# Electronic – Why the two coils don’t repel each other

coilelectromagnetism

I'm experimenting with two coils. I drive them with two MOSFETs and one consumes around 1A and the second ~0.7A (12V power supply able to deliver 15A). While the coils work with permanent magnets and repel and attract them as opposed, I observe no interaction when I bring them close together. I mean not even a hint of movement. Unfortunately I don't have a way to take a picture of these, the first one is around 1 cm diameter and 50 turns and the second around 2cm diameter and 50 turns. Is the power very limited? How should I estimate electromagnet force in Tesla units, and how to measure permanent magnet (small neodymium) magnetic force?

There will be some interaction but very small compared to using a permanent magnet and a coil. The magnetic field produced by the coils will be individually weak hence no apparent interaction but the magnetic field strength from a permanent magnet will be massive in comparison and this is enough to produce a noticeable effect.

Have you tried winding the coils around an iron core to increase the flux density? Here's a formula that should give you some general idea about the force from an electromagnet acting on a piece of magnetizable metal: -

Force = \$(N\cdot I)^2\cdot 4\pi 10^{-7}\cdot \dfrac{A}{2g^2}\$

• F = Force
• I = Current
• N = Number of turns
• g = Length of the gap between the solenoid and the magnetizable metal
• A = Area

Detail above taken from here