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Lesson_4.3

# Lesson_4.3 - Electromagnetic Induction You have seen in the...

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Electromagnetic Induction You have seen in the last two lessons that a current carrying conductor in a magnetic field experiences a force and hence moves. An obvious question here is, can the reverse of this process occur? In other words, if a conductor placed in a magnetic field is moved by applying a force, will that make the free charges in the conductor move generating a current in the conductor. A current is cause by an emf. Can an emf be generated by moving a conductor in a magnetic field? Michael Faraday answered this question in 1830 by showing that an emf and hence a current can be induced in a coil of conducting wire by simply changing the magnetic field around it. Magnetic flux linked with a conductor. When a conducting wire is placed in a magnetic field, there will be a certain amount of magnetic flux ( φ B ) linked with it which is a measure of the number of lines of force crossing the conductor at right angles. When the conductor is moved inside the field, the flux linked with the conductor changes. It is this change in magnetic flux linked with the conductor that is responsible for the induced emf. http://micro.magnet.fsu.edu/electromag/java/faraday2/ The figure shows a magnetic filed at right angles to the plane of the paper and a conducting wire moved at right angles to it. This will cause an emf to be induce in the conducing wire. The magnitude of the induced emf can be increased by increasing the speed of motion of the conductor. Reversing the direction of motion will reverse the direction of induced current.

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If a rectangular coil of N turns is rotated in a magnetic field produced by a bar magnet as shown in the figure on the right, free charges (electrons) in the coil of wire will experience a force and will move causing a current to flow in the coil. Remember, the direction of the current is opposite to the direction of electron drift. For one complete rotation of the coil, the direction of the induced current will reverse. From our discussion on magnetic flux in lesson 5.1, you know that the magnetic flux φ m crossing an area A at right angles to the surface is φ B = B A If B makes an angle θ with the normal to the surface, the expression for the flux becomes φ B = B A Cos θ A coil of N turns of area A placed in a magnetic field B will have an amount of flux linked with it given by φ B = NB A Cos θ Faraday’s Law of Electromagnetic Induction Faraday investigated the factors that influence the magnitude of the induced emf when the magnetic flux linked with a coil of wire is allowed to change. He showed that the magnitude of the induced emf (E) is equal to the rate of change of magnetic flux. B d E dt φ = Lenz’s Law
http://www.launc.tased.edu.au/ONLINE/SCIENCES/physics/Lenz's.html The direction of the induced emf is given by Lenz’s law. The induced emf causes a current to flow in the conductor. This current will produce a magnetic filed which will oppose the changing magnetic field that is the cause of the induction. In other words, the direction of the induced emf is such as to oppose the cause of induction. Combining Lenz’s law with Faraday’s law we can

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