Question 451
Residual magnetism is the magnetic flux density remaining when:
Options:
A) Current is maximum
B) Magnetizing force becomes zero
C) Saturation occurs
D) Voltage is removed
Answer: B) Magnetizing force becomes zero
Step-by-Step Solution:
Even after removing the magnetizing force (H = 0), some magnetic flux remains.
This remaining flux is called residual magnetism.
Important Notes:
- Residual magnetism is also called remanence.
✔ Answer: B) Magnetizing force becomes zero
Question 452
The ability of a magnetic material to retain magnetism is called:
Options:
A) Coercivity
B) Retentivity
C) Permeability
D) Reluctivity
Answer: B) Retentivity
Step-by-Step Solution:
Retentivity is the property by which a magnetic material retains magnetism after the external field is removed.
Important Notes:
- High retentivity → Good permanent magnets.
✔ Answer: B) Retentivity
Question 453
The reverse magnetizing force required to remove residual magnetism is called:
Options:
A) Saturation
B) Retentivity
C) Coercive force
D) Reluctance
Answer: C) Coercive force
Step-by-Step Solution:
The reverse magnetic field required to reduce residual magnetism to zero is called coercive force.
Important Notes:
- Symbol: Hc
✔ Answer: C) Coercive force
Question 454
Coercive force is applied in which direction?
Options:
A) Same direction
B) Opposite direction
C) Perpendicular direction
D) Random direction
Answer: B) Opposite direction
Step-by-Step Solution:
Residual magnetism can only be removed by applying a magnetic field in the opposite direction.
Important Notes:
- Reverse magnetization removes residual flux.
✔ Answer: B) Opposite direction
Question 455
The closed curve obtained between B and H is called the:
Options:
A) Magnetization curve
B) Hysteresis loop
C) B-H graph
D) Flux curve
Answer: B) Hysteresis loop
Step-by-Step Solution:
Repeated magnetization produces a closed B-H curve known as the hysteresis loop.
Important Notes:
- Also called B-H loop.
✔ Answer: B) Hysteresis loop
Question 456
The energy lost during magnetic hysteresis is proportional to the:
Options:
A) Height of the loop
B) Width of the loop
C) Area of the hysteresis loop
D) Perimeter of the loop
Answer: C) Area of the hysteresis loop
Step-by-Step Solution:
The area enclosed by the hysteresis loop represents the energy lost per magnetization cycle.
Important Notes:
- Larger area → Greater hysteresis loss.
✔ Answer: C) Area of the hysteresis loop
Question 457
Hysteresis loss appears in the form of:
Options:
A) Light
B) Heat
C) Sound
D) Radiation
Answer: B) Heat
Step-by-Step Solution:
The energy lost due to hysteresis is converted into heat inside the magnetic material.
Important Notes:
- Causes heating of transformer cores.
✔ Answer: B) Heat
Question 458
A soft magnetic material has a:
Options:
A) Wide hysteresis loop
B) Narrow hysteresis loop
C) Circular loop
D) Square loop
Answer: B) Narrow hysteresis loop
Step-by-Step Solution:
Soft magnetic materials have low hysteresis loss and therefore a narrow hysteresis loop.
Important Notes:
- Used where magnetization changes frequently.
✔ Answer: B) Narrow hysteresis loop
Question 459
Soft magnetic materials are generally made of:
Options:
A) Iron and Silicon Steel
B) Copper
C) Aluminium
D) Brass
Answer: A) Iron and Silicon Steel
Step-by-Step Solution:
Iron and silicon steel possess high permeability and low hysteresis loss.
Important Notes:
- Common transformer core material.
✔ Answer: A) Iron and Silicon Steel
Question 460
Soft magnetic materials have:
Options:
A) High coercivity
B) High retentivity
C) Low coercivity
D) High hysteresis loss
Answer: C) Low coercivity
Step-by-Step Solution:
Soft magnetic materials are easily magnetized and demagnetized.
Therefore, they have low coercive force.
Important Notes:
- Ideal for AC machines.
✔ Answer: C) Low coercivity
Question 461
Hard magnetic materials have a:
Options:
A) Narrow hysteresis loop
B) Wide hysteresis loop
C) Straight line
D) No hysteresis loop
Answer: B) Wide hysteresis loop
Step-by-Step Solution:
Hard magnetic materials retain magnetism strongly.
Hence, they have a wide hysteresis loop.
Important Notes:
- Used for permanent magnets.
