How is physics involved in Onboard energy system of a spacecraft?

Question

How is physics involved in Onboard energy system of a spacecraft?

Answer 1

It depends on what you mean by energy system.

For simple rocket propulsion it's basically just the conservation of momentum.

For solar cells it's solid state physics with some crystal and material science thrown in.

For radioisotope based generators there is a bit of nuclear physics.  

Plus all will have been simulated using various physics modeling codes to test there behavior in unusual circumstances.