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why magnetic field near the end of a solenoid is less than magnetic field at the centre
Think about the magnetic field of a single loop of wire. It is maximum right in the middle and gets smaller as you go to either side of it along its axis. A solenoid is equivalent to a long string of loops placed next to each other. The net magnetic field is the sum from each individual loop and is maximum in the middle of the solenoid because that point minimizes the average distance to each loop. At the end of the solenoid the average distance to all the loops is greater, hence the magnetic field is less. When a solenoid is very long, the field at the end is close to one half that in the center.
(published on 08/26/07)
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