Re: Solenoid operation

R.Spinks wrote: 
I'm trying to understand basic solenoid operation a little better. I was
under the impression that if I applied DC to a coil (inductor) and the
inductor had a metal object (like a nail or something) say in it and
slightly out to one side the current through the inductor would create a
right hand rule flux in the inductor and the nail would move in the flux
direction. Then if I switched the polarity the nail would move the opposite
direction as the current direction would be opposite. I tried building a
little coil out of magnet wire and put a nail in it - then applied 14V - but
it didn't move (the nail did get a bit magnetic as I touched it w/another
nail). Is my understanding of the theory right or is there still something
missing (like does the nail have to be a permanent magnet or something
instead of just a 'ferrite')? Also -- I read on line something about the
coil always pulls to the center... but ... the right hand rule should apply
so .. I think it should have a direction not just to center of coil.



Generally, solenoids have a spring that pulls or pushes
the plunger away from the center of the coil:
                                  >
               COIL    PLUNGER SPRING
             )))))))))=========ssssssss

In the diagram above, the tension spring exerts a force
toward the right. When energized, the coil exerts a
larger force, toward the left:
             <<<<-----           -->
               COIL             SPRING
             )))))))))===== s s ssssss

Note that the plunger has begun to move inside the coil,
and the spring has stretched a bit. The arrow showing the
spring's rightward force is now stronger, but the
attraction of the coil on the plunger is strong enough
to overcome it.

             <<<---        --->>>
               COIL          SPRING
            =)))))))))= s s s s s s s s

Finally, the force exerted by the coil to move the plunger
toward the left equals the force exerted on the plunger
by the spring to move the plunger to the right, so the
plunger stops moving.  When you de-energize the coil, the
spring pulls the plunger back to the right.

If you had no spring and started with this:

               COIL    PLUNGER
             )))))))))=========

And then energized the coil, you would end up with this:

               COIL
            =)))))))))=

The plunger would move in to the coil and stop when the
magnetic center of the coil and the magnetic center of
the plunger lined up.  Unless there is a force to pull
the plunger out of the coil - another coil, air pressure,
a spring, whatever - the plunger will stay there when the
coil is de-energized.

Ed
.

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