Faraday's law of induction

Faraday's law of induction, or simply Faraday's law, is a law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (emf). This phenomenon, known as electromagnetic induction, is the fundamental operating principle of transformers, inductors, and many types of electric motors, generators and solenoids.

Faraday's experiment showing induction between coils of wire: The liquid battery (right) provides a current which flows through the small coil (A), creating a magnetic field. When the coils are stationary, no current is induced. But when the small coil is moved in or out of the large coil (B), the magnetic flux through the large coil changes, inducing a current which is detected by the galvanometer (G).

The Maxwell–Faraday equation (listed as one of Maxwell's equations) describes the fact that a spatially varying (and also possibly time-varying, depending on how a magnetic field varies in time) electric field always accompanies a time-varying magnetic field, while Faraday's law states that emf (electromagnetic work done on a unit charge when it has traveled one round of a conductive loop) appears on a conductive loop when the magnetic flux through the surface enclosed by the loop varies in time.

Once Faraday's law had been discovered, one aspect of it (transformer emf) was formulated as the Maxwell–Faraday equation. The equation of Faraday's law can be derived by the Maxwell–Faraday equation (describing transformer emf) and the Lorentz force (describing motional emf). The integral form of the Maxwell–Faraday equation describes only the transformer emf, while the equation of Faraday's law describes both the transformer emf and the motional emf.

History

Electromagnetic induction was discovered independently by Michael Faraday in 1831 and Joseph Henry in 1832. Faraday was the first to publish the results of his experiments.

Faraday's 1831 demonstration

Faraday's notebook on August 29, 1831 describes an experimental demonstration of electromagnetic induction that wraps two wires around opposite sides of an iron ring (like a modern toroidal transformer). His assessment of newly-discovered properties of electromagnets suggested that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. Indeed, a galvanometer's needle measured a transient current (which he called a "wave of electricity") on the right side's wire when he connected or disconnected the left side's wire to a battery. This induction was due to the change in magnetic flux that occurred when the battery was connected and disconnected. His notebook entry also noted that fewer wraps for the battery side resulted in a greater disturbance of the galvanometer's needle.

Within two months, Faraday had found several other manifestations of electromagnetic induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk").

Faraday's disk, the first electric generator, a type of homopolar generator

Michael Faraday explained electromagnetic induction using a concept he called lines of force. However, scientists at the time widely rejected his theoretical ideas, mainly because they were not formulated mathematically. An exception was James Clerk Maxwell, who in 1861–62 used Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's papers, the time-varying aspect of electromagnetic induction is expressed as a differential equation which Oliver Heaviside referred to as Faraday's law even though it is different from the original version of Faraday's law, and does not describe motional emf. Heaviside's version is the form recognized today in the group of equations known as Maxwell's equations.

Lenz's law, formulated by Emil Lenz in 1834, describes "flux through the circuit", and gives the direction of the induced emf and current resulting from electromagnetic induction (elaborated upon in the examples below).

According to Albert Einstein, much of the groundwork and discovery of his special relativity theory was presented by this law of induction by Faraday in 1834.

Faraday's law

Alternating electric current flows through the solenoid on the left, producing a changing magnetic field. This field causes, by electromagnetic induction, an electric current to flow in the wire loop on the right.

The most widespread version of Faraday's law states:

The electromotive force around a closed path is equal to the negative of the time rate of change of the magnetic flux enclosed by the path.

Mathematical statement

The definition of surface integral relies on splitting the surface Σ into small surface elements. Each element is associated with a vector dA of magnitude equal to the area of the element and with direction normal to the element and pointing "outward" (with respect to the orientation of the surface).

