Magnetic Materials MCQ Quiz - Objective Question with Answer for Magnetic Materials - Download Free PDF

Explanation:

Diamagnetic Materials

Diamagnetic materials are those that create an opposing magnetic field when exposed to an external magnetic field. This phenomenon causes them to be weakly repelled by the magnetic field. Diamagnetism is a fundamental property of all materials and arises from the orbiting electrons creating tiny current loops, which produce magnetic fields that oppose the applied field.

Characteristics:

• Diamagnetic materials are repelled by magnetic fields.
• The induced magnetic field within diamagnetic materials opposes the external magnetic field.
• They exhibit negative magnetic susceptibility, meaning their magnetization is in the opposite direction to the applied field.

Examples: Some common examples of diamagnetic materials include bismuth, copper, gold, silicon, and water.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: They are strongly magnetized in the same direction as the applied magnetic field.

This description is characteristic of ferromagnetic materials, not diamagnetic ones. Ferromagnetic materials, such as iron, cobalt, and nickel, become strongly magnetized in the presence of a magnetic field and the magnetization aligns in the same direction as the applied field.

Option 2: They exhibit no effect in a magnetic field.

This option is incorrect as diamagnetic materials do exhibit an effect when placed in a magnetic field. They generate an induced magnetic field that opposes the external magnetic field, leading to weak repulsion.

Option 4: They exhibit a strong attraction to the magnetic field.

This description is characteristic of paramagnetic materials, not diamagnetic ones. Paramagnetic materials are weakly attracted to magnetic fields and align their magnetic dipoles in the same direction as the applied field.

Conclusion:

Understanding the behavior of different types of magnetic materials is crucial for accurately identifying their characteristics. Diamagnetic materials, as explained, exhibit weak repulsion from an applied magnetic field due to the induced magnetic field opposing the external field. This property distinguishes them from ferromagnetic and paramagnetic materials, which are either strongly attracted to or weakly attracted to magnetic fields, respectively.

Concept:

In a magnetic circuit, the magnetomotive force (MMF) is the force that drives magnetic flux through the circuit, similar to how voltage drives current in an electrical circuit.

The relationship is given by:

\( \text{MMF} = \Phi \times \mathcal{R} \)

Where:

• \(\Phi\) = Magnetic flux (in Webers)
• \(\mathcal{R}\) = Reluctance (in AT/Wb)
• MMF = Magnetomotive Force (in Ampere-Turns or AT)

Calculation:

Then, the MMF required is,

\( \text{MMF} = \Phi \times \mathcal{R} = 2 \times 5 = 10~\text{AT} \)

Explanation:

Manganin Properties and Applications

Definition: Manganin is an alloy composed mainly of copper (approximately 84%), manganese (approximately 12%), and nickel (approximately 4%). This alloy is primarily used in the manufacture of precision resistors, strain gauges, and other temperature-sensitive applications.

Correct Option Analysis: The correct option is option 3: Low temperature coefficient of resistance.

The low temperature coefficient of resistance is the most significant property that makes manganin particularly useful in precision resistors and temperature-sensitive applications. The temperature coefficient of resistance (TCR) is a measure of how the resistance of a material changes with temperature. A low TCR means that the resistance of the material remains relatively stable over a range of temperatures, which is crucial for applications that require high precision and stability.

Why Low Temperature Coefficient of Resistance is Important:

• Precision Resistors: In precision resistors, it is essential that the resistance remains consistent regardless of temperature changes. Any fluctuation in resistance due to temperature variations can lead to inaccuracies in measurements and signals. Manganin's low TCR ensures that the resistors maintain their specified resistance value over a wide temperature range, providing reliable and accurate performance.
• Temperature-Sensitive Applications: In applications where temperature changes are expected or unavoidable, materials with low TCR are preferred. Manganin's low TCR minimizes the effect of temperature changes on the resistance, making it ideal for use in environments where temperature stability is critical. This property is particularly beneficial in strain gauges and other sensors where precise measurements are required.

