Quoted from jodini:
My daughter the other day was asking for help with her science (4th grader) and the question was, "Which is one way to increase the strength of an electromagnet?" I told her it was a trick question and to answer "C: Remove some coils of wire" Because what do we do when want to increase the power to a coil in the EM world....remove windings. I know, not too many....but I know it works. My daughter gets her paper back with a big RED check mark and says "WRONG"!
So can someone please explain to me "why"? It does make sense if I want to create a magnetic nail, that I would increase the windings, but in our EM world we remove?
A year later I know, but here you go.... there are two ways to increase the electromagnetic force.
1.) Increase the voltage which will increase the current going through the same resistance (Ohm's Law). Be aware that coil resistance will change as the wire heats up and will affect the current.
2.) Increase the number of turns (number of times the coil is wrapped around the core) up to a point where diminishing returns make it ineffective because the coil becomes to far aware from the core to add much to the field.
Removing windings (reducing the number of turns) will reduce the electromagnetic force. However, you are also reducing the resistance when you remove windings, so you are increasing the current (assuming voltage remains constant and you don't fry the coil). As a result, there is a trade off. There is the possibility that removing some of the turns that are the furthest from the core may have a small net benefit in force if the current increases more than the # turns are reduced. The may be why the "EM world" trick of removing windings could work up to a point before it starts to reduce the force.
Calculate the force by writing the equation:
F = (n x i)2 x magnetic constant x a / (2 x g2)
Where, F = force, i = current, g = length of the gap between the solenoid and a piece of metal, a = Area, n = number of turns in the solenoid, and the magnetic constant = 4 x PI x 10-7.
Analyze your electromagnet to determine its dimensions and the amount of current you will be running through it. For example, imagine you have a magnet with 1,000 turns and a cross-sectional area of 0.5 neters that you will operate with 10 amperes of current, 1.5 meters from a piece of metal. Therefore:
N = 1,000, I = 10, A = 0.5 meters, g = 1.5 m
Plug the numbers into the equation to compute the force that will act on the piece of metal.
Force = ((1,000 x 10)2 x 4 x pi x 10-7 x 0.5) / (2 x 1.52) = 14 Newtons (N).