![]() ![]() It is the constant of proportionality of the Josephson effect, relating the potential difference across a Josephson junction to the frequency of the irradiation. The inverse of the flux quantum, 1/Φ 0, is called the Josephson constant, and is denoted K J. The quantization of magnetic flux is closely related to the Little–Parks effect, but was predicted earlier by Fritz London in 1948 using a phenomenological model. The phenomenon of flux quantization was discovered experimentally by B. Its value is, therefore, the same for any superconductor. ×10 −15 Wb is a combination of fundamental physical constants: the Planck constant h and the electron charge e. The (superconducting) magnetic flux quantum Φ 0 = h/(2 e) ≈ 2.067 833 848. However, if one deals with the superconducting loop or a hole in a bulk superconductor, the magnetic flux threading such a hole/loop is quantized. Both B and S can be arbitrary, meaning Φ can be as well. The magnetic flux, represented by the symbol Φ, threading some contour or loop is defined as the magnetic field B multiplied by the loop area S, i.e. Experiments revealed that there is a crucial quantity called the magnetic flux, \boldsymbol induces an emf-the process is defined to be electromagnetic induction.Quantized unit of magnetic flux CODATA values So we see that changing the magnitude or direction of a magnetic field produces an emf. Note the generator is very similar in construction to a motor. This is the basic construction of a generator, where work done to turn the coil is converted to electric energy. Rotation of a coil in a magnetic field produces an emf. Note that the generator is remarkably similar in construction to a motor (another symmetry). A coil is rotated in a magnetic field, producing an alternating current emf, which depends on rotation rate and other factors that will be explored in later sections. The method of inducing an emf used in most electric generators is shown in Figure 3. The greater the speed, the greater the magnitude of the emf, and the emf is zero when there is no motion. The same emfs are produced if the coil is moved relative to the magnet. Movement of a magnet relative to a coil produces emfs as shown. The faster the motion, the greater the emf, and there is no emf when the magnet is stationary relative to the coil. The same results are produced if the coil is moved rather than the magnet-it is the relative motion that is important. Emfs of opposite signs are produced by motion in opposite directions, and the emfs are also reversed by reversing poles. An emf is induced in the coil when a bar magnet is pushed in and out of it. No current flows through the galvanometer when the switch remains closed or open.Īn experiment easily performed and often done in physics labs is illustrated in Figure 2. When the switch is opened and closed, the galvanometer registers currents in opposite directions. A change in the field produced by the top coil induces an emf and, hence, a current in the bottom coil. Faraday’s apparatus for demonstrating that a magnetic field can produce a current. The current is a result of an emf induced by a changing magnetic field, whether or not there is a path for current to flow. More basic than the current that flows is the emf that causes it. It is the change in magnetic field that creates the current. Closing and opening the switch induces the current. Interestingly, if the switch remains closed or open for any length of time, there is no current through the galvanometer. (You can also observe this in a physics lab.) Each time the switch is opened, the galvanometer detects a current in the opposite direction. ![]() It was found that each time the switch is closed, the galvanometer detects a current in one direction in the coil on the bottom. ![]() The galvanometer is used to detect any current induced in the coil on the bottom. When the switch is closed, a magnetic field is produced in the coil on the top part of the iron ring and transmitted to the coil on the bottom part of the ring. The apparatus used by Faraday to demonstrate that magnetic fields can create currents is illustrated in Figure 1. Describe methods to produce an electromotive force (emf) with a magnetic field or magnet and a loop of wire.Calculate the flux of a uniform magnetic field through a loop of arbitrary orientation.
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