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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Comparing hysteresis loss in a core worked at 1 T with the same core at 1.5 T, the loss at the higher flux density is roughly
Answer: C
Hysteresis loss rises roughly as the flux density to the power 1.6, so a factor of 1.5 in flux gives about \( 1.5^{1.6} = 1.9 \) in loss. That steep dependence is why cores are worked well below saturation and why a small increase in supply voltage raises the no-load loss noticeably.
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A cyclotron works because the period of a charged particle's circular motion is
Answer: D
The radius grows in proportion to the speed but so does the circumference, so the time for one revolution depends only on the mass, charge and flux density. A fixed accelerating frequency therefore keeps step with the particle throughout, until relativistic mass increase breaks the condition at high energies.
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A velocity selector uses crossed electric and magnetic fields so that particles pass undeflected when
Answer: A
Balancing \( QE \) against \( QvB \) gives a speed of \( E/B \) that is independent of both charge and mass, so the arrangement selects by velocity alone. Combining it with a mass spectrometer stage then separates by mass unambiguously.
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The Hall voltage across a current-carrying strip in a transverse field arises because
Answer: B
Deflected carriers accumulate on one side, and the transverse electric field they build up grows until it just cancels the magnetic force on the remaining carriers. Its sign reveals whether the carriers are electrons or holes, which is how semiconductor type is determined.
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A moving coil instrument develops a torque proportional to the current because
Answer: C
The cylindrical pole faces and soft iron core produce a field that is radial everywhere the coil travels, so the angle between the conductors and the field stays at ninety degrees. The torque is then simply proportional to current, which is what makes the scale uniform.
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A conductor carrying current in a slot experiences a force that acts to
Answer: D
The flux crossing the slot is largely radial and the current axial, so the cross product gives a force in the circumferential direction, across the slot. It is taken by the slot walls and the wedge, and it is what drives the rotor round.
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Two coils carrying current attract or repel according to
Answer: A
Coaxial coils with currents in the same sense behave as two magnets with unlike poles facing and attract; reversing one reverses the force. This is the principle of the electrodynamometer movement and of the current balance formerly used to realise the ampere.
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The demagnetising field within a short bar magnet is larger than in a long one because
Answer: B
The field the poles produce within the material opposes the magnetisation, and bringing the poles closer strengthens it. A long thin magnet therefore holds its magnetisation better than a short stubby one of the same material, which is why permanent magnets were traditionally made long.
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Magnetic screening of a steady field by a conducting shell is
Answer: C
A steady field induces nothing, so a copper box does nothing against it, whereas an electrostatic field is screened completely. High permeability material is needed for a steady magnetic field, and conducting shells become useful only at higher frequencies.
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