For a loop of wire in a magnetic field, the magnetic flux Φ_B is defined for any surface Σ whose boundary is the given loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is the surface integral: $$\Phi _{B}=\iint _{\Sigma (t)}\mathbf {B} (t)\cdot \mathrm {d} \mathbf {A} \,,$$ where dA is an element of area vector of the moving surface Σ(t), B is the magnetic field, and B · dA is a vector dot product representing the element of flux through dA. In more visual terms, the magnetic flux through the wire loop is proportional to the number of magnetic field 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 traveled once around the wire loop. (Although some sources state the definition differently, this expression was chosen for compatibility with the equations of special relativity.) 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: $${\mathcal {E}}=-{\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}},$$ where ${\mathcal {E}}$ is the electromotive force (emf) and Φ_B is the magnetic flux.

The direction of the electromotive force is given by Lenz's law.

The laws of induction of electric currents in mathematical form were established by Franz Ernst Neumann in 1845.

Faraday's law contains the information about the relationships between both the magnitudes and the directions of its variables. However, the relationships between the directions are not explicit; they are hidden in the mathematical formula.

A Left Hand Rule for Faraday's Law. The sign of ΔΦ_B, the change in flux, is found based on the relationship between the magnetic field B, the area of the loop A, and the normal n to that area, as represented by the fingers of the left hand. If ΔΦ_B is positive, the direction of the emf is the same as that of the curved fingers (yellow arrowheads). If ΔΦ_B is negative, the direction of the emf is against the arrowheads.

It is possible to find out the direction of the electromotive force (emf) directly from Faraday’s law, without invoking Lenz's law. A left hand rule helps doing that, as follows:

• Align the curved fingers of the left hand with the loop (yellow line).
• Stretch your thumb. The stretched thumb indicates the direction of n (brown), the normal to the area enclosed by the loop.
• Find the sign of ΔΦ_B, the change in flux. Determine the initial and final fluxes (whose difference is ΔΦ_B) with respect to the normal n, as indicated by the stretched thumb.
• If the change in flux, ΔΦ_B, is positive, the curved fingers show the direction of the electromotive force (yellow arrowheads).
• If ΔΦ_B is negative, the direction of the electromotive force is opposite to the direction of the curved fingers (opposite to the yellow arrowheads).

For a tightly wound coil of wire, composed of N identical turns, each with the same Φ_B, Faraday's law of induction states that $${\mathcal {E}}=-N{\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}}$$ where N is the number of turns of wire and Φ_B is the magnetic flux through a single loop.

Maxwell–Faraday equation

An illustration of the Kelvin–Stokes theorem with surface Σ, its boundary ∂Σ, and orientation n set by the right-hand rule

The Maxwell–Faraday equation states that a time-varying magnetic field always accompanies a spatially varying (also possibly time-varying), non-conservative electric field, and vice versa. The Maxwell–Faraday equation is

$\nabla \times \mathbf {E} =-{\frac {\partial \mathbf {B} }{\partial t}}$

(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:

$\oint _{\partial \Sigma }\mathbf {E} \cdot \mathrm {d} \mathbf {l} =-\int _{\Sigma }{\frac {\partial \mathbf {B} }{\partial t}}\cdot \mathrm {d} \mathbf {A}$

where, as indicated in the figure, Σ is a surface bounded by the closed contour ∂Σ, dl is an infinitesimal vector element of the contour ∂Σ, and dA is an infinitesimal vector element of surface Σ. Its direction is orthogonal to that surface patch, the magnitude is the area of an infinitesimal patch of surface.

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 line integral around ∂Σ is called circulation. A nonzero circulation of E is different from the behavior of the electric field generated by static 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.

The integral equation is true for any path ∂Σ through space, and any surface Σ for which that path is a boundary.

If the surface Σ is not changing in time, the equation can be rewritten: $$\oint _{\partial \Sigma }\mathbf {E} \cdot \mathrm {d} \mathbf {l} =-{\frac {\mathrm {d} }{\mathrm {d} t}}\int _{\Sigma }\mathbf {B} \cdot \mathrm {d} \mathbf {A} .$$ The surface integral at the right-hand side is the explicit expression for the magnetic flux Φ_B through Σ.