✔ Answer: B) Wide hysteresis loop
Question 462
Hard magnetic materials possess:
Options:
A) Low retentivity
B) High coercivity
C) Low saturation
D) Low permeability
Answer: B) High coercivity
Step-by-Step Solution:
A large reverse magnetic field is needed to demagnetize hard magnetic materials.
Therefore, they have high coercivity.
Important Notes:
- Suitable for permanent magnets.
✔ Answer: B) High coercivity
Question 463
Magnetic materials having a wide hysteresis loop are used in:
Options:
A) Transformers
B) Relays
C) Magnetic tapes and hard disks
D) Solenoids
Answer: C) Magnetic tapes and hard disks
Step-by-Step Solution:
Wide hysteresis loops indicate high retentivity.
Therefore, such materials are used for magnetic storage devices.
Important Notes:
Examples:
- Hard disk
- Credit card
- Magnetic tape
- Audio recording
✔ Answer: C) Magnetic tapes and hard disks
Question 464
Magnetic materials having a narrow hysteresis loop are commonly used in:
Options:
A) Permanent magnets
B) Hard disks
C) Transformers and relays
D) Credit cards
Answer: C) Transformers and relays
Step-by-Step Solution:
Transformers and relays operate under alternating magnetic fields.
Hence, they require materials with low hysteresis loss and narrow hysteresis loops.
Important Notes:
Applications of soft magnetic materials:
- Transformers
- Electromagnets
- Solenoids
- Relays
✔ Answer: C) Transformers and relays
Question 465
The phenomenon in which magnetic flux density (B) lags behind the magnetizing force (H) is known as:
Options:
A) Magnetic Saturation
B) Magnetic Hysteresis
C) Magnetic Induction
D) Magnetic Retentivity
Answer: B) Magnetic Hysteresis
Step-by-Step Solution:
Magnetic Hysteresis is the phenomenon in which the magnetic flux density (B) does not immediately follow the magnetizing force (H).
Instead, during one complete cycle of magnetization, B always lags behind H.
Therefore, the correct answer is Magnetic Hysteresis.
Important Notes:
- B lags behind H.
- Occurs during cyclic magnetization.
- Responsible for hysteresis loss.
✔ Answer: B) Magnetic Hysteresis
Question 466
The word "Hysteresis" is derived from the Greek word meaning:
Options:
A) Magnetism
B) Rotation
C) To lag behind
D) Attraction
Answer: C) To lag behind
Step-by-Step Solution:
The word Hysteresis is derived from the Greek word "Hysterein", which means to lag behind.
This correctly describes the lag of magnetic flux density behind the magnetizing force.
Important Notes:
- Greek word: Hysterein
- Meaning: To lag behind
✔ Answer: C) To lag behind
Question 467
The hysteresis loop is mainly exhibited by:
Options:
A) Diamagnetic materials
B) Paramagnetic materials
C) Ferromagnetic materials
D) Antiferromagnetic materials
Answer: C) Ferromagnetic materials
Step-by-Step Solution:
Although several types of magnetic materials exist, the hysteresis loop is mainly associated with ferromagnetic materials because of their magnetic domain structure.
Important Notes:
Examples of ferromagnetic materials:
- Iron
- Nickel
- Cobalt
✔ Answer: C) Ferromagnetic materials
Question 468
Before the application of a magnetic field, the magnetic dipoles of a ferromagnetic material are:
Options:
A) Perfectly aligned
B) Randomly oriented
C) Parallel to each other
D) Perpendicular to each other
Answer: B) Randomly oriented
Step-by-Step Solution:
Initially, without an external magnetic field, the magnetic dipoles are randomly arranged.
Therefore, the material has almost no net magnetization.
Important Notes:
- No external field → Random dipoles.
- External field → Dipoles align.
✔ Answer: B) Randomly oriented
Question 469
When a magnetic field is applied to a ferromagnetic material, the magnetic dipoles:
Options:
A) Become random
B) Reverse completely
C) Align in one direction
D) Disappear
Answer: C) Align in one direction
Step-by-Step Solution:
Application of a magnetic field causes the magnetic domains to align in the direction of the field.
This greatly increases the magnetization.
Important Notes:
- Domain alignment increases magnetic flux density.
- Produces strong magnetization.
✔ Answer: C) Align in one direction
Question 470
Magnetic saturation occurs when:
Options:
A) Current becomes zero
B) Flux becomes zero
C) Nearly all magnetic domains become aligned
D) Voltage becomes maximum
Answer: C) Nearly all magnetic domains become aligned
Step-by-Step Solution:
As the magnetizing force increases, more magnetic domains align.
