MCQs on Engineering Materials

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01․ The core of a transformer for microwave frequency should be made of
silicon carbide.

Microwave frequency range is in terms of 500 KHz. So, there would be a chance to create more hysteresis losses and eddy current losses due to its high frequency (because frequency is directly proportional to both two losses). So, we need to choose a high permeability material to minimize the hysteresis losses. Hence, the laminated ferrite core is best material to for microwave frequency in order to reduce those losses because this material has high permeability in nature.

02․ A ferrite core has low eddy current loss than an iron core because
ferrites are magnetic.
ferrites have low permeability.
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ferrites have hig hysteresis.

We know that the high resistance cannot allow the current to flow through the circuit. Due to the high permeability property of ferrite core, it should be having high reluctance. This is reluctance is electrical analogous to resistance. Hence, the formation of eddy current losses in the core is reduced due to the high resistance of the ferrite soft iron core.

03․ Ferrites are a sub-group of
paramagnetic materials.
ferromagnetic materials.
non-magnetic materials.
ferri-magnetic materials.

Ferrites are a subgroup of ferromagnetic materials. For example, from the following some ferromagnetic materials such as iron, nickel, cobalt, iron alloys, nickel alloys, cobalt alloys, perm alloy, and several steels, the ferromagnetic materials such as magnetite, nickel-zinc ferrite, manganese-zinc ferrite, and copper-zinc ferrite are developed.

04․ A ferromagnetic core subjected to cycles of magnetisation will exhibit hysteresis when the cycle is
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If we apply a alternating cycle to the material, it generates alternating magnetic field. That alternating magnetic field would trace out a loop that is called a hysteresis loop. As the frequency of the alternating magnetic field increases, other losses caused by eddy currents and magnetic viscosity are added to hysteresis losses. At high frequencies the area of the hysteresis loop increases correspondingly. Hence, the cycles of magnetization exhibits the hysteresis when the cycle is alternating.

05․ In order to minimise loss due to hysteresis, the magnetic material should have
high resistivity.
low hysteresis coefficient.
large B-H loop area.
high retentivity.

The hysteresis coefficient is defined as the ratio of loss of energy per cycle to the field intensity or the intensity of electromagnetic stress in the material. Hence, the low hysteresis coefficient leads to reduce the hysteresis loss in that magnetic material.

06․ Piezoelectric effect is carried out in
composite filter.
crystal filter.
m derived.
constant k prototype.

In piezoelectric effect when a mechanical strain is applied to one face of a suitably cut face of a piezoelectric crystal, it causes an emf to be developed in opposite surface of that piece. Reverse is also true.

07․ Dielectric constant for vacuum is

Dielectric constant of vacuum is unity i.e 1.

08․ It is difficult to magnetize steel because it has
high permeability.
high permittivity.
low susceptibility.
low permeability.

The steel is very hard magnetic material. All the hard magnetic materials have low permeability. That means the magnetic lines of force do not easily distribute themselves readily through the steel material. Due to this, it reduces the ability to concentrate the magnetic flux in that magnetic material.

09․ Piezoelectric effect is carried out in
composite filter.
constant k prototype filter.
crystal filter.
M derived filter.

In piezoelectric effect when a mechanical strain is applied to one the face of a suitably cut face of a piezoelectric crystal, it causes an EMF to be developed in opposite surface of that piece. It usually happens in crystal filters.

10․ In electrical machines the material preferred for pole shoes of electromagnets is
pure iron.

For making an electromagnet, the material should not get the saturated soon. Pure iron (or) soft iron does not get saturated in short period of time. That is why pure iron is preferred for the pole shoes of an electromagnet.