Sonic logging

Template:Orphan In physics, Felici's law allows one to calculate the net charge through a circuit configuration in which there is a current induced by a variable magnetic field, such a conductor coil immersed in a variable magnetic field. This law makes it possible to calculate net charge in a period using initial flux and final flux.

$q(t)=\frac{1}{\mathrm{R}}[\mathrm{\Phi }(0)-\mathrm{\Phi }(t)]$

Demonstration

$q(t)=\underset{0}{\overset{t}{\int }}i(\tau )d\tau =\frac{1}{\mathrm{R}}\underset{0}{\overset{t}{\int }}{f}_{em}(\tau )d\tau =\frac{1}{\mathrm{R}}[\mathrm{\Phi }(0)-\mathrm{\Phi }(t)]$

When $f_{em}(t)=-\frac{d\mathrm{\Phi }(t)}{dt}$