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Question 8: Explain eddy current in terms of Lenz’s law. Also by drawing suitable diagram show the direction of eddy current and the polarity produced in the sheet as a result of magnetic field.


Eddy Currents

Eddy currents are currents induced in metals moving in a magnetic field or metals that are exposed to a changing magnetic field. These are like eddies swirling in water. (ایډي معنا پهٔ پشتو کې د اوبو غُولۍ، پهٔ فارسۍ کې ګرداب او اُردو کې بهنور ده)

Eddy current is proportional to the magnetic field, area of the loop, rate of change of magnetic flux and is inversely proportional to the resistivity of the conductor.

Now current in a conductor produces its own magnetic field around it. Lenz’s law states the direction of the induced current (i-e, eddy current in this case) will be such that the magnetic field it produces will oppose the change of the magnetic field which created it (i-e, the magnetic field in which the metal is moving).

Consider a solid metallic cylinder rotating in a magnetic field as shown in the figure below.

B-field is from right to left. Cylinder is rotating clockwise.

Let ∆Ф is the change in the magnetic flux during time ∆t. The induced emf is then,

Similarly, if R is the resistance of the metallic cylinder, then the eddy current produced is given by

Polarity of eddy current and Lenz’s law

The direction of the eddy current is determined by the Lenz law.

According to Lenz’s law, the eddy currents must flow in a direction to oppose the cause which produces it. The cause is the changing magnetic flux. Therefore, the direction of the eddy current will be such that the polarity of the magnetic field produced by the eddy current is opposing the change in the magnetic flux which links the metal. If the changing magnetic flux linking the metal is increasing, the eddy current will have the direction to decrease this external increasing flux. This means the same pole of the magnet due to eddy current is facing the changing (increasing) flux. If the external magnetic flux is decreasing, the eddy current will flow in the direction such that the magnetic field produced is increasing the external flux which is decreasing now. This means the opposite pole of the magnet due to eddy current is facing the changing (now decreasing) flux.

Therefore, in the figure above, the magnetic force due to the eddy currents will be such to resist the rotation of the cylinder. (Because magnetic flux is changed due to the rotation of the cylinder). See the figure again. The cylinder is rotating clockwise. Therefore, the magnetic force due to the interaction of the two magnetic fields must try to move the cylinder in anti-clockwise direction. Therefore, the force must be out of the paper on the left and into the paper on the right as shown in the figure.(Fleming’s LHR). In order to produce such an effect the eddy current loops should move perpendicular to the plane of the external magnetic field in the directions shown in the figure.

Drawbacks of the eddy currents

Eddy currents have a major drawback of producing heat. The heat produced (I2Rt) is usually big enough to cause damage. Therefore, it is important to reduce eddy currents in such cases.

One method to reduce the unwanted effects of eddy currents is using a laminated conductor. A laminated substance is one that is built in thin sheets or layers that have been bonded together with some insulating adhesive (glue) or pressed under heat. The insulation between the sheets increases the resistance of the conductor and thus reducing the amount of current. The reduced amount of current then produces less heat loss.



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