Electronic – Why does adding more coils make the electromagnet weaker

electromagneticelectromagnetism

I've built a couple of simple electromagnets, using a nail (not sure of the material) and copper wire connected to a 1.5V AA battery. I'm really confused because the electromagnets with less coils are noticeably more powerful than the electromagnets with far more coils and I wanted to understand why. Could this be because of the added resistance of the coils? What can I change in my set-up to get higher magnetic field with increased coils?

Best Answer

Yes, it’s increased resistance. Why? With a few simplifications, the principles are simple to understand.

  • The generated magnetic field is proportional to the current through the coils and to the number of coils (per unit length). B ~ n*I
  • The resistance of the electromagnet is proportional to the number of coils and to radius of the coils (technically to the length, but that’s just a 2pi constant). R ~ n*r
  • Being at steady state, the current through the electromagnet is inversely proportional to the total resistance. I ~ 1/R

So, if all of the turns are the same radius (which would be physically impossible) you would have: B ~ n/(n*r) ~ 1/r regardless of the number of turns.

But as the radius of the turns increase as you add more layers of wire, in reality you have: B ~ n/(n1*r1+n2*r2+n3*r3...) where r1 < r2 < r3... and n=n1+n2+n3... Which decreases as you add more layers because of the increasing radius.

The optimum number of turns will be achieved when the increase in coil resistance is exactly balanced by the increased voltage provided due to the reduction of load to the battery.

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