# Eddy Current Calculations

 PF Gold P: 947 Maybe it would help to look at a simple special case: a disc. By symmetry, the eddy currents will follow circular paths centred on the centre, O, of the disc. The emf acting in a circular path of radius r will, from Faraday’s law be given by $$\varepsilon = \frac{d\Phi}{dt} = \pi r^2 \dot B$$ But if the current density is J around a path of radius r, we have $$\varepsilon = \rho 2 \pi r J$$, in which $\rho$ is the resistivity. So $$J = \frac{r \dot B}{2\rho}$$. Say if this isn't clear.
 P: 4 This Helps a Ton and it's very clear! thank you. I'm still a little confused about how to derive the induced magnetic field. The current density that you've given is now a function of radius. Does this mean we can use Ampere's law to model the Magnetic Field as a function of radius? $$J=\frac{r \dot B}{2\rho}$$ $$\oint B \cdot \partial L = \mu_0 I_{enc}$$ $$I_{enc} = \int J \cdot \partial A = J \pi r^2$$ $$\int \int_0^{2\pi} Br \cdot \partial \theta \cdot \partial r = \pi \mu_0 \int Jr^2 \cdot \partial r$$ Or is this just bad calculus?
 PF Gold P: 947 There are only a few cases where Ampère's law can be used to find B. These are cases in which there's enough symmetry for B to be effectively the same all along a particular integration path. Looking at your post, it seems that you don't have a particular path in mind. And the bad news is that for these circular eddy currents, as for a single circular loop of wire, there is no path along which B is constant. Ampère's law, beautiful though it is, can't help. In fact the general problem of finding B at points in the vicinity of the disc, as for a circular loop, is very difficult. The only easy cases are for points on the axis of the disc, and, simplest of all, at its centre. To find the field at the centre of the disc (of thickness b, say), think if it as made up of annuli of cross-sectional area b dr. Then the current in an annulus is Jbdr. But from the Biot-Savart law we know that the field at the centre of a ring carrying current I is $\mu$0I/2r. Using the J from my previous post, and integrating for a disc of radius a, I find $$B_{ind} = \frac{\mu_0 ba \dot B}{4\rho}$$ A neat result, I thought. But I'm prone to slips...
 PF Gold P: 947 Hadn't thought of the field ending; was thinking of the whole disc being subjected to a uniform normal field. But if the field 'covered' only an inner part, DB, of the disc, I wouldn't expect the induced field to drop to zero at exactly the edge of DB. Disc annuli outside DB will still have emfs induced in them, because changing flux will still be linked with them. The emf will be $$\varepsilon = \pi r_B^2 \dot B$$ in which rB is the radius of DB. But beyond DB, the current density will fall because of the increasing value of 2$\pi$r, and at some point, I think that B will indeed drop to zero and then reverse, as would happen for an ordinary current-carrying loop. Thanks for such an interesting question.