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The kinetic theory of gases allows us to use a very simple mechanical model to derive the relation between the pressure, volume and temperature of a gas.

(Audio glitch fixed.) We have an intuitive feel for the concepts of work and energy. In physics we give these precise definitions. For at least some systems we find that we can define different forms of energy, potential and kinetic, such that the total energy remains constant even as these specific forms vary.

We have an intuitive idea of the concepts of work and energy. In physics we make these mathematically precise. We find that, at least for certain system, we can define two types of energies, potential and kinetic, such that their sum, the total energy, is conserved.

Here we complete the "quantum field theory" of a vibrating string.

In the previous video we saw how the quantum harmonic oscillator provides a model system in which we can describe the creation and destruction of energy quanta. In 1925 Born, Heisenberg and Jordan presented a way to apply these ideas to a continuous field.

Here we complete our discussion of the concepts of heat and temperature.

We begin our investigation into thermodynamics with a consideration of the phenomena of heat and temperature.

Arguably the most important equation in all of physics is F = ma, force equals mass times acceleration. Newton presented this law of Nature in 1687.

A mass attached to spring is an example of a "harmonic oscillator." For the quantum harmonic oscillator we can find creation and destruction operators which create and destroy one quantum of energy. This seems like a promising development in our quest to develop a rigorous quantum description of photons.

The theory of quantum mechanics we developed in the previous series has some loose ends. Notably: 1) We talk about photons being emitted (created) and absorbed (destroyed) but we haven't given a rigorous description of this process. 2) The Dirac equation implies the presence of a "Dirac sea" of invisible, negative-energy electrons. Exciting a negative-energy electron to a positive-energy state appears as the creation of an electron-positron pair. A "better" theory would predict this creation directly without the bizarre, invisible Dirac sea.

The theory developed to tie up these loose ends is called Quantum Electrodynamics. It's the first (of several) quantum field theories.

Continuation of: https://youtu.be/6vCvVOqid-A

Here we extend our video 5a (https://youtu.be/kN_d7eknfYk) treatment of the twin "paradox" of special relativity.