The electric vector field induced by a changing magnetic flux, the solenoidal component of the overall electric field, can be approximated in the non-relativistic limit by the volume integral equation $$\mathbf {E} _{s}(\mathbf {r} ,t)\approx -{\frac {1}{4\pi }}\iiint _{V}\ {\frac {\left({\frac {\partial \mathbf {B} (\mathbf {r} ',t)}{\partial t}}\right)\times \left(\mathbf {r} -\mathbf {r} '\right)}{|\mathbf {r} -\mathbf {r} '|^{3}}}d^{3}\mathbf {r'}$$

Derivation from microscopic equations

The four Maxwell's equations, together with the Lorentz force law, form a complete foundation for classical electromagnetism. From these, Faraday's law can be derived directly.

The derivation begins by considering the time derivative of the magnetic flux through a surface Σ(t) that may vary with time: $${\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}}={\frac {\mathrm {d} }{\mathrm {d} t}}\int _{\Sigma (t)}\mathbf {B} (t)\cdot \mathrm {d} \mathbf {A} .$$ The magnetic flux can change for two reasons: the magnetic field itself may vary with time, and the surface may move or change shape, enclosing a different region of space. Both effects are captured by the three-dimensional version of the Leibniz integral rule, sometimes referred to as the "flux theorem": $${\frac {\mathrm {d} }{\mathrm {d} t}}\int _{\Sigma (t)}\mathbf {B} \cdot \mathrm {d} \mathbf {A} =\int _{\Sigma (t)}\left({\frac {\partial \mathbf {B} }{\partial t}}+(\nabla \cdot \mathbf {B} )\mathbf {v} _{c}\right)\cdot \mathrm {d} \mathbf {A} -\oint _{\partial \Sigma (t)}(\mathbf {v} _{c}\times \mathbf {B} )\cdot \mathrm {d} \mathbf {l}$$ Here, ∂Σ(t) is the moving boundary of the surface and $\mathbf {v} _{c}$ is the local velocity of the boundary at each point. By Gauss's law for magnetism ($\nabla \cdot \mathbf {B} =0$), the second term under the area integral vanishes. Applying the Maxwell–Faraday equation to the remaining term, $$\int _{\Sigma (t)}{\frac {\partial \mathbf {B} }{\partial t}}\cdot \mathrm {d} \mathbf {A} =-\oint _{\partial \Sigma (t)}\mathbf {E} \cdot \mathrm {d} \mathbf {l} ,$$ and combining the two line integrals gives $${\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}}=-\oint _{\partial \Sigma (t)}\left(\mathbf {E} +\mathbf {v} _{c}\times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} .$$ This is an exact result, derived from Maxwell's equations and vector calculus.

However, the quantity inside the integral is not the full Lorentz force per unit charge, because the velocity $\mathbf {v} _{c}$ represents the motion of loop boundary, not the actual velocity of the charge carriers. To recover the physical electromotive force, we must distinguish between these velocities. Let us choose the integration path to coincide with the physical circuit. The velocity of a charge carrier in the conductor is then given by

$\mathbf {v} (\mathbf {r} ,t)=\mathbf {v} _{c}(\mathbf {r} ,t)+\mathbf {v} _{d}(\mathbf {r} ,t)$,

where $\mathbf {v} _{c}$ is the velocity of the conductor (the ions in the material), and $\mathbf {v} _{d}$ is the drift velocity of the electrons relative to the material. This decomposition assumes nonrelativistic (Galilean) addition of velocities.

The emf ${\mathcal {E}}$ associated with the Lorentz force is defined as $${\mathcal {E}}=\oint _{\partial \Sigma (t)}\left(\mathbf {E} +\mathbf {v} \times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} .$$

Substituting the expression for the carrier velocity and the above result yields:

${\mathcal {E}}=-{\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}}+\oint _{\partial \Sigma (t)}\left(\mathbf {v} _{d}\times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} .$

Equivalently, this can be expressed as

${\mathcal {E}}=-\int _{\partial \Sigma (t)}{\frac {\partial \mathbf {B} }{\partial t}}\cdot {\rm {d}}\mathbf {A} +\oint _{\partial \Sigma (t)}\left(\mathbf {v} \times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} ,$

where the first term is the "transformer emf" due to a time-varying magnetic field, and the second term is the "motional emf" due to the magnetic Lorentz force by the motion of the charges in the magnetic field.

