# Why my two coils don't repel each other?

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?

• What do you drive them with? AC or DC? – winny Aug 6 '17 at 11:15
• @winny DC 12V. dd – John Am Aug 6 '17 at 11:23

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

• Thanks. Yes, I tried to place an iron bar inside both of them. I still haven't observed even a tiny bit of force between them. Does a permanent magnet create so much greater magnetic force? With the 1A coil I can attract the permanent magnet from 3 cm away. – John Am Aug 6 '17 at 11:13
• A permanent magnet will generate a massively greater field than the coils you have mentioned. Electromagnets in relays have thousands of turns. – Andy aka Aug 6 '17 at 11:15
• If you (as in the OP) have a relay lying around, take it apart and see how much force it generates compared to your coils... – Solar Mike Aug 6 '17 at 11:16
• How much area? Think again and work in metres not cm. Force can be measured by a spring balance but you need to define your experiment to get meaningful answers. – Andy aka Aug 6 '17 at 11:43
• @JohnAm To reason in a practical way about measurements in newtons think of this: the force of 1N is the force that you would perceive as weight if you held a mass of about 100g in your hand. – Lorenzo Donati Aug 6 '17 at 13:12