Question

If an ideal flask containing hot coffee is shaken, the temperature of the coffee will:
A. decrease
B. increase
C. remain same
D. decrease if temperature is below \[4C\] and increase if temperature is equal to or more than \[4C\].

Answer 1

Hint: The concept of thermodynamics' first law gives us the relationship between heat supplied, work done, and change in the given flask's internal energy. We will use the relation between these quantities to determine the effect of shaking on coffee temperature.

Complete step by step answer:
It is given that an ideal glass containing hot coffee is shaken; we have to define the effect of this process on the temperature of the coffee.

Let us consider the given ideal flask as a thermodynamic system. If we shake the flask containing hot coffee, that means we do work on the system. It is also given that the flask is ideal, which means that the exchange of heat between the system and surrounding will be zero.

From the concept of thermodynamics' first law, we can say that heat supplied to the given flask is equal to the summation of work done and internal energy change.
\[Q = \Delta U + W\]
Here \[\Delta U\] is the change in internal energy, and W is the work done.

Substitute \[0\]for Q in the above expression.
\[0 = \Delta U + W\]

We know that work done on a thermodynamic system is negative. Therefore, substitute the negative value of work in the above expression.
\[0 = \Delta U + \left( { - W} \right)\]

On rearranging the above expression, we can write:
\[W = \Delta U\]

Based on the above expression, we can say that the work done on the flask by shaking it is converted into a rise in its internal energy. We also know that internal energy is a function of temperature only.

Therefore, if an ideal flask containing hot coffee is shaken, the coffee's temperature will increase

So, the correct answer is “Option B”.

Note:
Alternate method: If we shake an ideal flask containing hot coffee, its kinetic energy will increase because of increased velocity. And we know that temperature is directly proportional to kinetic energy. Therefore, we can conclude that the temperature of the given flask will increase