Eventually, almost all domains are aligned, and further increase in H produces very little increase in B.
This condition is called magnetic saturation.
Important Notes:
- Saturation = Maximum practical magnetization.
- Further increase in H gives negligible increase in B.
✔ Answer: C) Nearly all magnetic domains become aligned
Question 471
The portion 'OA' of the hysteresis curve represents:
Options:
A) Demagnetization
B) Initial magnetization
C) Residual magnetism
D) Coercive force
Answer: B) Initial magnetization
Step-by-Step Solution:
Starting from an unmagnetized state, increasing the magnetizing force causes the flux density to increase until saturation at point A.
Thus, OA represents the initial magnetization curve.
Important Notes:
- OA → Initial magnetization.
- Ends at saturation point.
✔ Answer: B) Initial magnetization
Question 472
Residual magnetism is the magnetic flux density remaining when:
Options:
A) H is maximum
B) H becomes zero
C) Current is maximum
D) Saturation occurs
Answer: B) H becomes zero
Step-by-Step Solution:
After reducing the magnetizing force to zero, some magnetic flux still remains.
This remaining flux is called Residual Magnetism.
Important Notes:
- Residual magnetism exists even when H = 0.
- Also called remanence.
✔ Answer: B) H becomes zero
Question 473
The ability of a magnetic material to retain residual magnetism is called:
Options:
A) Coercivity
B) Reluctivity
C) Retentivity
D) Permeability
Answer: C) Retentivity
Step-by-Step Solution:
Retentivity is the property by which a magnetic material retains magnetic flux after the magnetizing force has been removed.
Important Notes:
- High retentivity → Permanent magnets.
- Low retentivity → Transformer cores.
✔ Answer: C) Retentivity
Question 474
The reverse magnetizing force required to remove residual magnetism is known as:
Options:
A) Retentivity
B) Saturation
C) Coercive Force
D) Permeability
Answer: C) Coercive Force
Step-by-Step Solution:
Residual magnetism is removed by applying a reverse magnetic field.
The required reverse magnetizing force is called Coercive Force.
Important Notes:
- Symbol: Hc
- Measured in A/m.
✔ Answer: C) Coercive Force
Question 475
On the hysteresis loop, the path 'BC' represents:
Options:
A) Saturation
B) Demagnetization due to reverse magnetic field
C) Initial magnetization
D) Positive saturation
Answer: B) Demagnetization due to reverse magnetic field
Step-by-Step Solution:
When the current direction is reversed, the reverse magnetizing force increases.
The magnetic flux density decreases to zero along the BC path.
Important Notes:
- BC removes residual magnetism.
- Ends at B = 0.
✔ Answer: B) Demagnetization due to reverse magnetic field
Question 476
Eddy currents are produced due to:
Options:
A) Constant magnetic field
B) Changing magnetic field
C) Constant electric field
D) Electrostatic induction
Answer: B) Changing magnetic field
Step-by-Step Solution:
According to Faraday's Law of Electromagnetic Induction, whenever a conducting material is subjected to a changing magnetic field, an EMF is induced within it.
Since the magnetic core is also a conductor, this induced EMF causes circulating currents inside the material. These circulating currents are known as eddy currents.
Therefore, eddy currents are produced due to a changing magnetic field.
Important Notes:
- Eddy currents are induced currents.
- They occur only when magnetic flux changes.
- Based on Faraday's Law.
✔ Answer: B) Changing magnetic field
Question 477
The circulating currents induced inside a conducting magnetic material are called:
Options:
A) Leakage currents
B) Magnetizing currents
C) Eddy currents
D) Displacement currents
Answer: C) Eddy currents
Step-by-Step Solution:
When magnetic flux changes inside a conducting magnetic material, induced EMFs produce closed-loop currents.
These circulating currents are called eddy currents.
Important Notes:
- Eddy currents circulate inside the material.
- They do not perform useful work.
- They produce heat.
✔ Answer: C) Eddy currents
Question 478
Eddy current loss is mainly a:
Options:
A) Mechanical loss
B) Copper loss
C) I²R loss in the core
D) Friction loss
Answer: C) I²R loss in the core
Step-by-Step Solution:
The induced eddy currents flow through the resistance of the magnetic core.
Hence, power is dissipated as
P = I²R
This power loss is called eddy current loss.
Important Notes:
- Eddy current loss is a core loss.
- It appears as heat.
- It is an I²R loss.
