This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Soft and Hard Magnetic Materials”.
1. The hysteresis loss in soft magnetic materials must be kept ______
Explanation: Soft magnetic materials are used in cases where regular reversal of magnetization path is required. The hysteresis losses in such materials must be kept to a minimum. This is due to a smaller hysteresis area.
2. The supermaloy primarily composed of ________________
Explanation: Permalloy is a magnetic material invented by the scientist Gustav Elmen. It is composed of just about 80% nickel and 5% molybdenum. It has a high relative permeability of 105.
3. Which of the following is an example of soft magnetic material?
Explanation: Permalloy is a soft magnet mainly used in electrical and electronic applications. It is composed of roughly 45% nickel. It has a high relative permeability of around 2700.
4. What is the relative permeability of iron?
Explanation: Relative permeability is the measure of the effective permeability of a material. Commercial iron has a relative permeability of 250. The values of relative permeability of Fe-Si, permalloy, and supermaloy are 1500, 2700, and 100000 in that order.
5. The heat treatment of alnico alloys at _____ results in phase separation.
Explanation: Some materials are heat treated in a magnetic field to enhance their properties. At just about 800oC, alnico undergoes a phase separation into two phases having different compositions and amount of magnetizations.
6. What is the hysteresis loss of permalloy?
Explanation: Hysteresis loss of soft magnetic materials is generally kept low due to the small area. This value resides at 120 J m-3 for permalloy and even lower at 21 J m-3 for supermaloy.
7. Which of the following is a property of a hard magnetic material?
a) Low hysteresis
b) Low eddy loss
c) Low coercive force
d) High residual induction
Explanation: Hard magnetic materials are used to produce permanent magnets. These permanent magnets have a high residual induction and large coercive force. Low hysteresis and Low eddy loss are properties of soft magnetic materials.
8. A _____ cooling rate solidifies metallic glass made from iron- base alloys.
a) 10oC s-1
b) 100oC s-1
c) 1000oC s-1
d) 10000oC s-1
Explanation: Metallic glasses produced from iron-base alloys are used to provide a reduction in core losses. When such an alloy is cooled at 104oC s-1, it solidifies into a ribbon-shaped metallic glass instead of crystallization.
9. Which of the following is not a use of magnesium-manganese ferrites?
a) Microwave isolator
d) Memory core of computer
Explanation: Magnesium-manganese ferrites having high resistivity are used as microwave insulators and gyrators in the KHz and MHz range. They are also used in memory cores of the computer when there is higher manganese to magnesium ratio.
10. Yttrium-iron-garnet is used as microwave isolators in the ____ range.
Explanation: Garnets such as Y3Fe5O12 (yttrium-iron-garnet)have a narrow resonance line width. These are popularly used as microwave isolators in the GHz range.
11. What is the coercive force of high carbon steel?
Explanation: Coercive force is defined as the ability of a material to withstand a magnetic field. It is denoted by Hc and must be larger for permanent magnets (hard magnetic material). High carbon steel has a coercive force of 3.98 kA m-1.
12. What is the value of residual induction of alnico alloys?
Explanation: Alnico is a common hard magnetic material containing aluminum, nickel, and cobalt (Al-Ni-Co). It has a residual induction (Br) of 0.8 to 1.2 Wb m-2 and a coercive force (Hc) of 60-12 kA m-1.
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