# A magnet that won't stop spinning?

A magnet that won't stop spinning??

what happens if a horseshoe magnet is fixed to a a coil such that there is a constant field through the coil?? The initial torque due to the field will spin the coil,but since the magnet is connected to the coil,it will keep sending a field through the coil at the same angle,further spinning the coil and then the process continues.......the coil-magnet apparatus will keep on spinning. Isn't this possible??

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SteamKing
Staff Emeritus
Homework Helper

By Jove! I think you've got something! Now, run with it!

russ_watters
Mentor

No, clearly this violates conservation of energy, so it won't work.

Why would it violate the law of conservation of energy?The magnet is the source of energy.We are simply using the energy provided by the magnet.No energy is being "magically produced".

russ_watters
Mentor

No, nagnets do not expend energy by exerting a static force. A magnet is no more a source of continuous power than a spring.

Perhaps if you drew yourself a picture you'd see that what you are suggesting is like lifting yourself off the ground by pulling on your own shoelaces.

But where does the magnet-coil model fail?What would actually happen if the set up is made and placed in a zero gravity environment.As long as the the magnet exerts force,the coil would spin.

russ_watters
Mentor

Nothing will happen. It will not spin because there is no external force to spin it.

Hmm..but if the magnet were to be detached for a split second,it would spin.That brings me to a more fundamental question-why is there such a big difference between internal and external forces in this case?Why doesn't the internal force in the attached apparatus do anything?After all,it is a field through a magnet[is it because the coil applies an equal and opposite force?]

Ryan_m_b
Staff Emeritus

Hmm..but if the magnet were to be detached for a split second,it would spin.That brings me to a more fundamental question-why is there such a big difference between internal and external forces in this case?Why doesn't the internal force in the attached apparatus do anything?After all,it is a field through a magnet[is it because the coil applies an equal and opposite force?]
What do you mean "spin"? What you are trying to describe just wont work. You are trying to create perpetual motion.

By spin ,i mean that the coil will rotate so as to minimize magnetic flux through it.
Then again,why wouldn't the magnet-coil apparatus spin-is it because an equal force would be exerted by the coil,thus cancelling out torque due to the magnet?

uart

By spin ,i mean that the coil will rotate so as to minimize magnetic flux through it.
Then again,why wouldn't the magnet-coil apparatus spin-is it because an equal force would be exerted by the coil,thus cancelling out torque due to the magnet?
Yes. If the coil is correctly orientated within the magnet then there is certainly a torque ,due to the current, acting on the coil. For this same orientation and current direction however, the coil itself will induce it's own magnetic poles, which are perpendicular to the main magnet poles and with a N-S polarity directed so as to produce a counter torque on the main magnet.

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russ_watters
Mentor
That brings me to a more fundamental question-why is there such a big difference between internal and external forces in this case?Why doesn't the internal force in the attached apparatus do anything?
Internal forces always sum to zero. Try this:

Sit on a swivel chair.
Pull.

Do you start to spin?

oh yes!! i have realized my mistake! i had not thought about the opposite magnetic field.thank you very much

Internal forces always sum to zero. Try this:

Sit on a swivel chair.
Pull.

Do you start to spin?
I've been sitting in my office chair trying this for a while now and can't seem to do it.

Am I doing it wrong? Do I pull or push my shoulder?

Ryan_m_b
Staff Emeritus