Naked Science Forum
Non Life Sciences => Technology => Topic started by: teragram on 02/08/2008 17:38:44
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I have a limited knowledge of magnetism, can someone help:-
Imagine a toroidal transformer having one of its windings energised by direct current to the point where its core became permanently magnetised (assume that the core material allowed this).
The magnetic field in the core would then resemble that around a current carrying conductor.
If placed in a magnetic field, would the core then experience the same force as does the current carrying conductor ie., a force at right angles to the direction of the magnetic field.
It seems to me that the answer cannot be yes, as this would imply a way round the rules that forbid the creation of energy.
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Hi teragram
I think this is the basic answer to your question.
Your model is not different, in essence, from having any piece of iron next to any wire. Consider this simpler, model first.
The actual field pattern around the wire would start off as concentric circles; none of the lines actually go through the wire. It would be modified by the permeability of the nearby steel but, still, the field would not be 'cutting' the wire - which is what you need for the motor effect.
Whether your metal is soft iron or has remnance to get permanently magnetised, the situation after the current has established itself would be for no force on the wire.
For a long solenoid the field tends to force the coils together (to shorten the coil) and, for a toroid, this will tend to shrink the coil. Adding iron to the core would just increase this force (permanent magnetisation doesn't alter the situation). In both these cases, the force is between the coils and not between coils and core.
However, with an iron / steel core, the actual magnetic energy is greater and this comes from the electrical energy. The self inductance of the coil is how this manifests itself - an increased 'back emf' is generated at switch on for the magnetic circuit with greater reluctance.
The force on the coils is not energy. Energy is only transferred if there is work done - i.e. motion. If the coil was a spring, it would return some of this energy as it opened again when the current was reduced else the energy goes into friction as the wire is distorted plastically.
What I am saying is that there is no real violation of Energy conservation. As in many of these problems, it may appear as if there is violation but there's always an explanation!
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Hi Sophie Centaur
Thanks for your reply. However, I think I may not have compiled my original question clearly enough. My example attempted to describe a toroidal transformer having its core (ferrite or some other material) permanently magnetised, the wires in the transformer thereafter playing no further part (take them off if you like). The magnetised core would be a ring magnet with the field lines following a circular path along the material of the ring, with no defined north and south poles (similar to an element in the old type computer magnetic memory matrix, but permanently magnetised). This circular magnetic field would then resemble the field around a straight conductor.
What I attempted to ask was, if this ring magnet is now placed in a permanent magnetic field, would it experience a force at right-angles to the lines of force in that field? In other words, not attracted to either pole, but trying to move parallel to each pole.
Perhaps your reply did answer this, and I’m too thick to understand it. If that is the case, or if I'm still not making sense, ignore me! In the meantime I'll work on trying to get a sketch into the message (I'm a computer duffer!)
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This circular magnetic field would then resemble the field around a straight conductor.
No, it wouldn't. It would be a single 'loop', where the field around a long wire is a set of coaxial cylinders - not 'a ring' like they show you in textbooks.
And, if you actually put the toroid into a magnetic field, if there were to be any external field (i.e. not an ideal toroid, which would have no external field) you would have to be doing WORK in putting it there. That's where your energy would be coming from.
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Hi Sophiecentaur
Thanks for your further reply.
That makes sense.
teragram