A thermos flask contains coffee. It is vigorously sheken, considering the coffee as the system. (a) Does its temperature rise ? (b) Has heat been added to it ? (c ) Has work been done on it ?

Options

A. decreases
B. increases
C. remains same
D. decreases if temperature is below `4^(@)C` and increses if temperature is equal to or more than `4^(@)C`

Explanation

To solve the problem step by step, we will analyze each part of the question regarding the thermos flask containing coffee that is being vigorously shaken.

### Step 1: Analyzing Heat Transfer
- **Concept**: A thermos flask is designed to be insulated, which means it prevents heat transfer between the contents (coffee) and the surroundings.
- **Conclusion**: Since the flask is insulated, no heat is added to the coffee from the surroundings.

**Answer (a)**: No heat has been added to the coffee.

### Step 2: Determining Work Done
- **Concept**: When the coffee is shaken, work is done on the liquid due to the viscous forces that arise within the liquid. This work is done against these forces as the liquid moves.
- **Conclusion**: Work is indeed done on the coffee as it is shaken, and this work is considered to be negative because it is done on the system.

**Answer (b)**: Yes, work has been done on the coffee.

### Step 3: Analyzing Temperature Change
- **Concept**: According to the first law of thermodynamics, the change in internal energy (∆U) of a system is equal to the heat added to the system (Q) plus the work done on the system (W). In equation form:
\[ \Delta U = Q + W \]
Since the thermos flask is insulated, Q = 0. Therefore, we have:
\[ \Delta U = 0 + W \]
Since work is done on the system (W is negative), this results in an increase in internal energy (∆U is positive).
- **Conclusion**: An increase in internal energy indicates that the temperature of the coffee rises.

**Answer (c)**: Yes, the temperature of the coffee rises.

### Summary of Answers:
(a) No, heat has not been added to it.
(b) Yes, work has been done on it.
(c) Yes, the temperature rises.