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A current can be induced in a conducting loop if it is exposed to a changing magnetic field. This change may be produced in several ways; you can change the strength of the magnetic field, move the conductor in and out of the field, alter the distance between a magnet and the conductor, or change the area of a loop located in a stable magnetic field. No matter how the variation is achieved, the result, an induced current, is the same. The strength of the current will vary in proportion to the change of magnetic flux, as suggested by Faradayís law of induction. The direction of the current can be determined by considering Lenzís law, which says that an induced electric current will flow in such a way that it generates a magnetic field that opposes the change in the field that generated it. In other words, if the applied magnetic field is increasing, the current in the wire will flow in such a way that the magnetic field that it generates around the wire will decrease the applied magnetic field.
In the above tutorial, a coil of wire connected to an Ammeter is placed in a stable magnetic field; imagine a flux line heading directly into each of the xís on the board. The area of the coil can be altered by adjusting the Coil Area slider, thus increasing or decreasing the area inside the coil through which the magnetic field is passing.
Notice that moving the slider produces an electric current, as shown by the Ammeter; the direction of the current is both reflected in the ammeter reading (positive or negative) and in the black arrows that appear. Notice that the response of the ammeter also varies depending on how quickly you move the coil shape slider. Since the strength of an induced current depends in part on the rate of change in the magnetic flux, changing the coil shape very quickly produces higher readings on the Ammeter than when the coil is adjusted slowly.
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