In circuits made of thin, one-dimensional wires, the drift velocity is aligned with the wire, and hence with the integration element ${\textstyle {\rm {d}}\mathbf {l} }$. In that case, the cross product ${\textstyle \mathbf {v} _{d}\times \mathbf {B} }$ is perpendicular to ${\textstyle {\rm {d}}\mathbf {l} }$, and the term proportional to the drift velocity vanishes. This recovers the standard form of Faraday's law: $${\mathcal {E}}=-{\frac {\mathrm {d} \Phi _{B}}{\mathrm {d} t}}$$ In this case, the emf can also be expressed as a sum $${\mathcal {E}}=-\int _{\Sigma (t)}{\frac {\partial \mathbf {B} }{\partial t}}\cdot \mathrm {d} \mathbf {A} +\oint _{\partial \Sigma (t)}\left(\mathbf {v} _{c}\times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} .$$However, in extended systems not made of thin wires, the drift velocity term ${\textstyle \oint _{\partial \Sigma (t)}\left(\mathbf {v} _{d}\times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} }$ in the above may not be negligible. For example, in the Hall effect, the flux term ${\textstyle \mathrm {d} \Phi _{B}/\mathrm {d} t}$ vanishes, and the observed emf arises entirely from the drift velocity term.

Exceptions

See also: Faraday paradox and Hering's Paradox

It is tempting to generalize Faraday's law to state: If ∂Σ is any arbitrary closed loop in space whatsoever, then the total time derivative of magnetic flux through Σ equals the emf around ∂Σ. This statement, however, is not always true. As noted in the previous section, Faraday's law is not guaranteed to work unless the velocity of the abstract curve ∂Σ matches the actual velocity of the material conducting the electricity. If the conductor is not an infinitely thin wire, one may also have take into account the velocity of charges with respect to the material. The two examples illustrated below show that one often obtains incorrect results when Faraday's law is applied too broadly.

• 
  Faraday's homopolar generator. The disc rotates with angular rate ω, sweeping the conducting radius circularly in the static magnetic field B (which direction is along the disk surface normal). The magnetic Lorentz force v × B drives a current along the conducting radius to the conducting rim, and from there the circuit completes through the lower brush and the axle supporting the disc. This device generates an emf and a current, although the shape of the "circuit" is constant and thus the flux through the circuit does not change with time.
• 
  A wire (solid red lines) connects to two touching metal plates (silver) to form a circuit. The whole system sits in a uniform magnetic field, normal to the page. If the abstract path ∂Σ follows the primary path of current flow (marked in red), then the magnetic flux through this path changes dramatically as the plates are rotated, yet the emf is almost zero. After Feynman Lectures on Physics

One can analyze examples like these by taking care that the path ∂Σ moves with the same velocity as the material. The electromotive force can always be correctly calculated by combining the Lorentz force law with the Maxwell–Faraday equation: $${\mathcal {E}}=\int _{\partial \Sigma }(\mathbf {E} +\mathbf {v} \times \mathbf {B} )\cdot \mathrm {d} \mathbf {l} =-\int _{\partial \Sigma (t)}{\frac {\partial \mathbf {B} }{\partial t}}\cdot {\rm {d}}\mathbf {A} +\oint _{\partial \Sigma (t)}\left(\mathbf {v} \times \mathbf {B} \right)\cdot \mathrm {d} \mathbf {l} ,$$ where v is the velocity of the conductor in the frame of reference in which B in described. The time derivative cannot in general be moved outside the integral since the position or shape of the loop may be a function of time.

