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Energy/Feynman: a certain quantity which we can find out numerically, because it does not change with the various changes in nature.
This is a very abstract idea, because it is a mathematical principle: there is a numerical quantity that does not change when something happens.
E.g. Analogy: Building blocks are thrown into dirty water, the water level can be calculated.
Important Point: the building blocks can be abstracted from this example.
Kinds of Energy: gravitational energy, kinetic energy, thermal energy, elastic energy, electrical energy, chemical energy, radiation energy, nuclear energy, mass energy.
In today's physics, we do not know what energy is!
Negative Energy/Feynman: means that the electron in the atom has less energy than if it were free. It means that it needs the bonding energy and that energy is required to knock it out. (Scale: 13.6 eV, for a hydrogen atom one "Rydberg").
Energy/Equilibrium/Statistical Mechanics/Feynman: Energy can be generated or lost by circulating the atoms in circular paths for which the realized work is not zero, and no equilibrium can be maintained like that.
The thermal equilibrium cannot exist if the external forces on the atoms are not conservative.
Vom Wesen physikalischer Gesetze München 1993
Vorlesungen über Physik I München 2001