Thermodynamics | Class 11 Physics Notes

11.4 Heat, internal energy and work

Heat is energy transferred between systems due to temperature difference. Internal energy (U) of a system is the total microscopic kinetic and potential energy of its molecules, excluding the kinetic energy of the system as a whole. Internal energy is a state variable, depending only on the current state of the system (pressure, volume, temperature), not on the path taken to reach that state. Heat and work are modes of energy transfer that change the internal energy. For example, in a gas contained in a cylinder with a movable piston, internal energy can increase by heat flow from a hotter body or by work done on the gas by compressing the piston. Conversely, the gas can lose internal energy by doing work on the piston or by transferring heat to a colder body. Heat is energy in transit due to temperature difference, while work is energy transfer by mechanical means such as moving a piston. Heat and work are not state variables; only internal energy is. This distinction is crucial for understanding thermodynamics.

📊 Diagram: Fig. 11.3 (a) Internal energy U of a gas is the sum of the kinetic and potential energies of its molecules when the box is at rest. Kinetic energy due to various types of motion (translational, rotational, vibrational) is to be included in U. (b) If the same box is moving as a whole with some velocity, the kinetic energy of the box is not to be included in U. Fig. 11.4 Heat and work are two distinct modes of energy transfer to a system that results in change in its internal energy. (a) Heat is energy transfer due to temperature difference between the system and the surroundings. (b) Work is energy transfer brought about by means (e.g. moving the piston by raising or lowering some weight connected to it) that do not involve such a temperature difference.

🔗 Connection: This section prepares the ground for the First Law of Thermodynamics, which relates heat, work, and internal energy changes.