The charge "q"2 can also act on "q"1 in a similar manner, by which it returns some of the momentum that it received from "q"1. This back-and-forth component of momentum contributes to magnetic inductance. The closer that "q"1 and "q"2 are, the greater the effect. When "q"2 is inside a conductive medium such as a thick slab made of copper or aluminum, it more readily responds to the force applied to it by "q"1. The energy of "q"1 is not instantly consumed as heat generated by the current of "q"2 but is also stored in "two" opposing magnetic fields. The energy density of magnetic fields tends to vary with the square of the magnetic field's intensity; however, in the case of magnetically non-linear materials such as ferromagnets and superconductors, this relationship breaks down.