https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction
Faraday's law of induction
is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF)—a phenomenon called electromagnetic induction. It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids.[1][2]
The Maxwell–Faraday equation is a generalization of Faraday's law, and forms one of Maxwell's equations.
The most widespread version of Faraday's law states:
The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.[14][15]
This version of Faraday's law strictly holds only when the closed circuit is a loop of infinitely thin wire,[16] and is invalid in other circumstances as discussed below. A different version, the Maxwell–Faraday equation (discussed below), is valid in all circumstances.
Faraday's law of induction makes use of the magnetic flux ΦB through a hypothetical surface Σ whose boundary is a wire loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is defined by a surface integral:
where dA is an element of surface area of the moving surface Σ(t), B is the magnetic field (also called "magnetic flux density"), and B·dA is a vector dot product (the infinitesimal amount of magnetic flux through the infinitesimal area element dA). In more visual terms, the magnetic flux through the wire loop is proportional to the number of magnetic flux lines that pass through the loop.
When the flux changes—because B changes, or because the wire loop is moved or deformed, or both—Faraday's law of induction says that the wire loop acquires an EMF, , defined as the energy available from a unit charge that has travelled once around the wire loop.[16][17][18][19] Equivalently, it is the voltage that would be measured by cutting the wire to create an open circuit, and attaching a voltmeter to the leads.
Faraday's law states that the EMF is also given by the rate of change of the magnetic flux:
where is the electromotive force (EMF) and ΦB is the magnetic flux. The direction of the electromotive force is given by Lenz's law.
For a tightly wound coil of wire, composed of N identical turns, each with the same ΦB, Faraday's law of induction states that[20][21]
where N is the number of turns of wire and ΦB is the magnetic flux through a single loop.
The Maxwell–Faraday equation is a generalisation of Faraday's law that states that a time-varying magnetic field is always accompanied by a spatially-varying, non-conservative electric field, and vice versa. The Maxwell–Faraday equation is
(in SI units) where is the curl operator and again E(r, t) is the electric field and B(r, t) is the magnetic field. These fields can generally be functions of position r and time t.
The Maxwell–Faraday equation is one of the four Maxwell's equations, and therefore plays a fundamental role in the theory of classical electromagnetism. It can also be written in an integral form by the Kelvin-Stokes theorem:[22]
where, as indicated in the figure:
Both dℓ and dA have a sign ambiguity; to get the correct sign, the right-hand rule is used, as explained in the article Kelvin-Stokes theorem. For a planar surface Σ, a positive path element dℓ of curve ∂Σ is defined by the right-hand rule as one that points with the fingers of the right hand when the thumb points in the direction of the normal n to the surface Σ.
The integral around ∂Σ is called a path integral or line integral.
Notice that a nonzero path integral for E is different from the behavior of the electric field generated by charges. A charge-generated E-field can be expressed as the gradient of a scalar field that is a solution to Poisson's equation, and has a zero path integral. See gradient theorem.
The integral equation is true for any path ∂Σ through space, and any surface Σ for which that path is a boundary.
If the path Σ is not changing in time, the equation can be rewritten:
The surface integral at the right-hand side is the explicit expression for the magnetic flux ΦB through Σ.
Reflection on this apparent dichotomy was one of the principal paths that led Einstein to develop special relativity:
It is known that Maxwell's electrodynamics—as usually understood at the present time—when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Take, for example, the reciprocal electrodynamic action of a magnet and a conductor.
The observable phenomenon here depends only on the relative motion of
the conductor and the magnet, whereas the customary view draws a sharp
distinction between the two cases in which either the one or the other
of these bodies is in motion. For if the magnet is in motion and the
conductor at rest, there arises in the neighbourhood of the magnet an
electric field with a certain definite energy, producing a current at
the places where parts of the conductor are situated.
But if the magnet is stationary and the conductor in motion, no electric
field arises in the neighbourhood of the magnet. In the conductor,
however, we find an electromotive force, to which in itself there is no
corresponding energy, but which gives rise—assuming equality of relative
motion in the two cases discussed—to electric currents of the same path
and intensity as those produced by the electric forces in the former
case.
Examples of this sort, together with unsuccessful attempts to discover
any motion of the earth relative to the "light medium," suggest that the
phenomena of electrodynamics as well as of mechanics possess no
properties corresponding to the idea of absolute rest.