Induced emf By: Mr. Baughman Created: 28 February 2005

Remember Faraday's Law Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, a voltage will be generated. No matter how the change is produced, a voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

Definition: emf The energy per unit charge that is converted reversibly from chemical, mechanical, or other forms of energy into electrical energy in a battery or dynamo. The energy per unit charge that is converted reversibly from chemical, mechanical, or other forms of energy into electrical energy in a battery or dynamo. What is the SI unit for Energy/Charge? What is the SI unit for Energy/Charge? Volts Volts Electrifying

cos 

Calculating emf For induction the emf depends on four variables: For induction the emf depends on four variables: N – number of turns N – number of turns A – area A – area B – Magnetic field B – Magnetic field  - the angle between the magnetic field and the normal to the plane of the loop  - the angle between the magnetic field and the normal to the plane of the loop B B(cos  ) Normal 

Implications of Faraday We can use any changing magnetic field to produce electricity We can use any changing magnetic field to produce electricity When we change the direction of the magnetic field we also change the direction of the current When we change the direction of the magnetic field we also change the direction of the current So it is either positive (decreasing magnetic field) or negative (increasing magnetic field) So it is either positive (decreasing magnetic field) or negative (increasing magnetic field)

Lenz’s Law When an emf is generated by a change in magnetic flux according to F F F F F aaaa rrrr aaaa dddd aaaa yyyy '''' ssss L L L L aaaa wwww, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In these examples, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

AC/DC Changing the direction produces alternating current. Did you know? The band AC/DC got their name when Young's sister Margaret suggested something she'd seen on the back of a vacuum cleaner: 'AC/DC'. According to Angus, "It had something to do with electricity, so it seemed to fit...".

Generators By moving the loop instead of the field we can do the same thing By moving the loop instead of the field we can do the same thing In this orientation the loop passes more field lines in the beginning but as it rotates, the wire pass fewer field lines in a given time, therefore, lowing the emf to zero In this orientation the loop passes more field lines in the beginning but as it rotates, the wire pass fewer field lines in a given time, therefore, lowing the emf to zero Voltage Top view Side view

Calculating the emf For a rotating wire the emf depends on four variables For a rotating wire the emf depends on four variables N – number of turns N – number of turns A – area A – area B – Magnetic field B – Magnetic field  – angular frequency (  frequency in hertz)  – angular frequency (  frequency in hertz)

Alternating current This changing emf gives us a sinusoidal graph that is periodic (meaning is repeats) This changing emf gives us a sinusoidal graph that is periodic (meaning is repeats) Emf (V) Time (s) Side view

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