Question

Let there be four articles having colours blue, red, black and white. When they are heated together and allowed to cool, the article that cools earlier is:
A. Blue
B. Red
C. Black
D. White

Answer 1

Hint: The properties of the body/object that can be used to solve this problem are absorptivity and emissivity. Absorptivity deals with the rate of absorption of a body/object. Similarly, emissivity deals with the rate at which the body/object emits radiation.

Complete answer:
From the given information we have the data as follows.
The four articles having colours blue, red, black and white are heated together and allowed to cool.
As the same amount of heat is supplied to all the articles having colours blue, red, black and white, so, the temperature of all the articles will be the same.
The properties of the body/object that define the heating and cooling are given as follows.
Absorptivity is the property of a body/object that measures how much of the radiation is absorbed by it. Emissivity is the property of a body/object that measures how much thermal radiation is emitted by it to the environment.
These properties are directly related to each other, that is, the body/object that has higher absorptivity (the rate at which the body absorbs the heat) will have a higher emissivity (the rate at which the body emits the heat).
As we know that the colour black represents a black body. Thus, as the black colour absorbs and emits the heat at a faster rate compared to all other colours, so, the black colour article will cool earlier.

\[\therefore \]As the black article absorbs and emits the heat at a faster rate compared to others, thus, option (C) is correct.

Note:
In this question, the temperature, that is, the heat supplied to all the coloured articles is the same, so it becomes easier to compare the articles using the properties of body/object such as absorptivity and emissivity.