Active Figure 31.26 Eddy Currents In the animation below, a flat conducting plat

Question

Active Figure 31.26 Eddy Currents In the animation below, a flat conducting plate attached to a rigid bar swings back and forth through a magnetic field. As the plate enters the field, the changing magnetic flux through the plate induces voltages within the plate, and this causes charge carriers within the plate to move. The resulting currents resemble eddy's in a flowing stream, and thus are called eddy currents. The eddy currents create an induced magnetic field. Since, by Lenz's law, induced currents always oppose the change in flux, the magnetic force that develops retards the motion of the plate. Thus, the eddy currents cause a breaking effect that slows the motion of the plate. If slots are cut in the plate, then the overall strength of the eddy currents are reduced. Instructions: Use the slider to adjust the magnetic field magnitude. Click "start", and observe the eddy currents induced in the plate. Click the "prongs" button to give a plate with slots. Explore Eddy currents are induced currents set up in a conductor when it moves through a non-uniform magnetic field. In the animation, the magnetic field is non-uniform at the edges of the field region. The field region is defined by the blue x's that indicate that the magnetic field points into the page. Consider an instant where the plate is moving from left to right, as diagrammed in the figure to the below. At position 1, the pendulum is moving from a region where there is no magnetic field into a region where the field B_in is directed into the page. In this case, the change in flux is: an increasing flux caused by magnetic field lines pointing into the page. To oppose this change, current must flow in the conductor to keep this inward flux from increasing. Thus, the induced current must induce a magnetic field that points outward. Right-Hand Rule #2 indicates that the induced current at position 1 is counter-clockwise. This induced magnetic field is of the opposite polarity as B_in, and the resulting magnetic force F_m is a repulsive force that tends to push the plate out of the field region. At position 2, the pendulum is moving from a region where the field B_in is directed into the page into a region where there is no field. In this case, the change in flux is: a decreasing flux caused by magnetic field lines pointing into the page. To oppose this change, current must flow in the conductor to keep this inward flux from decreasing. Thus, the induced current must induce a magnetic field that points inward. Right-Hand Rule #2 indicates that the induced current at position 1 is clockwise. This induced magnetic field is of the same polarity as B_in, and the resulting magnetic force F_m is an attractive force that tends to push the plate back into the field region. In both cases, the magnetic force opposes the plate's motion, and this results in a breaking force that slows the motion of the plate. Clicking the "prongs" button causes slots to be cut into the plate that reduce the overall strength of the eddy currents, thus decreasing their breaking effect. Exercise 31.AF.26 Match each item on the left with the most appropriate item on the right. Magnetic force points to the right Magnetic force is consistently small Magnetic flux decreases Magnetic force points to the left Magnetic flux increases

Explanation / Answer

if the magnetic force to the right,
(plate swings from right to the left) becouse the force is directed oppese to the motion.

magnetic flux is consistently (small means plate has slots)

if the magnetic flux decreases (the plate is leaving the magnetic field rigion)as it reduses the area of magnetic flux

if the magnetic force pointed to the left means (plate swings from left to right) becouse the force is directed oppose to the motion.

magnetic flux increases when (plate is entering the magnetic field rigion) becouse the area increases.

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