Question

A disc spinning with 24J of kinetic energy is kept in a container containing a fluid having a heat capacity of 2000J/K. When the disc comes to rest what will be the change in internal energy if ΔU = 3.5ΔT.

(a) 0

(b) 0.042J

(c) 0.021J

(d) 0.028J

The question was posed to me in semester exam.

This intriguing question originated from First Law of Thermodynamics topic in division Thermodynamics of Physics – Class 11

Answer 1

Right answer is (b) 0.042J

The best I can explain: When the disc comes to rest all its kinetic energy will have been converted into internal energy of the system.

The fluid has a heat capacity of 2000J/K which means that for 24J the change in temperature will be 24/2000 = 0.012K.

Given: ΔU = 3.5ΔT

⇒ ΔU = 3.5*0.012

= 0.042J.