reference specific heat values and mechanisms of heat loss, but there are unresolved assumptions regarding the overall impact of these factors on coffee temperature retention.

Who May Find This Useful

This discussion may be of interest to those exploring practical applications of thermodynamics, particularly in everyday scenarios like beverage temperature management.

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Today, I read a short essay from a magazine about how to preserve the coffee as hot as possible in a short time (if you don't want to drink it right away) by adding the cream into it as soon as possible. It reads: if you add the cream soon, some amount of heat being transferred to the cream so the temperature b/w the coffee and air will be decreased, this is also make the convection process slower. I am thinking ... what about the sugar? In this sense, should I also add the sugar asap (without stirring of course)? And of course, if I use a black cup instead of light-color one, it will also help to preserve the temperature a bit due to radiation, right? What about the color of cream, will dark one better than light one?
 
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Adding the cream will cool the coffee.
The rate of cooling is proportional to the temperature difference between the coffee and surroundings.
So the coffee cools fastest when it is hottest, as it cools the temperature difference is less and the cooling is slower. That might be what you are thinking about by adding the cream first.

To keep it hot the longest you need to reduce the mecahnisms for heat loss.
Conduction - that's why you use a styrofoam cup.
Radiation - probably not a major factor at these temperatures, but a shiny cup will help.
Convection - the hot coffee is at the top, not much you can do about this except put the lid on which stops hot air escaping.
 
Good explanation. Thanks a lot. From your clue, will we use a cup with dark color is better? What about the color the cream being added? I discussed this problem with my friend, he said we should also add the sugar soon (the reason is just like that of adding cream), but I think we shouldn't because sugar will absorb too much heat to dissolve, it will low the temperature too much. What do you think?
 
Sugar will absorb some heat to dissolve, the heat of solution of sucrose is - 16.1 J/g
A teaspoon of sugar will take in about 80J, it takes 4200J to cool a Kg of water by 1degC
So this will cool a large cup of coffee by about 0.05deg C
 
Regarding color, no, dark is worse because dark colors radiate heat more efficiently than light ones. This is why good coffee thermoses are polished metal on the inside (plus glass and a vacuum): to block radiation.
 

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