# Design of Electrical Machines Questions and Answers – Magnetic Materials

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Magnetic Materials”.

1. What is the property of magnetic materials?
a) Resistivity
b) Conductivity
c) Permeability
d) Ductility

Explanation: There are many properties of magnetic materials, and permeability is one among them. The other 3 properties are related to other materials like conducting and insulating materials.

2. What is the property of permeability in magnetic materials?
a) how easily the magnetic flux is broken/clear
b) how easily the magnetic flux is set up
c) how long the magnetic flux takes to form
d) how long the magnetic flux takes to clear

Explanation: The basic operation of magnetic material is to form magnetic flux. Permeability is the ability of the material to determine how easily the magnetic flux is set up.

3. What is the representation of permeability?
a) coercivity/retentivity
b) flux/flux density
c) magnetic force/magnetic flux density
d) magnetic flux density/magnetic force

Explanation: Permeability is the property which deals, with the relationship with magnetic flux density and magnetic force. Magnetic force/Magnetic flux density deals with the reciprocal of permeability. Coercivity/Retentivity deals with the terms of B-H curve.

4. How should the permeability and number of ampere turns for good magnetic materials be?
a) high permeability, high ampere turns
b) high permeability, low ampere turns
c) low permeability, low ampere turns
d) low permeability, high ampere turns

Explanation: High permeability is always required in magnetic materials for its good operation. At the same time high permeability leads to less ampere turns in the materials.

5. Is retentivity associated with B-H curve?
a) Yes
b) No

Explanation: B-H curve deals with the concepts of retentivity and coercivity. The property of retentivity can be shown in the B-H curve by an increasing curve in the curve.

6. What is the property of retentivity in magnetic materials?
a) After removal of external magnetic fields, magnetization exists
b) After removal of external magnetic fields, magnetization doesn’t exist
c) After removal of internal magnetic fields, magnetization exists
d) After removal of internal magnetic fields, magnetization doesn’t exist

Explanation: Magnetic materials have the property of retentivity in which the magnetic flux produced acts according to the external magnetic field. When the external field is removed, the magnetization in the materials doesn’t deform immediately.

7. What is coercivity force in magnetic materials?
a) The force required to add upon the existing magnetization
b) The force required to remove the existing magnetization
c) The force required to produce magnetic flux
d) The force required to break magnetic flux

Explanation: Magnetic materials generally have the property of retaining magnetization, even if the external magnetic field is removed. So, coercive force is the force that is required to reduce the magnetization.

8. What are magnetic hard materials?
a) High retentivity, low coercivity
b) High retentivity, high coercivity
c) Low retentivity, low coercivity
d) Low retentivity, high coercivity

Explanation: High retentivity is required for protecting the magnetic materials from losing its magnetic property. High coercivity is required to reduce the effect of retentivity to protect the material.

9. What is reluctance in magnetic materials?
a) Allows the buildup of magnetic flux
b) Reduces the buildup of magnetic flux
c) Resists the buildup of magnetic flux
d) Increases the buildup of magnetic flux

Explanation: Reluctance, as the name suggests, is something which is reluctant or hesitant to do. As per the magnetic terms it resists the building up of magnetic flux in the materials.

10. High Reluctance affects the performance of magnetic materials.
a) True
b) False

Explanation: High reluctance means the materials resist in building up the magnetic flux to a higher extent. So, for the proper functioning the reluctance values should be as low as possible.

11. What is the unit of reluctance in magnetic materials?
a) Henry/m
b) Weber/m2
c) Ampere-turns/Weber
d) Ampere-turns/m

Explanation: Henry/m deals with the unit of permeability. Weber/m2 deals with the unit of magnetic field. Reluctance is the opposite of permeance.

12. How many classifications of magnetic materials are present?
a) 3
b) 4
c) 5
d) 6

Explanation: There are basically 4 properties in magnetic materials and 5 classifications. They are diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic.

13. What is the property of ferromagnetic materials?
a) Negative magnetization
b) Magnetization slightly less than 1
c) Magnetization slightly greater than 1
d) Magnetization very much higher than 1

Explanation: Negative magnetization denotes the property of Diamagnetic materials. Magnetization slightly greater than 1 denotes the property of Paramagnetic materials. Ferromagnetic materials have magnetization in the range of 1000+.

14. What is the example of diamagnetic materials?
a) Quartz
b) Pyrite
c) Montmorillonite
d) Biotite

Explanation: The other 3 materials are paramagnetic in nature, which means magnetization is slightly above 1. Quartz is a diamagnetic material in which the magnetization is negative.

15. What is the example of ferromagnetic materials is?
a) Magnetite
b) Hematite
c) Nickel
d) Biotite

Explanation: Hematite denotes the example of antiferromagnetic materials. Nickel denotes an example of ferromagnetic materials. Biotite denotes the example of paramagnetic materials.

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