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I am currently working on a project that aims to use the attraction & repulsion between two electromagnets as an alternative to weights, from a physiotherapy and rehabilitation perspective to help people build muscle and strength but who would not necessarily be able to lift actually weights. They would be available in 2 pairs in adjustable bands so that a user may have one positive band around an arm and a negative around their bicep to create an attraction, then doing repetition exercises to work certain muscles. Alternatively the user may have one positive band and the other positive band around each wrist to give repulsion for example.

Unfortunately I have a very limited knowledge of electromagnets and the research I have done I find overwhelming. A few answers to my questions would be massively appreciated or some direction into areas I could research would equally be helpful.

How large do electromagnets need to be to create a certain amount of attraction, is there a correlation or any possible tables that could help? would the electromagnets be able to fit into bands around the arm and have enough strength to be useful, the force wouldn't need to be huge but the larger the force the more appealing the final product would be as it would replace are large range of dumbbells.

How simple is it, if at all possible to change/control the strength or of an electromagnet, could the control over the electricity input effect it?

Thank you!

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Your challenge will be maintaining a constant force of attraction (or repulsion) while the person is moving against the force. With just one electromagnet, moving against the force requires a change of distance to the electromagnet and this change of distance will dramatically change the force.

So what you need is a torque motor. This is a special type of motor that is not meant to turn very much but to maintain a certain fixed rotational force (i.e., torque) regardless of the position or motion of its rotor. See https://en.m.wikipedia.org/wiki/Torque_motor

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The force on a magnetic moment (magnet) in a magnetic field depends on the magnetic field gradient. (How fast the field is changing in space.) So you don't really want a big field, but a big field gradient. The force from a single coil has a rather complicated dependence on position. Maybe I can sell you one of these?

http://teachspin.com/brochures/Magnetic%20Force.pdf

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