Other Properties and Options Analysis:

• High tensile strength and ductility (Option 1): While high tensile strength and ductility are valuable properties for materials used in various engineering applications, they are not the primary reasons for using manganin in precision resistors and temperature-sensitive applications. These properties do not directly influence the stability of resistance with temperature changes.
• High electrical conductivity (Option 2): High electrical conductivity is a desirable property for materials used in electrical wiring and components where efficient conduction of electricity is required. However, for precision resistors, stability of resistance with temperature is more critical than high conductivity. Manganin does not have particularly high electrical conductivity compared to other materials like copper or silver, but its low TCR makes it suitable for precision resistors.
• High thermal coefficient of resistance (Option 4): A high thermal coefficient of resistance would mean that the resistance of the material changes significantly with temperature. This is the opposite of what is desired in precision resistors and temperature-sensitive applications. Manganin is chosen specifically because it has a low TCR, ensuring that resistance remains stable with temperature changes.

Conclusion:

Manganin's low temperature coefficient of resistance is the key property that makes it invaluable in the manufacture of precision resistors and temperature-sensitive applications. This property ensures that the resistance remains stable over a range of temperatures, providing accurate and reliable performance. While other properties like tensile strength, ductility, and electrical conductivity are important for various applications, they do not provide the same level of stability and precision required in these specific uses.

Explanation:

Ferromagnetic Materials

Ferromagnetic materials are substances that exhibit strong magnetic properties. These materials have unpaired electrons, which cause their atoms to act like tiny magnets. The key feature of ferromagnetic materials is that they can retain their magnetization even after the external magnetic field is removed, a property known as hysteresis.

Examples: Common examples of ferromagnetic materials include iron, cobalt, nickel, and their alloys. These materials are widely used in various applications due to their strong magnetic properties.

Hysteresis: One of the distinctive characteristics of ferromagnetic materials is hysteresis. When the material is exposed to a varying magnetic field, the magnetization of the material does not follow the applied field precisely. Instead, it exhibits a lag, creating a hysteresis loop. This property is crucial for applications like magnetic storage devices, transformers, and electric motors.

Analysis of Other Options:

Option 2: "They are weakly repelled by a magnetic field and have negative magnetic susceptibility." This statement is true for diamagnetic materials, not ferromagnetic materials. Diamagnetic materials develop a weak magnetic moment in opposition to an applied magnetic field, causing them to be repelled.

Option 3: "They show no effect in a magnetic field and are considered neutral." This statement is true for materials that are neither paramagnetic nor diamagnetic, such as some non-magnetic materials. Ferromagnetic materials, however, show a strong attraction to magnetic fields.

Option 4: "They are strongly attracted to a magnetic field but do not exhibit hysteresis." This statement is true for paramagnetic materials, which are attracted to magnetic fields but do not retain their magnetization once the external field is removed. Ferromagnetic materials, in contrast, do exhibit hysteresis.

In conclusion, the correct option is Option 1: "They retain their magnetisation even after the external magnetic field is removed." This is the defining characteristic of ferromagnetic materials, distinguishing them from other types of magnetic materials.

Explanation:

In the study of magnetic materials, particularly when dealing with the phenomenon of eddy current loss, it's important to understand the factors that influence this type of loss. Eddy current loss occurs when alternating magnetic fields induce circulating currents within the material, leading to energy dissipation in the form of heat. This type of loss is significant in the design and application of transformers, electric motors, and other electromagnetic devices.

The given statement asks which of the following factors is directly proportional to the eddy current loss in magnetic materials:

• 1) Magnetic field frequency
• 2) Thickness of the material
• 3) Temperature of the material
• 4) Permeability of the material

The correct answer is option 2, "Thickness of the material". Let's delve into the detailed solution and explanation of why this is the case.

Correct Option Analysis:

Thickness of the Material:

Eddy current loss in magnetic materials is significantly influenced by the thickness of the material. The relationship is directly proportional, meaning that as the thickness of the material increases, the eddy current loss also increases. This can be understood by examining the nature of eddy currents and their formation within conductive materials subjected to changing magnetic fields.

When an alternating magnetic field is applied to a conductive material, it induces circulating currents (eddy currents) within the material. These currents flow in loops perpendicular to the magnetic field and create their own magnetic fields, which oppose the original magnetic field. The magnitude of these eddy currents is dependent on several factors, including the material's thickness. The thicker the material, the larger the loops of circulating currents, and consequently, the greater the energy loss due to these currents. This loss manifests as heat within the material, leading to inefficiencies.

Mathematically, eddy current loss (P_e) can be expressed as:

P_e = K * B² * f² * t²

Where:

• K is a constant that depends on the material properties.
• B is the magnetic flux density.
• f is the frequency of the alternating magnetic field.
• t is the thickness of the material.

From this equation, it is clear that eddy current loss is directly proportional to the square of the thickness (t²). Therefore, increasing the thickness of the material results in a quadratic increase in eddy current loss.

