To understand the how the MRI works, we must first understand the conditions inside a perfectly conducting fluid in motion. This is often a good approximation to astrophysical gases. In the presence of a magnetic field formula_55 a moving conductor responds by trying to eliminate the Lorentz force on the free charges. The magnetic force acts in such a way as to locally rearrange these charges to produce an internal electric field of formula_56 In this way, the direct Lorentz force on the charges formula_57 vanishes. (Alternatively, the electric field in the local rest frame of the moving charges vanishes.) This induced electric field can now itself induce further changes in the magnetic field formula_3 according to Faraday's law,