Correct Answer - Option 1 : It has high permeability and low retentivity.
CONCEPT:
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Electromagnets are materials that behave as magnets when a current-carrying wire is wound on it.
- Electromagnets generally lose their magnetic property once the current flow through the wire is stopped.
- They also acquire magnetic property quickly, as soon as the current starts flowing without any time lag.
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Retentivity is the ability to retain a magnetic field after the removal of the externally applied magnetic field.
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Magnetic susceptibility (χm): It is the property of the substance which shows how easily a substance can be magnetized.
It is defined as the ratio of the intensity of magnetization (M) in a substance to the magnetic intensity (H) applied to the substance, i.e.
\(\chi = \frac{M}{H}\)
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Magnetic permeability (μ): It is the degree or extent to which magnetic lines of force can enter a substance.
The relation between permeability and susceptibility:
B = Bo + Bm
B = μ0H + μ0M = μ0H (1 + χ)
μr = 1 + χ
Where B in a material is the sum of magnetic flux density in vacuum Bo produced by magnetizing force and magnetic flux density due to magnetization of the material Bm
EXPLANATION:
- Since electromagnetic materials acquire magnetic property quickly, they have to be highly susceptible to magnetism.
- Also, as soon as the current flow ceases, the material stops behaving as an electromagnet. This indicates that the material has a low capacity to retain the magnetic properties and hence, it must have low retentivity.
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Soft iron is known to have low retentivity and high susceptibility to magnetism, as there is a direct relation between permeability and susceptibility, so it will have high permeability.
Therefore electromagnets are made using soft iron.