This principle is particularly important in the design of transformers and electric motors, where minimizing eddy current loss is crucial for efficient operation. Engineers often use laminated cores made of thin sheets of magnetic material, insulated from each other, to reduce the effective thickness and thereby minimize eddy current loss.

Analysis of Other Options:

1) Magnetic Field Frequency:

While the frequency of the magnetic field does affect eddy current loss, it is not directly proportional. Eddy current loss is proportional to the square of the frequency (f²). As the frequency increases, the rate of change of the magnetic field increases, which induces stronger eddy currents. However, the relationship is not directly proportional but rather quadratic.

3) Temperature of the Material:

Temperature can influence the resistivity of the material, which in turn can affect the magnitude of eddy currents. However, the relationship between temperature and eddy current loss is not direct. Instead, it is mediated by changes in material properties like resistivity. Generally, as temperature increases, resistivity increases, which can reduce the eddy currents and thus the loss, but this effect is not linear or directly proportional.

4) Permeability of the Material:

The permeability of the material affects the magnetic flux density within the material. While a higher permeability can lead to higher magnetic flux and stronger induced eddy currents, the relationship is more complex and not directly proportional. Eddy current loss depends on the square of the magnetic flux density, which is influenced by permeability, but this is not a straightforward direct proportionality.

In conclusion, the thickness of the material is the factor that has a directly proportional relationship with eddy current loss in magnetic materials. This understanding is essential for designing efficient electromagnetic devices, where minimizing energy loss is crucial for optimal performance.

Soft magnetic materials have a small area of the hysteresis loop.

Hysteresis Loop (B.H Curve):

• Consider a completely demagnetized ferromagnetic material (i.e. B = H = 0)
• It will be subjected to the increasing value of magnetic field strength (H) and the corresponding flux density (B) measured the result is shown in the below figure by the curve O-a-b.
• At point b, if the field intensity (H) is increased further the flux density (B’) will not increase anymore, this is called saturation b-y is called solution flux density.
• Now if field intensity (H) is decreased, the flux density (B) will follow the curve b-c. When field intensity (H) is reduced to zero, flux remains the iron this is called remanent flux density or remanence, it is shown in fig. O-C.
• Now if the H increased in the opposite direction the flux density decreases until the point d here the flux density (B) is zero.
• The magnetic field strength (points between O and d) require to remove the residual magnetism i.e. reduce B to zero called a coercive force.
• Now if H is increased further in the reverse direction causes the flux density to increase in the reverse direction all the saturation point e.
• If H is varied backwords OX to O-Y, the flux Density (B) follows the curve b-c-d-d.
• From the figure the clear that flux density changes ‘log behind the changes in the magnetic field strength this effect is called hysteresis.
• The closed figure b-c-d-e-f-g-b is called the hysteresis loop.

• The energy loss associated with hysteresis is proportional to the area of the hysteresis loop.
• The area of the hysteresis loop varies with the type of material.
• For hard material: hysteresis loop area large → hysteresis loss also more → high remanence (O-C) and large coercivity (O-d).
• For soft material: hysteresis loop area small → hysteresis loss less → large remanence and small coercivity.

Note:

The difference between soft magnetic materials & hard magnetic materials is as shown:

The schematic arrangements of spins of four different types of magnetic materials are as follows:

The correct answer is option 3):(Diamagnetic)

Concept:

• Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field and are repelled by the applied magnetic field. The opposite behaviour is exhibited by paramagnetic materials. 

Properties:

• These substances are repelled by a magnet Atomic orbitals of these substances are completely filled
• It develops weak magnetization in a direction opposite to the direction of the applied magnetic field As soon as the magnetizing field is removed,
• it loses its magnetization When placed in a non-uniform magnetic field, it tends to move from stronger to weaker regions of the magnetic field
• When placed in a uniform magnetic field, it aligns itself perpendicular to the direction of the magnetic field Magnetic susceptibility is a small negative value Relative permeability is close to one and always less than 1
• Magnetic permeability is slightly less compared to free space

Additional Information

• Ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field when a magnetic field is applied to it.
• In antiferromagnetic materials, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighbouring spins pointing in opposite directions. 
• Ferrimagnetic materials display a weak form of ferromagnetism associated with parallel but the opposite alignment of neighbouring atoms. 

