Lenz' law

CC Image
Illustration of Lenz law. The time derivative $\scriptstyle {\dot {\mathbf {B} }}$ describes an increase of B (vectors parallel) or a decrease of B (vectors antiparallel). In the first case the field due toi_ind weakens B and in the second case it reinforces B.

In electromagnetism Lenz' law states that a time-dependent change in magnetic induction B gives an induced current that opposes this change. According to Faraday's law of magnetic induction the time derivative of B

${\dot {\mathbf {B} }}\equiv {\frac {d\mathbf {B} }{dt}}$

induces a current i_ind in a conducting loop. The direction of i_ind is such that the change in field B is opposed. Recall here that the direction of current i and field B are connected through the right-hand screw rule, that is, a screw is driven in the direction of B and rotated in the direction of i.

The law is named for the Estonian physicist Heinrich Friedrich Emil Lenz (1804 – 1865).