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Ch 29 Electromagnetic Induction

# Ch 29 Electromagnetic Induction - In 1831 the Law of...

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In 1831 the Law of Electromagnetic Induction was discovered by Michael Faraday and independently at the same time by Joseph Henry. Induction coil from 1800s used to demonstrate induction in physics classes dt d B Φ - = ε Understanding of this fundamental law of nature allows humankind to build generators. Simply stated this law is given by:

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dt d B Φ - = ε What Faraday discovered was that changing magnetic flux ( B · A ) can induce an electromotive force (or voltage difference) in a conductor. Another way of expressing electromagnetic induction is to say time-varying magnetic fields can act as a source of electric fields . Likewise time-varying electric fields can induce magnetic fields.
Experiment shows when a magnet is moved toward or away form a conductor (say a coil of wire), current flows in the coil of wire . We call this an induced current and the corresponding emf required to cause this current is called an induced emf .

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Current through a wire or coil produces a magnetic B field. Changing the B field by moving it closer or further away from a second coil produces a current in the second coil .
In conclusion, if we connect a coil of wire to a galvanometer , then place the coil between the poles of an electromagnet whose magnetic field we can vary , then the following can occur: 1. When there is no current in the electromagnet, so that B = 0, the galvanometer shows no current. 2. When the electromagnet is turned on, there is a momentary current through the meter as B increases.

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3. When B level’s off at a steady state, the current in the coil drops to zero. 4. With the coil in a horizontal plane, if the coil’s cross-sectional area is increased or decreased (say by squeezing or stretching it) the meter will detect current flowing during the deformation of the coil, not before or after . 5. If we rotate the coil a few degrees about the horizontal axis, the meter detects current during the rotation, and in the same direction as when we decreased the area of the coil. We the coil is rotated in the opposite way, the current flows in the opposite direction.
6. If we jerk the coil out of the magnetic field, there is a current during the motion, in the same direction as when the area of the coil is decreased. 7. If we decrease the number of turns in the coil by unwinding one or more turns, there is a current during the unwinding. If we wind more turns onto the coil, there is a current in the opposite direction (to the unwinding) during the winding. 8. When the magnet is turned off there is a momentary current in the direction opposite to that when the magnet was turned on.

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9. The faster we carry out any of these change, the greater the current. 10. If all these experiments are repeated with a coil that has the same shape but different material and different resistance, the current in each case in inversely proportional to the total circuit resistance.
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