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Lesson_4.3_Printable_PPT

# Lesson_4.3_Printable_PPT - Electromagnetic induction When a...

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Electromagnetic induction 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 made to move across the magnetic filed, the magnetic flux linked with the conductor changes. http://micro.magnet.fsu.edu/electro mag/java/faraday2/ This change in magnetic flux linked with the conductor induces an emf in the conductor. 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. For one complete rotation of the coil, the direction of the induced current will reverse.
If a magnetic field B crosses at right angles to an area A, the magnetic flux φ B crossing the area A 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 A Cos θ B 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

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Example 1 A 40 cm long solenoid has 600 turns and a radius of 2.5 cm. If it carries a current of 7.5 A, what is the magnetic flux through it? The number of turns per unit length n = 600 turns/0.4 m = 1500 turns per meter, I = 7.5 A, area of the coil A = π r 2 = π (0.025m) 2 = 0.00196 m 2 The filed inside the solenoid is B = μ o n I = 4 π × 10 -7 TmA -1 × 1500 turns per m × 7.5 A = 0.01414 T Magnetic flux through the solenoid is φ B = NBA = 600 × 0.014 T × 0.00196 m 2 = 0.0166 Wb
Faraday’s Law of Electromagnetic Induction Faraday’s investigation on electromagnetic induction showed that the magnitude of the induced emf (E) is equal to the rate of change of magnetic flux. B d E dt φ = 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 .

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Combining Lenz’s law with Faraday’s law we can write B d E dt φ = - If a coil of N turns is subjected to a changing magnetic flux, the induced emf will be B d E N dt φ = -
When the north pole of a magnet is moved towards a ring, the flux linked with the ring conductor increases. This induces a current in the ring which in turn produces a magnetic field. The induced B-field opposes the increasing B-field of the bar magnet which is the cause of induction.

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