Saturday 8 September 2018

magnetic fields - Solenoid vs coil of wire


This may sound pretty basic, but:


I'm studying for my AP Physics exam, and I am confused.


Apparently, the formula for the magnetic field in the center of a loop of wire with $n$ turns is:



$B=\frac{\mu_0NI}{2R}$.


However, the formula for the magnetic field in the center of a solenoid of length $L$ with $n$ turns is $B=\frac{\mu_0NI}{L}$.


Those two formulas are clearly not the same, but I do not see how a loop of wire with many turns is different from a solenoid. Both consist of current-carrying wire tightly wrapped into many coils, right? (Well, I get that the loop of wire is not necessarily tightly wrapped, but I am not aware of anything that says it cannot be.)


What's going on?


Thanks.




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