Electromagnets and conservation of energy

  • #1
Right, I'll try to get this question across.
Say you have 2 bar electromagnets, aligned so when you turn them on they are attracted to each other. You start with them touching, turn them on and pull them apart. You then turn of the current and put them back together. If you continue to repeat this, you are putting a lot of energy in, but where does it go? The only two answers I can think of are heat and EM radiation.

Answers and Replies

  • #2
Some of the energy also gets dumped back into the circuit when you turn off the electricity. Lenz's law will tell you that when you turn off the current that is supporting the magnetic field, then the field will try to keep a current to support itself. Some of the field's energy gets dumped back into the electromagnet in order to sustain itself and this gets dissipated as heat through ohmic losses. The changing magnetic fields and currents when you turn on and off the magnets will produce a small amount of electromagnetic radiation. The one you miss is the work done in separating the magnets. The magnets repulse each other over a distance producing work, this energy comes out of the magnetic fields (which is replenished by the currents in the electromagnet). The resulting kinetic energy is expended as heat due to friction.

So yes, in the end you eventually end up with waste heat and some EM radiation but the energy goes through various transfers until that point.

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