# Physics Questions and Answers – Thermodynamics – Heat, Internal Energy and Work

«
»

This set of Physics Online Test for Class 11 focuses on “Thermodynamics – Heat, Internal Energy and Work”.

1. Internal energy of a system is defined as?
a) The sum of kinetic energies of all molecules of the system
b) The sum of kinetic and potential energies of all molecules of the system
c) The sum of potential energies of the system
d) The average kinetic energy of all molecules

Explanation: The internal energy of a system corresponds to the energy possessed by all molecules. Thus it is the sum of kinetic and potential energies of all molecules in the system considered. Also note that potential energy is frame dependent, so we choose a frame in which the centre of mass is at rest.

2. Select the correct statement.
a) Internal energy is a path variable
b) Heat is a path variable
c) Work done is a state variable
d) Internal energy is a microscopic variable

Explanation: Internal energy corresponds to the mechanical energy of molecules. It is a state variable as it doesn’t depend on the path taken. While heat is the energy in transit, so it is a path variable. Internal energy is a thermodynamic variable and is therefore macroscopic as thermodynamics deals with bulk systems.

3. A box contains 105 molecules. The average kinetic energy of each molecule is 0.5*10-6J. With respect to an observer at rest the box, having a mass of 1kg, is moving with a velocity of 2m/s. What is the value of kinetic energy that will contribute to the internal energy?
a) 0.05J
b) 2.05J
c) 0
d) 2J

Explanation: When calculating internal energy of molecules, the system is to be studied from a frame in which the centre of mass is at rest. So the kinetic energy due to movement of the entire box will not be considered. Only the individual kinetic energies of molecules will be considered, and their value is: 105*0.5*10-6
= 0.05J.

4. If some heat is provided to a system and work is done on it, it is possible that internal energy will decrease. True or False?
a) True
b) False

Explanation: Internal energy corresponds to the mechanical energy of the molecules. When these molecules are provided with heat and are compressed their mechanical energy will increase. Both factors support the increase of internal energy hence the given statement is false.

5. Which of the following types of kinetic energies contribute to internal energy?
a) Translational
b) Rotational
c) Vibrational
d) Translational, Vibrational & Rotational

Explanation: Internal energy consists of the total kinetic energy of all molecules, which means all types, namely translational, rotational & vibrational kinetic energies contribute to internal energy.

6. A body has 15J of heat energy in a particular state. At the same state it has an internal energy of 30J. Assume no work is done, what will happen to the internal energy if 5J of heat is added to the system?
a) Heat energy = 20J, Internal energy = 35J
b) Heat energy = 15J, Internal energy = 30J
c) Heat energy = 15J, Internal energy = 35J
d) A body cannot have heat energy in a particular state

Explanation: Heat energy is the energy in transit. A system’s state cannot be defined by heat energy, it is defined by internal energy. Heat and work are path functions. Heat can be supplied or removed from a system but can’t define a system in a state.

Sanfoundry Global Education & Learning Series – Physics – Class 11. 