# Question of conservation of energy in this thought experiment

Gold Member
Hi,

I have a somewhat silly question about something I cannot figure out properly.

I have a cylinder with a piston inside. The cylinder is sealed and filled with air. The piston is free to move back and forth inside the cylinder, but are also sealed so no air can escape no matter how much the piston compress the air inside the cylinder.

If this system is submerged into a fluid, water for example, the water pressure will push the piston further into the cylinder so the air inside becomes compressed according to the water column above.

When I pull the system up to the water surface, the air inside the piston finally expands to initial volume.

So to the question:
If the piston is a magnet, and the cylinder is a coil, and I repeat the experiment, the moving magnet piston will generate an electric current through the coil if the coil is shorted or loaded.

We all know that energy is allways conserved, so:
What difference in work, and how would it "feel like", to put the system in and out of the water if the coil is:

Br.

Vidar

Dale
Mentor
2021 Award

The piston has a certain amount of buoyancy, so it will take work to push it under water. The change in volume will reduce that buyoancy as the cylinder goes further down and therefore reduce the amount of energy per meter it takes to push it down. On the way up work can be extracted from the piston. The amount of energy extracted per meter will increase as the piston expands and the buoyancy increases. Neglecting friction and viscous losses, the energy required to push the coil down will equal the energy recovered in raising it.