Electromagnets and Conversation of Energy

[treehouse]

Running an electric current through a copper wire induces a magnetic field. Wrapping that wire around a ferromagnetic material amplifies its magnetic field (this is the basis on which electromagnets are designed*). This does not violate Newton's first law of thermodynamics because magnetic fields are local - the flux lines of a magnetic field do not permeate the entire universe.

I hypothesize that if the same voltage is run through the wire with and without the ferromagnetic core for long enough, the difference in the strength of the magnetic field will be greater than the amount of work required to place the core in the wire.

*http://en.wikipedia.org/wiki/Magnet...d_due_to_moving_charges_and_electric_currents
http://en.wikipedia.org/wiki/Electromagnet#History

 

[sophiecentaur]

Why do you say the lines don't permeate the whole Universe? They certainly can permeate a long way before getting mixed up with the fields due to other currents. Is it relevant to the situation? I'm not sure.

When you APPLY the voltage across (not "run it through") the coil, energy is transferred, as you say, as the current builds up. The Energy stored will be I^2 L/2 (where L is the inductance of the coil). The ferromagnetic core will produce more of a back emf, resulting in a smaller effective PD to cause the current to increase. As the current increases over a longer time so the integral of VdI over time will be greater with a core than without one.

(Just another of the many ways of looking at this sort of thing.)

 

[Drakkith]

I don't see why you are comparing the strength of a magnetic coil with and without an iron core to work done to place the core in the coil. Which isn't much unless it was already turned on. Work and force are not the same thing.