Choose the correct option for the following Material science assertion & reason questions.
- Both A and R are true and R is a correct explanation of A
- Both A and R are true but R is not a correct explanation of A
- A is true but R is false
- A is false but R is true
Q. (A): Storing of electrical energy in dielectric materials is analogous to stretching of a spring.
(R): The storage takes place by a shift in the relative positions of the internal positive and negative charges against the normal molecular and atomic forces.
Q. (A): Iron, cobalt and nickel are ferromagnetic.
(R): Spin of electrons in third band is responsible for ferromagnetism.
Q. (A): Diamagnetic effect is present in all the materials.
(R): The applied magnetic field alters the orbital motion of the constituent electrons of the materials.
Q. (A): Alnico (aluminium-nickel-iron-cobalt) alloys are the most important of hard magnetic materials.
(R): Alnico magnet alloys have the lowest energy per unit of volume of any permanent magnet materials available commercially.
Q. (A): Soft magnetic materials cannot be used for the construction of cores for the electrical machines.
(R): Soft magnetic materials have relatively small and narrow hysteresis loop.
Q. (A): Hard magnetic materials are used for making permanent magnets.
(R): Hard magnetic materials have relatively small and narrow hysteresis loop.
Q. (A): Hard magnetic materials are used for making cores of electrical machines.
(R): Hard magnetic materials after magnetization store a considerable amount of magnetic energy, and retain it for a long time.
Q. (A): Magnetic leakage is considered undesirable in electric machines.
(R): Power efficiency of electric machines is lowered due to magnetic leakage.
Q. (A): It is usually difficult to magnetise steel.
(R): Steel has high specific gravity.
Q. (A): Wrought iron and cast steel are most suitable for permanent magnets.
(R): Wrought iron and cast steel have high magnetic permeability and good retentivity.
Q. (A): Hard steel is most suitable for transformer core and armatures.
(R): Hard steel has high retentivity power and large coercive force.
Q. (A) Silicon alloys and permalloy are most suitable for core of electromagnets.
(R): Silicon alloys and permalloy have very high permeability and low hysteresis loss.
Q. (A): A laminated iron core has reduced iron losses.
(R): The laminations are insulated from each other and results in high resistance to eddy currents.