- Faraday paradox
:"This article describes the Faraday paradox in electromagnetism. There is a different Faraday paradox in electrochemistry: see
Faraday paradox (electrochemistry)."
The Faraday paradox (or Faraday's paradox) is an experiment that illustrates
Michael Faraday's law of electromagnetic induction. Faraday deduced this law in 1831, after inventing the first electromagnetic generator or dynamo, but was never satisfied with his own explanation of the paradox.
The experiment requires a few simple components (see Figure 1): a cylindrical
magnet, a conducting disc with a conducting rim, a conducting axle, some wiring, and a galvanometer. The disc and the magnet are fitted a short distance apart on the axle, on which they are free to rotate about their own axes of symmetry. An electrical circuit is formed by connecting sliding contacts: one to the axle of the disc, the other to its rim. A galvanometer can be inserted in the circuit to measure the current.
The experiment proceeds in three steps. First, the magnet is held to prevent it from rotating, while the disc is spun on its axis. The result is that the galvanometer registers a
direct current. The apparatus therefore acts as a generator, variously called the Faraday generator, the Faraday disc, or the homopolar (or unipolar) generator.
In the second step, the disc is held stationary while the magnet is spun on its axis. The result is that the galvanometer registers no current.
In the third step, the disc and magnet are spun together. The galvanometer registers a current, as it did in step 1.
Why is this paradoxical?
The experiment is described by some as a "paradox" as it seems, at first sight, to violate Faraday's law of electromagnetic induction, because the flux through the disc appears to be the same no matter what is rotating. Hence, the EMF is predicted to be zero in all three cases of rotation. The discussion below shows this viewpoint stems from an incorrect choice of surface over which to calculate the flux.
The paradox appears a bit different from the lines of flux viewpoint: in Faraday's model of electromagnetic induction, a
magnetic fieldconsisted of imaginary lines of magnetic flux, similar to the lines that appear when iron filings are sprinkled on paper and held near a magnet. The EMF is proposed to be proportional to the rate of cutting lines of flux. If the lines of flux are imagined to originate in the magnet, then they would be stationary in the frame of the magnet, and rotating the disc relative to the magnet, whether by rotating the magnet or the disc, should produce an EMF, but rotating both of them together should not.
In Faraday's model of electromagnetic induction, a circuit received an induced current when it cut lines of magnetic flux. According to this model, the Faraday disc should have worked when either the disc or the magnet was rotated, but not both. Faraday attempted to explain the disagreement with observation by assuming that the magnet's field, complete with its lines of flux, remained stationary as the magnet rotated (a completely accurate picture, but maybe not intuitive in the lines-of-flux model). In other words, the lines of flux have their own frame of reference. As we shall see in the next section, modern physics (since the discovery of the
electron) does not need the lines-of-flux picture and dispels the paradox.
Using the Lorentz force
After the discovery of the
electronand the forces that affect it, a microscopic resolution of the paradox became possible. See Figure 1. The metal portions of the apparatus are conducting, and confine a current due to electronic motion to within the metal boundaries. All electrons that move in a magnetic field experience a Lorentz forceof F = v × B, where "v" is the velocity of the electrons. This force is perpendicular to both the velocity of the electrons, which is in the plane of the disc, and to the magnetic field, which is normal to the disc. An electron at rest in the frame of the disc moves circularly with the disc relative to the "B"-field, and so experiences a radial Lorentz force. In Figure 1 this force (on a "positive" charge, not an electron) is outward toward the rim according to the right-hand rule.
Of course, this radial force, which is the cause of the current, creates a radial component of electron velocity, generating in turn its own Lorentz force component that opposes the circular motion of the electrons. This reaction is a form of
Le Chatelier's principle, tending to slow the disc's rotation, but the electrons retain a component of circular motion that continues to drive the current via the radial Lorentz force.
This mechanism agrees with the observations: an EMF is generated whenever the disc moves relative to the magnetic field, regardless of how that field is generated.
Relation to Faraday's law of induction
The flux through the portion of the path from the brush at the rim, through the outside loop and the axle to the center of the disc is always zero because the magnetic field is in the plane of this path (not perpendicular to it), no matter what is rotating, so the integrated emf around this part of the path is always zero. Therefore, attention is focused on the portion of the path from the axle across the disc to the brush at the rim.
Faraday's law of induction can be stated in words as: [See, for example,cite book
author=M N O Sadiku
title=Elements of Electromagnetics
page =§9.2 pp. 386 ff
publisher=Oxford University Press
isbn=0-19-530048-3] Quotation|The induced
electromotive forceor EMF in any closed circuit is equal to the time rate of change of the magnetic fluxthrough the circuit. Mathematically, the law is stated:
where ΦB is the flux, and "d" A is a vector element of area of a moving surface Σ("t") bounded by the loop around which the EMF is to be found.
How can this law be connected to the Faraday disc generator, where the flux linkage appears to be just the "B"-field multiplied by the area of the disc?
One approach is to define the notion of "rate of change of flux linkage" by drawing a hypothetical line across the disc from the brush to the axle and asking how much flux linkage is swept past this line per unit time. See Figure 2. Assuming a radius "R" for the disc, a sector of disc with central angle θ has an area:
so the rate that flux sweeps past the imaginary line is
with ω = "d" θ / "dt" the angular rate of rotation. The sign is chosen based upon
Lenz's law: the field generated by the motion must oppose the change in flux caused by the rotation. [ For example, the circuit with the radial segment in Figure 2 according to the right-hand rule "adds" to the applied B-field, tending to increase the flux linkage. That suggests that the flux through this path is decreasing due to the rotation, so "d" θ / "d t " is "negative.]
This flux-cutting result for EMF can be compared to calculating the work done per unit charge making an infinitesimal test charge traverse the hypothetical line using the Lorentz force / unit charge at radius "r", namely |v × B | = "B v = B r" ω:
which is the same result.
The above methodology for finding the flux cut by the circuit is formalized in the flux law by properly treating the time derivative of the bounding surface Σ ( "t" ). Of course, the time derivative of an integral with time dependent limits is "not" simply the time derivative of the integrand alone, a point often forgotten; see Leibniz integral rule and Lorentz force.
In choosing the surface Σ ( "t" ), the restrictions are that (i) it be bounded by a closed curve around which the EMF is to be found, and (ii) it capture the relative motion of all moving parts of the circuit. It is emphatically "not" required that the bounding curve correspond to a physical line of flow of the current. On the other hand, induction is all about relative motion, and the path emphatically "must" capture any relative motion. In a case like Figure 1 where a portion of the current path is distributed over a region in space, the EMF driving the current can be found using a variety of paths. Figure 2 shows two possibilities. All paths include the obvious return loop, but in the disc two paths are shown: one is a geometrically simple path, the other a tortuous one. We are free to choose whatever path we like, but a portion of any acceptable path is "fixed in the disc itself" and turns with the disc. The flux is calculated though the entire path, return loop "plus" disc segment, and its rate-of change found.
In this example, all these paths lead to the same rate of change of flux, and hence the same EMF. To provide some intuition about this path independence, in Figure 3 the Faraday disc is unwrapped onto a strip, making it resemble a sliding rectangle problem. In the sliding rectangle case, it becomes obvious that the pattern of current flow inside the rectangle is time-independent and therefore irrelevant to the rate of change of flux linking the circuit. There is no need to consider exactly how the current traverses the rectangle (or the disc). Any choice of path connecting the top and bottom of the rectangle (axle- to-brush in the disc) and moving with the rectangle (rotating with the disc) sweeps out the same rate-of-change of flux, and predicts the same EMF. For the disc, this rate-of-change of flux estimation is the same as that done above based upon rotation of the disc past a line joining the brush to the axle.
Whether the magnet is "moving" is irrelevant in this analysis, as it does not appear in Faraday's law. In fact, rotating the magnet does not alter the "B"-field. Likewise, rotation of the magnet "and" the disc is the same as rotating the disc and keeping the magnet stationary. The crucial relative motion is that of the disk and the return path, not of the disk and the magnet.
This becomes clearer if a modified Faraday disk is used in which the return path is not a wire but another disk. That is, mount two conducting disks just next to each other on the same axle and let them have sliding electrical contact at the center and at the circumference. The current will be proportional to the relative rotation of the two disks and independent of any rotation of the magnet.
Configuration without a return path
A Faraday disk can also be operated with neither a galvanometer nor a return path. When the disk spins, the electrons collect along the rim and leave a deficit near the axis (or the other way around). It is possible in principle to measure the distribution of charge, for example, through the
electromotive forcegenerated between the rim and the axle (though not necessarily easy). This charge separation will be proportional to the magnetic field and the rotational velocity of the disk. The magnetic field will be independent of any rotation of the magnet. In this configuration, the polarisation is determined by the absolute rotation of the disk, that is, the rotation relative to an inertial frame. The relative rotation of the disk and the magnet plays no role.
Inapplicability of Faraday's law
Figure 4 shows a translating rectangle of material with a narrow conducting strip subject to a magnetic field. This strip of material is rendered conducting at a fixed location by, for example, a strong light beam at this location. The magnetic field also is confined to the same strip. The Lorentz force drives a current from the top rail to the bottom rail through this strip, and the circuit is completed through leads attached to the top and bottom conducting rails. In this example, the circuit does not move, and the magnetic flux through the circuit is not changing, so Faraday's law suggests no current flows. However, the Lorentz force law suggests a current does flow. This example is based upon one devised by
Richard Feynmanto illustrate the inapplicability of Faraday's law of inductionto certain situations (that is, the version of Faraday's law of induction which relates EMF to magnetic flux, which he terms the "flux rule"). Referring to his example, Feynman said:cite book
author=Richard Phillips Feynman, Leighton R B & Sands M L
title=The Feynman Lectures on Physics
page =Vol. II, pp. 17-2, 17-3
url=http://books.google.com/books?id=zUt7AAAACAAJ&dq=intitle:Feynman+intitle:Lectures+intitle:on+intitle:Physics&lr=&as_brr=0] Quotation |The "flux rule" does not work in this case. It must be applied to circuits in which the "material" of the circuit remains the same. When the material of the circuit is changing, we must return to the basic laws. The "correct" physics is always given by the two basic laws
:::|Richard P Feynman  "The Feynman Lectures on Physics"
Accordingly, he explains the phenomenon using the Lorentz force law, as described above. The point is that the flux law applies only to some situations, albeit some very practical ones.
References and notes
* [http://www.brera.unimi.it/old/Atti-Cosenza-2001/Giuliani.pdf "Electromagnetic induction: physics and flashbacks" (PDF)] by Giuseppe Giuliani - details of the Lorentz force in Faraday's disc
* [http://www.spots.ab.ca/~belfroy/Homopolars/homopoltext.html "Homopolar Electric Dynamo"] - contains derivation of equation for EMF of a Faraday disc
* [http://www.tinaja.com/glib/muse121.pdf Don Lancaster's "Tech Musings" column, Feb 1998] - on practical inefficiencies of Faraday disc
* [http://www.marmet.ca/louis/induction_faraday/kelly/KellyFa3.pdf "Faraday's Final Riddle; Does the Field Rotate with a Magnet?" (PDF)] - contrarian theory, but contains useful references to Faraday's experiments
*P. J. Scanlon, R. N. Henriksen, and J. R. Allen, "Approaches to electromagnetic induction," Am. J. Phys. 37, 698–708 (1969). - describes how to apply Faraday's law to Faraday's disc
*Jorge Guala-Valverde, Pedro Mazzoni, Ricardo Achilles "The homopolar motor: A true relativistic engine," Am. J. Phys. 70 (10), 1052–1055 (Oct. 2002). - argues that only the Lorentz force can explain Faraday's disc and describes some experimental evidence for this
*Frank Munley, Challenges to Faraday's flux rule, Am. J. Phys. 72, 1478 (2004). - an updated discussion of concepts in the Scanlon reference above.
*Richard Feynman, Robert Leighton, Matthew Sands, "The Feynman Lectures on Physics Volume II", Chapter 17 - In addition to the Faraday "paradox" (where linked flux does not change but an emf is induced), he describes the "rocking plates" experiment where linked flux changes but no emf is induced. He shows that the correct physics is always given by the combination of the
Lorentz forcewith the Maxwell-Faraday equation (see quotation box) and poses these two "paradoxes" of his own.
* [http://www.geocities.com/terella1/ The rotation of magnetic field] by Vanja Janezic - describes a simple experiment that anyone can do. Because it only involves two bodies, its result is less ambiguous than the three-body Faraday, Kelly and Guala-Valverde experiments.
Faraday's law of induction
Moving magnet and conductor problem
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