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Assume this kind of system, where a cylindrical permanent magnet, magnetized axially, sits over a flat coil (a pcb coil) with a given number of turns in which a costant current I is flowing. The aim of this system is to move the magnet upwards of a small amount. Is there a way to approximate the force the magnet is experiencing in function of the parameters above? I would like to design a simple actuator like this, I don't need precise dimensioning but I would like to have a rule-of-thumb to decide sizing of the parts without going through too much trial and error. Note that I omitted te "sizes" of the magnet: this is because I don't know what parameter to use. Some shops sell magnet by "kg of force", some by Gauss amount... they don't all make much sense to me.enter image description here

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  • $\begingroup$ How flows the current in the flat coil? How can there flow a current? $\endgroup$
    – user307025
    Jul 16 at 10:44
  • $\begingroup$ There is a source, which I did not draw. $\endgroup$
    – Vitomakes
    Jul 16 at 11:03
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An isolated permanent magnet experiences no net force or torque. Apparently, various internal forces compensate for the effects of its own field. In an external field each atomic dipole is subject to a torque which depends on the direction and magnitude of the external field, and each dipole is subject to a force which depend on the gradient of the external field. One approximation which might give a slightly simpler calculation would be to replace the magnet with a current carrying solenoid with the same size, shape, and dipole moment. Then that can be treated as a series of loops. If the magnet and the source of the external field are well separated, they can be approximated as a pair of dipoles.

If you just want to get some motion, buy or salvage a solenoid activated device. They control valves in your washing machine or switch large currents in your car. The current in a solenoid magnetized an iron core and then “sucks” it in. The force occurs mainly at the end of the solenoid where the field gradient is large. A spring may be used for reset.

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