Magnetic permeability: 

• It is the relative increase or decrease in the resultant magnetic field inside a material compared with the magnetizing field in which the given material is located.
• It is the property of a material that is equal to the magnetic flux density B established within the material by a magnetizing field divided by the magnetic field strength H of the magnetizing field. Magnetic permeability is thus defined as μ = B/H.
• A diamagnetic material has a constant relative permeability slightly less than 1. When a diamagnetic material such as bismuth is placed in a magnetic field, the external field is partly expelled and the magnetic flux density within it is slightly reduced.
• A paramagnetic material has a constant relative permeability slightly more than 1. When a paramagnetic material such as platinum is placed in a magnetic field, it becomes slightly magnetized in the direction of the external field.
• A ferromagnetic material such as iron does not have a constant relative permeability. The permeability of ferromagnetic materials varies greatly with field strength. As the magnetizing field increases, the relative permeability increases, reaches a maximum, and then decreases.
• In free space the magnetic flux density is the same as the magnetizing field because there is no matter to modify the field. The permeability B/H of space is dimensionless and has a value of 1.

Note:

Paramagnetic Substances: The substances which are weekly magnetized when placed in an external magnetic field, in the same direction of the applied field are called paramagnetic substances.

Example: Sodium, aluminium, calcium, manganese, platinum

Properties:

• These substances are attracted by a magnet
• Atomic orbitals of these substances are partially filled
• It develops weak magnetization in the direction of the applied magnetic field
• After removing the magnetizing field, it loses its magnetization
• When placed in a non-uniform magnetic field, it tends to move from weaker to stronger regions of the magnetic field
• When placed in a uniform magnetic field, it aligns itself in the direction of the magnetic field
• Magnetic susceptibility is a small positive value
• Relative permeability is close to one and always greater than 1
• Magnetic permeability is slightly more compared to free space

Curie Law

Curie point is the temperature above which certain materials lose their permanent magnetic properties.

The Curie temperature is also known as the critical temperature.

The temperature above which a ferromagnetic material behaves like a paramagnetic material is defined as Curie temperature.

The Curie temperature is given by:

\(T_c={C\over ξ}\)

where, T_c = Curie Temperature

C = Material-specific Curie constant

ξ = Magnetic susceptibility

DC Resistivity

• ​DC Resistivity is a surface geophysical method that provides information about subsurface electrical resistivity distribution through the injection of an electric current into the earth and the measurement of the corresponding voltage.
• In a Direct Current Resistivity (DCR) experiment, a generator is used to inject current into the earth. 
• Two electrodes are used to send a current into the ground and a series of other electrodes measure the voltage at different points on the Earth.
• The current path depends upon the variation of conductivity or its reciprocal, the electrical resistivity.

The DC of ferrites is the highest among all magnetic materials. The decreasing order of magnetic material as per their DC resistivity value is given below:

Ferrites > Ferromagentic > Paramagnetic > Diamagnetic

Concept:

Magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field.

And the relation between Magnetic susceptibility and Temperature can be given as

\(\chi = \frac{C}{{T - {T_c}}}\)

Curie’s constant = C

Magnetic susceptibility = χ

Curie temperature = Tc

The temperature at any given instance = T

Explanation:

Effect of Temperature Ferromagnetism:

The magnetic susceptibility decreases with an increase in temperature.

So, the ferromagnetism decreases with rising temperature.

It is maximum at absolute zero temperature and becomes zero at Curie temperature.

Above this temperature, the ferromagnetic material behaves as a paramagnetic substance.

The correct answer is option 1):(Anti ferromagnetic materials)

Concept:

• Magnetic susceptibility is defined as the property that indicates the degree of magnetization of a material in response to an applied magnetic field
• Magnetic susceptibility indicates whether a material is attracted to or repelled out of a magnetic field. 
• Antiferromagnetism is a form of magnetism in which the magnetic moments of neighbouring atoms are arranged anti-parallel.
• As with ferromagnetism, there are materials that show magnetic ordering below a critical temperature.
• In Anti ferromagnetic materials, the susceptibility will decrease with an increase in temperature and they have relatively small susceptibility at all temperatures.
• Antiferromagnetism property depends on the spin of electrons 
• Manganese oxide, Chromium oxide, ( Cr 2 O 3 ), and Ferrous oxide  are examples of Anti ferromagnetic materials 

Additional Information

• Ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field
• Superparamagnetic substances do not retain any net magnetization once the external field has been removed.  they have no magnetic memory.
• A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains.