Question

The ferromagnetic core of electromagnets should have
A. A broad hysteresis loop
B. High permeability and high retentivity
C. Low permeability and low retentivity
D. Low permeability and high retentivity
E. High permeability and low retentivity

Answer 1

Hint We can think of this question as if we were to make an electromagnet and we are looking for suitable properties. We know electromagnet magnetises on flow of current and demagnetics as soon as current is switched off. Therefore, we need a material which magnetises and demagnetizes quickly.

 Complete step-by-step solution:
There are two properties which describe the nature of the material we use for making an electromagnet. These are retentivity and permeability.
We can see that the word retentivity itself means the ability of a substance to retain. Here we are concerned with the retention of magnetic behavior or in other words the strength of the magnetic field that remains in a sample after removal of the current coil.
We want our electromagnet to stop showing magnetic behavior as soon as current is switched off that means we want zero retention. So, the retentivity of ferromagnetic substances used must be low.
The second property is permeability. Permeability as the word suggests is the quality of being permeable. Here it means the relative increase or decrease in the resultant magnetic field inside a material as compared to the magnetizing field. We want materials to behave as a magnet as soon as it comes under the influence of a magnetic field generated by a current carrying coil. We can also say that we want our material to have high permeability.

So, option (E) is correct.

Note:- Deep conceptual knowledge and clarity of definitions of associated terms is required to link concepts to real life applications for better understanding.