Magnetic field doesnt do any work, so when an electromagnet lifts, say a car or something, where is the energy coming from?

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Conor Daniel · Accepted Answer

A constant magnetic field doesn&#039;t do any work on a moving charge. This, however, doesn&#039;t mean that a magnetic field can&#039;t do work if it is not constant. The field has an energy density of             1              2        μ                  B    2   and this energy can be converted into work if the field changes, e.g. by introducing a magnetic material. It&#039;s the change in the field strength when the magnetic material is being introduced that allows for such a system to perform mechanical work. In case of electromagnets the necessary energy comes from the electric current that powers the magnet.In a quasi-static configuration, i.e. when the magnetic fields change slowly, we can calculate the total energy of the system (i.e. the energy in the fields as well as the energy in the magnetized materials) as a function of the position of the magnetic materials. For linear magnetic materials we can use   B  =  μ  H and then the total energy of the system becomes      E          m      a      g      n      e      t      i      c        =            1      2        ∫  H  B  d  VThe system will be most stable in the configuration in which this total energy is the smallest, i.e. we can treat this almost like a potential problem. I say almost because we are usually dealing with extended magnetized bodies, i.e. we need to consider both the position and the orientation of the bodies, so it is a little more complicated than in the case of a gravitational or electric potential.

Magnetic field doesnt do any work, so when an electromagnet lifts, say a car or something, where is the energy coming from?

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