Faraday's law and relativity

Further information: Moving magnet and conductor problem

Two phenomena

Faraday's law is a single equation describing two different phenomena: the motional emf generated by a magnetic force on a moving wire, and the transformer emf generated by an electric force due to a changing magnetic field (described by the Maxwell–Faraday equation).

James Clerk Maxwell drew attention to this fact in his 1861 paper On Physical Lines of Force. In the latter half of Part II of that paper, Maxwell gives a separate physical explanation for each of the two phenomena.

A reference to these two aspects of electromagnetic induction is made in some modern textbooks. As Richard Feynman states:

So the "flux rule" that the emf in a circuit is equal to the rate of change of the magnetic flux through the circuit applies whether the flux changes because the field changes or because the circuit moves (or both) ...

Yet in our explanation of the rule we have used two completely distinct laws for the two cases – v × B for "circuit moves" and ∇ × E = −∂_tB for "field changes".

We know of no other place in physics where such a simple and accurate general principle requires for its real understanding an analysis in terms of two different phenomena.

— Richard P. Feynman, The Feynman Lectures on Physics

Explanation based on four-dimensional formalism

In the general case, explanation of the motional emf appearance by action of the magnetic force on the charges in the moving wire or in the circuit changing its area is unsatisfactory. As a matter of fact, the charges in the wire or in the circuit could be completely absent, will then the electromagnetic induction effect disappear in this case? This situation is analyzed in the article, in which, when writing the integral equations of the electromagnetic field in a four-dimensional covariant form, in the Faraday’s law the total time derivative of the magnetic flux through the circuit appears instead of the partial time derivative. Thus, electromagnetic induction appears either when the magnetic field changes over time or when the area of the circuit changes. From the physical point of view, it is better to speak not about the induction emf, but about the induced electric field strength ${\textstyle \mathbf {E} =-\nabla {\mathcal {E}}-{\frac {\partial \mathbf {A} }{\partial t}}}$, that occurs in the circuit when the magnetic flux changes. In this case, the contribution to $\mathbf {E}$ from the change in the magnetic field is made through the term ${\textstyle -{\frac {\partial \mathbf {A} }{\partial t}}}$ , where $\mathbf {A}$ is the vector potential. If the circuit area is changing in case of the constant magnetic field, then some part of the circuit is inevitably moving, and the electric field $\mathbf {E}$ emerges in this part of the circuit in the comoving reference frame K’ as a result of the Lorentz transformation of the magnetic field $\mathbf {B}$, present in the stationary reference frame K, which passes through the circuit. The presence of the field $\mathbf {E}$ in K’ is considered as a result of the induction effect in the moving circuit, regardless of whether the charges are present in the circuit or not. In the conducting circuit, the field $\mathbf {E}$ causes motion of the charges. In the reference frame K, it looks like appearance of emf of the induction ${\mathcal {E}}$, the gradient of which in the form of $-\nabla {\mathcal {E}}$, taken along the circuit, seems to generate the field $\mathbf {E}$.

Einstein's view

Reflection on this apparent dichotomy was one of the principal paths that led Albert 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.

— Albert Einstein, On the Electrodynamics of Moving Bodies

See also

• Eddy current
• Inductance
• Maxwell's equations
• Crosstalk
• Faraday paradox
• Felici's law

Further reading

• Clerk Maxwell, James (1881). A treatise on electricity and magnetism, Vol. II. Oxford: Clarendon Press. ch. III, sec. 530, p. 178. ISBN 0-486-60637-6. a treatise on electricity and magnetism. {{cite book}}: ISBN / Date incompatibility (help)

External links

• Media related to Faraday's law of induction at Wikimedia Commons
• A simple interactive tutorial on electromagnetic induction (click and drag magnet back and forth) National High Magnetic Field Laboratory
• Roberto Vega. Induction: Faraday's law and Lenz's law – Highly animated lecture, with sound effects, Electricity and Magnetism course page
• Notes from Physics and Astronomy HyperPhysics at Georgia State University
• Tankersley and Mosca: Introducing Faraday's law
• A free simulation on motional emf