Please help. My example:

You take a sheet of steel and suspend it in the air.

Then you take a permanent magnet (i.e. a neodymium magnet) and attach it to the sheet of steel and from that you suspend a 1kg weight. The weight and magnet will stay put providing that the room doesn’t change and that gravity is still acting on the objects.

Now you take a separate piece of steel (which is not magnetized) and try attach that and the 1kg weight to the suspended sheet of steel. You notice that without an external magnetic force you are unable to attach the two pieces together.

Now here is where my confusion lies. You then attach a electromagnet to the 2nd piece of steel and notice that it stick but the battery you have used to power the device drains. But all this time the permanent magnet with the same load and in the same conditions hasn’t why??

  • $\begingroup$ You confusion, if I am reading you right, comes from the misconception that one magnetic field requires energy and the other does not. It is not so. Neither magnetic field requires energy. The reason your battery is drained in the Electromagnet example is that to create the magnetic field, current has to flow through imperfect copper wires where Resistance turns electric energy into heat, according to P=R*I^2 $\endgroup$
    – shieldfoss
    Jun 25 '13 at 11:19

The permanent magnetic generates its magnetic field through ferromagnetism - basically, the magnetic dipoles of the atoms and electrons in the material are aligned in the same direction. Alternatively, the magnetic field generated by the electromagnet is made by generating a current loop through the steel sheet. This current requires energy to maintain; if the current loop is removed, the material does not always retain its magnetization.

To rephrase that, the permanent magnet creates a field due to the orientation of the electrons; their orbit and spin. Whereas the field from the electromagnet is created by a current - forcibly moving electrons around a loop. Since the electrons in the permanent magnet do not have to be moved to create their field, you do not have to provide energy to maintain the magnetic field. However, since the electromagnet only creates a field by moving electrons, you must provide the energy required to move them.

The physics of the situation goes a bit deeper than this, but this should help for a conceptual answer.

  • $\begingroup$ Thank you for your comment. So when you create a permanent magnetic in a factory, they use a electromagnet (usually connected to a large capacitor) which orientates the dipoles in the same direction. My understanding is that this requires energy. Since energy can't be destroyed, would it be possible to reclaim some of that energy back?? $\endgroup$ Jun 28 '13 at 19:00
  • $\begingroup$ Theoretically, yes. But in practice, this becomes very difficult. Most of that energy is lost as heat at various points in the cycle, so it is difficult to reclaim a usable amount when destroying a permanent magnet $\endgroup$
    – Jim
    Jul 2 '13 at 15:48

A perfect electromagnet wouldn't require any power as long as the piece of steel was stationary. An electromagnet is basically an inductor and perfect inductors don't dissipate any energy. In the real world the resistance of the wire in the electromagnet and eddy losses in it's core will dissipate energy so you need a continuous input of power to maintain the field.


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