<P> Once the relationship between H and B is known this equation is used to determine the work needed to reach a given magnetic state . For hysteretic materials such as ferromagnets and superconductors, the work needed also depends on how the magnetic field is created . For linear non-dispersive materials, though, the general equation leads directly to the simpler energy density equation given above . </P> <P> A changing magnetic field, such as a magnet moving through a conducting coil, generates an electric field (and therefore tends to drive a current in such a coil). This is known as Faraday's law and forms the basis of many electrical generators and electric motors . </P> <P> Mathematically, Faraday's law is: </P> <Dl> <Dd> E = − d Φ d t, (\ displaystyle (\ mathcal (E)) = - (\ frac (\ mathrm (d) \ Phi) (\ mathrm (d) t)),) </Dd> </Dl>

What is the formula of magnetic field intensity