In truth, they aren’t that practical at all. 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, 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. 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. The result is that the galvanometer registers no current. The disc is at a right angle to the magnetic field, so its rotation creates an electrical potential difference between the center of the disc and its rim. 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 fact it was shown that so long as a current loop was used to measure induced EMFs from the motion of the disk and magnet it is not possible to tell if the magnetic field does or does not rotate with the magnet. A homopolar generator is a type of electrical generator that produces direct current (DC) using an electrically conductive disc that rotates within a static magnetic field. In this example, all these paths lead to the same rate of change of flux, and hence the same EMF. The setup for this device (later called a homopolar generator) was simple, consisting of a copper disc that rotated in between the poles of a permanent magnet. Now that it has been proven that the magnetic field rotates with the magnet as discussed in the observation section, what is really causing the paradox? The magnet is held to prevent it from rotating, while the disc is spun on its axis. When the material of the circuit is changing, we must return to the basic laws. Nussbaum suggests that for Faraday's law to be valid work must be done in producing the change in flux. The point is that the flux law applies only to some situations, albeit some very practical ones. How does a generator work? 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. Please check your browser settings or contact your system administrator. The result is that the galvanometer registers a. There is no paradox or difficulty if one invokes the special theory of relativity. 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. We start with the switch on the left open and that on the right closed. In fact, rotating the magnet does not alter the B-field. Coated copper wire is generally made of multiple thin copper filaments woven into a rope-like wire. The ideas and opinions expressed on this forum do not necessarily reflect those of T-Bolts Group Inc or The Thunderbolts Project(TM), Unread post There are actually two modes of operation in which a homopolar generator can produce power. The work done is found from: If we plug in what we previously found for we get: The area covered by the displacement of the conductor is: The differential work can also be given in terms of charge and potential difference : By setting the two equations for differential work equal to each other we arrive at Faraday's Law. Good public relations versus good science. The force on wire 1 due to wire 2 is given by: The magnetic field from the second wire is given by: So we can rewrite the force on wire 1 as: Now consider a segment of a conductor displaced in a constant magnetic field. The perverse "consensus of leading scientists." The apparatus therefore acts as a generator, variously called the Faraday generator, the Faraday disc, or the homopolar (or unipolar) generator. Chronicles of scientific misbehavior. by D_Archer » Tue Oct 13, 2009 3:51 am, Unread post The perverse "consensus of leading scientists." There is no need to consider exactly how the current traverses the rectangle (or the disc). The polarity of the electrical current depends on the direction of rotation with respect to the magnetic field. The charge separation results from the Lorentz force on the free charges in the disk. 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. There is a galvanometer connected in the righthand loop, a magnet in the center of the lefthand loop, a switch in the lefthand loop, and a switch between the loops.  We start by calculating the force between two current carrying wires. The homopolar generator can be explained using Faraday's law in integral form or by its equivalent differential form which is not exactly the same as the watered down, popular version of Maxwell's equations given to us by Oliver Heaviside. Aether is a rotating magnetic field with a quantum spin of 2. This means that there is a change in flux. It has been used for generating very high currents at low voltages in applications such as welding, electrolysis and railgun research. 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.. 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: 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 ). In the case when the disk alone spins there is no change in flux through the circuit, however, there is an electromotive force induced contrary to Faraday's law. Is peer review working? This device consists of a conducting flywheel rotating in a magnetic field with one electrical contact near the axis and the other near the periphery. Figure 4 shows a translating rectangle of material with a narrow conducting strip subject to a magnetic field.

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