A thin aluminum ring jumped high up to the ceiling when an upward magnetic field was suddenly introduced to it. Describe why this is so.
Magnetic field will produce an electric field in the loop of conducting material if the magnetic field flux through the loop changes in time. The greater the rate of change of magnetic field flux, the larger is the induced electric field. The direction of the field obeys Lenz Law: the induced electric field will be directed in such a way so that the current due to the electric field will produce a magnetic field that opposes the change in the original magnetic field flux.
Answer and Explanation:
An upward magnetic field suddenly switched on will induce an electric field in the aluminum ring, according to the Faraday's Law. The induced electric field will have a direction such that the current produced by that field will produce its own magnetic field that will oppose the change in magnetic field flux through the ring. This means that since the original flux increased through the ring (from zero before the field is turned on to some finite value when it is on), the induced current will produce magnetic field directed downwards, to oppose the increase in the flux through the ring. This means the magnetic field of the ring will make it a magnet with northern pole facing downward, whereas the original field can be viewed as a magnet with its northern pole facing upward. Since two like poles of two magnets repel each other, the ring will feel repulsive force acting upward and will jump up if the force is strong enough to overcome the force of gravity on the ring.
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from GACE Physics (530): Practice & Study GuideChapter 23 / Lesson 9