# A magnet that won't stop spinning?

by ARAVIND113122
Tags: magnet, spinning, stop
 P: 54 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??
 P: 54 .......
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,756 By Jove! I think you've got something! Now, run with it!
 Mentor P: 22,316 A magnet that won't stop spinning? No, clearly this violates conservation of energy, so it won't work.
 P: 54 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".
 Mentor P: 22,316 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.
 P: 54 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.
 Mentor P: 22,316 Nothing will happen. It will not spin because there is no external force to spin it.
 P: 54 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?]
Mentor
P: 5,492
 Quote by ARAVIND113122 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.
 P: 54 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?
P: 2,751
 Quote by ARAVIND113122 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.
Mentor
P: 22,316
 Quote by ARAVIND113122 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.
Grab your left shoulder with your right hand.
Pull.

Do you start to spin?
 P: 54 oh yes!! i have realized my mistake! i had not thought about the opposite magnetic field.thank you very much
P: 1,103
 Quote by russ_watters Internal forces always sum to zero. Try this: Sit on a swivel chair. Grab your left shoulder with your right hand. 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?
Mentor
P: 5,492
 Quote by nitsuj 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?
Is this sarcasm? If so haha, if not it won't work anymore than reaching down and pulling your legs up will make you rise.

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