Chapter 22, Problem 34A

Suppose that a person at a restaurant is served coffee before he or she is ready to drink it. In order that the coffee be hottest when the person is ready for it, should cream be added to it right away or just before it is drunk?

To determine

To explain: If cream should be added right away to hot coffee or just before it is drunk to keep it warm for some time.

Explanation of Solution

Introduction:

According to Newton’s law of cooling, the heat lost by a body is directly proportional to the temperature difference between the body and its surrounding.

If cream is added to coffee, the temperature difference between coffee and the surrounding air becomes less. Hence, less heat is lost which keeps the coffee warm for a longer time. The temperature difference between hot coffee and the surrounding air is large and the coffee loses heat at a greater rate. It will become cold after sometime and thus adding cream just before drinking will not help in keeping the coffee warm.

Conclusion:

Cream should be added to the coffee right away and not just before drinking to keep it warm for a longer time.