Effects of lengths of solenoids

[aalnaif]

Hello,

In the design of electromagnets, why does the length of the solenoid matter? The magnetic field inside a solenoid, given by

$$B = \mu n I$$ ,

depends only on $$n$$, the number of turns per unit length, and not on the length itself. I know that the equation above applies only to the case where the solenoid is long enough that it can be approximated as an infinitely long solenoid, but what's the advantage of having a solenoid any longer than one that is "long enough"? Also, is there any rule of thumb for when a solenoid (containing an iron core) is long enough to be approximated as infinitely long?

 

[kuruman]

... but what's the advantage of having a solenoid any longer than one that is "long enough"?

The longer a solenoid relative to its diameter, the more uniform the field near its midpoint. I think that if the magnetic field is uniform enough over a working volume to be suitable for one's application, then the solenoid qualifies as "long enough" and does not need to be any longer. I don't know of a rule of thumb for a solenoid with an iron core.