Question

Two friends A and B are waiting for another friend for tea. A took the tea in a cup and mixed the cold milk and then waited. B took the tea in the cup and then mixed the cold milk when the friend came. Then tea will be hotter in the cup of
A.A
B.B
C.Tea will be equally hot in both cups
D.Friend’s cup

Answer 1

To solve this problem basic fundamentals of heat transfer will be used in which heat is transferred from high temperature body to low temperature body and we will see how the surrounding medium will play a significant role in changing the temperature of substance.

Complete answer:
Heat transfer is defined as the movement of heat due to difference in temperature of two mediums. The molecules or atoms of matter are in a state of motion with high speed which are responsible for heat or thermal energy to be transferred.
In the above problem for case one:
Friend A took the tea in a cup and added cold milk in it, so due to large temperature difference first it will start dissipating heat with a larger rate but after some time due to low difference in temperature between cup and surrounding it’s behavior will become slow.
Case two:
Friend B took the tea in a cup and waited for a friend to arrive after that he added cold milk in the cup. The important thing to notice in this case is after pouring tea in the cup it already starts dissipating heat due to difference in temperature which reduces the temperature by a large amount before adding cold milk in it, and further when he adds cold milk, due to large temperature difference the loss of heat increases rapidly.
Hence the Temperature in cup A will be more than that of B by above two conclusions from the cases.

$\therefore$ Option A will be correct (temperature of cup of A will be greater than B)

Note:
The rate of change in temperature is directly proportional to difference in temperature of substance and that of surrounding which is the key point to solve the above problem. The difference in temperature can be termed as ‘potential’ causes transfer of heat so heat is also known as flux.