Question

Hot coffee in a thermos flask is shaken vigorously, considering it as a system which of the statement is not true?

• A. Work is done on the system
• B. Temperature of the system rises
• C. Heat energy has been added to coffee
• D. Internal energy of the coffee increased

Answer 1

The correct option is C

Heat energy has been added to coffee

As thermos flask is insulated, so no heat is added to the system. Hence, ∆Q = 0.

Work is done on the system by shaking it against viscous force. Hence, ∆w is negative.

Temperature of the system rises because of increase in internal energy.

Internal energy of coffee ∆U = ∆Q - ∆W = 0 - (-∆W)

So, ∆U would be positive.