A current-carrying conductor is placed in the field of a permanent magnet and a force is observed. By Newton's third law the magnet
Answer: A
The interaction is mutual, so the magnet is pushed the other way with the same force, and in a motor that reaction is taken by the frame. Forgetting the reaction is a common slip in analysing the mechanics of a machine.
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A superconductor placed in a weak magnetic field
Answer: B
Surface currents flow without resistance and produce a field that exactly cancels the applied one inside, so the material behaves as a perfect diamagnet with a susceptibility of minus one. This Meissner effect is what allows a magnet to be levitated stably above a cooled superconductor.
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The magnetic moment of an electron arises from
Answer: C
Correct answer: its orbital motion and its intrinsic spin
Both contribute. The electron circulating in its orbital constitutes a current loop with a magnetic moment, and it also carries an intrinsic spin moment of about one Bohr magneton which has no classical analogue. In most ferromagnetic materials the spin contribution dominates, the orbital moment being largely quenched by the crystal field.
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A material is described as magnetically hard when it has
Answer: D
The term describes magnetic rather than mechanical behaviour, though the two happened to correlate in the steels where the usage began. Alnico, ferrite and the rare earth alloys are the modern hard materials, and a permanent magnet must be made of one.
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Annealing a cold-worked magnetic material after machining is done in order to
Answer: A
Dislocations and residual stress pin the domain walls, so a punched or bent lamination has a higher coercivity and a higher loss than the same material annealed. Final annealing after cutting is therefore a standard step in core manufacture.
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The magnetostriction of a magnetic material is
Answer: B
The strain is only a few parts per million but it reverses twice per cycle, and it is the source of the hum of a transformer. Reducing it is one of the objects of core material development and of the clamping arrangements used in a quiet transformer.
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The saturation flux density of ordinary transformer steel is about
Answer: C
Two teslas is the practical ceiling for iron and its alloys, and cores are worked at perhaps 1.5 to 1.7 teslas to stay below the sharp rise in magnetising current. No material available in bulk greatly exceeds it, which sets a hard limit on the compactness of every machine.
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A rotating machine cannot use grain-oriented steel to full advantage because
Answer: D
The flux in a stator tooth runs radially and in the back iron circumferentially, so no single rolling direction suits the whole lamination. Non-oriented steel, with more nearly equal properties in all directions in the plane, is used instead.
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Two identical bar magnets are placed end to end with like poles facing. They
Answer: A
Like poles repel exactly as like charges do, and the force falls rapidly with separation because each magnet is a dipole. Reversing one of them turns the repulsion into an attraction of the same magnitude at the same spacing.
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A soft iron keeper is placed across the poles of a horseshoe magnet in storage to
Answer: B
An open magnetic circuit subjects the magnet to its own demagnetising field, which slowly reduces the magnetisation. Closing the circuit with soft iron removes that field, which is why keepers are supplied with laboratory magnets and why a magnet is stored with its poles bridged.
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