Match the material class with its susceptibility.
1. Diamagnetic 2. Paramagnetic 3. Ferromagnetic
P. Small and positive Q. Small and negative R. Large and positive
Answer: C
The three classes differ by orders of magnitude as well as in sign, and each arises from a different mechanism: induced opposing moments, partial alignment of permanent moments, and cooperative alignment within domains.
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Match the loop feature with what it measures.
1. Loop area 2. Intercept on the flux axis 3. Intercept on the field axis
P. Coercivity Q. Energy lost per cycle R. Remanence
Answer: D
Correct answer: 1-Q, 2-R, 3-P
Each feature of the loop reports a different property. Its enclosed area is the energy dissipated per unit volume per cycle. Its intercept on the flux axis is the remanence, the flux left at zero field. And its intercept on the field axis is the coercivity, the reverse field needed to remove that flux. A transformer core wants a thin loop and a permanent magnet a fat one with large values of both intercepts.
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The steps of a magnetisation curve, taken from the unmagnetised state, are listed below. Put them in order.
1. Rotation of the magnetisation within domains 2. Reversible bending of domain walls 3. Approach to saturation 4. Irreversible domain wall motion
Answer: A
The easiest process comes first: small reversible bowing of the walls, then the irreversible jumps that give the steep part of the curve, then rotation against the crystal anisotropy, and finally the slow approach to saturation.
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Consider the following statements about magnetic materials.
1. A ferromagnetic material becomes paramagnetic above its Curie point 2. A ferrite has high resistivity 3. A diamagnetic material has relative permeability above unity
Answer: B
The first two are correct and both matter in practice. A diamagnetic material has a relative permeability just below one, since its susceptibility is small and negative, which is what makes it faintly repelled from a field.
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Consider the following statements about core loss.
1. Lamination reduces eddy current loss 2. Silicon raises the resistivity of the steel 3. Hysteresis loss rises as the square of the frequency
Answer: C
Lamination and silicon both attack the eddy current loss, one by restricting the current paths and the other by raising the resistance along them. Hysteresis loss rises with the first power of frequency; it is eddy current loss that goes as the square.
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Which of the following is NOT true of the Lorentz force?
Answer: D
The magnetic part is always perpendicular to the velocity, so it can never have a component along the direction of motion and can never change the speed. The other three statements are all correct descriptions of the force.
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Which change would NOT reduce the core loss of a transformer?
Answer: A
Loss rises steeply with flux density, roughly as its square for eddy currents and as a power near 1.6 for hysteresis, so working the core harder increases the loss. The other three all reduce it, which is why all three are used together in a modern core.
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A machine designer finds the iron loss of a core higher than predicted after assembly. A likely cause is
Answer: B
A burr from the punching operation can bridge several laminations and let eddy currents circulate over a much larger area than intended. Deburring and edge insulation are routine parts of core manufacture for exactly this reason.
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A permanent magnet is weakened by dropping it because the shock
Answer: C
Mechanical shock supplies energy that lets domains rotate into lower energy orientations, so the net moment falls. Modern rare earth magnets are far less susceptible than the older steel magnets, whose coercivity was low enough for the effect to be a practical nuisance.
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Comparing the electric and magnetic dipoles in a uniform field, both experience
Answer: D
The two cases are exactly parallel: a torque given by the cross product of the moment with the field, zero net force in a uniform field, and a net force toward stronger field only where the field varies. The analogy runs through the whole of dipole theory.
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