![]() (b) An approaching south pole induces a clockwise current with respect to the bar magnet. (a) An approaching north pole induces a counterclockwise current with respect to the bar magnet. This occurs when the induced current flows as shown, for then the face of the loop nearer the approaching magnet is also a north pole.įigure 13.7 The change in magnetic flux caused by the approaching magnet induces a current in the loop. Alternatively, we can determine the direction of the induced current by treating the current loop as an electromagnet that opposes the approach of the north pole of the bar magnet. Your fingers wrap in a counterclockwise direction as viewed from the bar magnet. By RHR-2, place your thumb pointing against the magnetic field lines, which is toward the bar magnet. Hence, the induced current circulates so that its magnetic field lines through the loop are directed from the back to the front of the loop. By Lenz’s law, the direction of the induced current must be such that its own magnetic field is directed in a way to oppose the changing flux caused by the field of the approaching magnet. A current is therefore induced in the loop. As the north pole of the magnet moves toward the loop, the flux through the loop due to the field of the magnet increases because the strength of field lines directed from the front to the back of the loop is increasing. We designate the “front” of the closed conducting loop as the region containing the approaching bar magnet, and the “back” of the loop as the other region. Let’s apply Lenz’s law to the system of Figure 13.7(a). The direction (or polarity) of the induced emf can now drive a conventional current in this direction.Use right-hand rule 2 (RHR-2 see Magnetic Forces and Fields) to determine the direction of the induced current I that is responsible for the induced magnetic field B →.Therefore, the induced magnetic field adds or subtracts to the applied magnetic field, depending on the change in magnetic flux. The induced magnetic field tries to reinforce a magnetic flux that is decreasing or opposes a magnetic flux that is increasing. ![]() Now determine the direction of the induced magnetic field B →.Determine whether its magnetic flux is increasing or decreasing.Determine the direction of the applied magnetic field B →.Make a sketch of the situation for use in visualizing and recording directions.To use Lenz’s law to determine the directions of induced magnetic fields, currents, and emfs: This will be developed through examples that illustrate the following problem-solving strategy. Finally, you can apply Lenz’s law to determine the sense of ε ε. The magnitude of ε ε is given by ε = | d Φ m / d t |. To determine an induced emf ε ε, you first calculate the magnetic flux Φ m Φ m and then obtain d Φ m / d t. Electric potential energy would have been created, violating the conservation of energy. If it were not the case that the induced field opposes the change in the flux, the magnet would be pulled in and produce a current without any work. We pushed a magnet against a field and did work on the system, and that showed up as current. If the induced current causes a magnetic field opposing the increase in field of the magnet we pushed in, then the situation is clear. If pushing a magnet into a coil causes current, the energy in that current must have come from somewhere. Lenz’s law can also be considered in terms of conservation of energy. The direction of the induced emf drives current around a wire loop to always oppose the change in magnetic flux that causes the emf.
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