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Q & A: magnetic field in solenoid

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Most recent answer: 10/22/2007
Q:
why is B non-zero inside a solenoid?
- Samia (age 16)
PAKISTAN
A:
Samia-

I'm not sure at what depth you want this answered, so I'll guess a sort of mid-level.

The magnetic B field near a current-carrying wire circles around the wire. The B fields from many wires just add together, but generally you have to be carefully to add them like vectors, remembering which way they point.

Say that our solenoid loops around some vertical axis. Look at some point in the middle of the solenoid. The B fields which loop around the wires in all horizontal directions from the middle all point the same way in the middle of the solenoid- either up or down. If you look at the fields from other parts of the wires, above or below the point we're interested in, they also have inward or outward components. However, when you add up the fields from all the wires around the solenoid, those in-out components all cancel, just leaving the upward or downward part.

You might find it easiest to picture this by making a few rings of wire to represent the solenoid. Then make some paper rings with arrows around them. You can see that if the arrows go the same way around the wires, the ones in the center all add up, either upwards or downwards.

Mike W.

(published on 10/22/2007)

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