PHY 1321 Lecture Notes - Lecture 2: James Prescott Joule, Internal Energy, Rotation Around A Fixed Axis
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Phy1331 Lecture 2
Thermodynamics –Historical Background
Thermodynamics and mechanics were distinct branches of physics.
▪Experiments by James Joule and others showed a connection between them.
• A connection was found between the transfer of energy by heat in thermal
processes and the transfer of energy by work in mechanical processes.
• The concept of energy was generalized to include internal energy.
• The principle of conservation of energy emerged as a universal law of nature
The first law of thermodynamics describes systems in which the only energy change is that of
internal energy. The transfers of energy are by heat and work
Internal energy is all the energy of a system that is associated with its microscopic components.
The system is viewed from a reference frame at rest with respect to the center of mass of the
• The kinetic energy due to its motion through space is not included.
1. Internal energy does include kinetic energies due to:
(1) Random translational motion
(2) Rotational motion
(3) Vibrational motion
Internal energy also includes potential energy between molecule
• Heat is defined as the transfer of energy across the boundary of a system due to a
temperature difference between the system and its surroundings.
• The term heat will also be used to represent the amount of energy transferred by this
• There are many common phrases that use the word “heat” incorrectly.
• Heat, internal energy, and temperature are all different quantities.
You cannot talk about the “heat of a system,” you can refer to heat only when energy has
been transferred as a result of a temperature difference.