[Physics] What does the minus sign in Maxwell’s third equation imply

electromagnetic-inductionelectromagnetismmaxwell-equations

If we write out Maxwell's equations with magnetic charges, we get

$$
\begin{align}
\nabla \cdot \mathbf{E} &= 4 \pi \rho_e \tag{1}\\
\nabla \cdot \mathbf{B} &= 4 \pi \rho_m \tag{2}\\
-\nabla \times \mathbf{E} &= \frac{\partial \mathbf{B}}{\partial t} + 4 \pi \mathbf{J}_m \tag{3}\label{Eq:Faraday}\\
\nabla \times \mathbf{B} &= \frac{\partial \mathbf{E}}{\partial t} + 4 \pi \mathbf{J}_e \tag{4}\label{Eq:Ampere}
\end{align}
$$

In particular, Faraday's law \eqref{Eq:Faraday} contains a minus sign that Ampere's law \eqref{Eq:Ampere} does not. This always struck me as odd because it's often said the fields are dual to each other (i.e. you can replace E with B and "get the same result"), but that requires a bit of mental recalibration to accommodate that minus. So I'm curious what the origin of that negative is and what it means. Are there any intuitive explanations for how to think about it?

Best Answer

In plain English, it is just Lenz’s law :

Lenz's law, named after the physicist Emil Lenz who formulated it in 1834, states that the direction of the electric current which is induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes changes in the initial magnetic field.

It is the basic principle behind all electric motors and dynamos, alternators, etc.

Wikipedia: Lenz's Law

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