✔ Answer: C) I²R loss in the core
Question 479
Eddy current loss causes an increase in the:
Options:
A) Voltage
B) Frequency
C) Temperature of the magnetic core
D) Power factor
Answer: C) Temperature of the magnetic core
Step-by-Step Solution:
Since eddy currents produce I²R losses inside the core, electrical energy is converted into heat.
Therefore, the temperature of the magnetic material increases.
Important Notes:
- Eddy current loss produces heating.
- It reduces machine efficiency.
✔ Answer: C) Temperature of the magnetic core
Question 480
Hysteresis loss and eddy current loss together are known as:
Options:
A) Copper losses
B) Mechanical losses
C) Core (Iron) losses
D) Stray losses
Answer: C) Core (Iron) losses
Step-by-Step Solution:
Both hysteresis loss and eddy current loss occur inside the magnetic core.
Hence, together they are called:
- Iron losses
- Core losses
- Magnetic losses
Important Notes:
Core Loss = Hysteresis Loss + Eddy Current Loss
✔ Answer: C) Core (Iron) losses
Question 481
Eddy currents are produced mainly in the:
Options:
A) Insulation
B) Magnetic core
C) Air gap
D) Windings only
Answer: B) Magnetic core
Step-by-Step Solution:
The magnetic core is a conducting material.
When alternating magnetic flux links with the core, circulating currents are induced inside it.
Hence, eddy currents are produced in the magnetic core.
Important Notes:
- Conducting core is necessary.
- Alternating flux induces eddy currents.
✔ Answer: B) Magnetic core
Question 482
If the magnetic core is made of solid iron, the eddy current loss will be:
Options:
A) Zero
B) Very small
C) Very large
D) Independent of the core
Answer: C) Very large
Step-by-Step Solution:
A solid iron core provides a large conducting area for circulating currents.
This results in high eddy current and therefore large eddy current loss.
Important Notes:
- Solid cores have high eddy current loss.
- Laminations reduce this loss.
✔ Answer: C) Very large
Question 483
The most common method of reducing eddy current loss is by:
Options:
A) Increasing current
B) Using laminated cores
C) Increasing frequency
D) Using thicker cores
Answer: B) Using laminated cores
Step-by-Step Solution:
The magnetic core is divided into thin insulated sheets called laminations.
This increases the resistance to eddy current flow and reduces their magnitude.
Therefore, laminated cores greatly reduce eddy current loss.
Important Notes:
- Laminations increase resistance.
- Eddy current decreases.
- Core heating reduces.
✔ Answer: B) Using laminated cores
Question 484
The laminations of a magnetic core are insulated from each other by:
Options:
A) Copper coating
B) Varnish or oxide film
C) Aluminium sheet
D) Plastic cover only
Answer: B) Varnish or oxide film
Step-by-Step Solution:
Each lamination is coated with a thin layer of varnish or oxide film.
This insulation prevents current from circulating freely between adjacent laminations.
Important Notes:
- Insulation increases resistance.
- Reduces eddy current flow.
✔ Answer: B) Varnish or oxide film
Question 485
Laminating the magnetic core primarily reduces:
Options:
A) Flux density
B) Magnetic permeability
C) Eddy current loss
D) Hysteresis loss
Answer: C) Eddy current loss
Step-by-Step Solution:
Laminations divide the core into thin insulated sections.
This:
- Reduces the area available for circulating currents.
- Increases resistance.
- Decreases eddy current.
Therefore, eddy current loss is significantly reduced.
Important Notes:
- Laminations do not significantly reduce hysteresis loss.
- They mainly reduce eddy current loss.
✔ Answer: C) Eddy current loss
Question 486
Residual Magnetism is defined as:
Options:
A) Magnetism produced by an alternating current
B) Magnetization remaining after removing the external magnetic field
C) Maximum magnetic flux density
D) Reverse magnetic field required for demagnetization
Answer: B) Magnetization remaining after removing the external magnetic field
Step-by-Step Solution:
Residual Magnetism is the amount of magnetization that remains in a magnetic material even after the external magnetizing field has been removed.
This remaining magnetism is also called Remanence.
Important Notes:
- Residual Magnetism remains after removing the external magnetic field.
- Also known as Remanence.
- Observed in ferromagnetic materials.
✔ Answer: B) Magnetization remaining after removing the external magnetic field
Question 487
The ability of a magnetic material to retain residual magnetism is called:
Options:
A) Permeability
B) Reluctance
C) Retentivity
D) Coercivity
Answer: C) Retentivity
Step-by-Step Solution:
Retentivity is the property of a magnetic material that enables it to retain magnetization after the external magnetic field is removed.
Higher retentivity means the material retains more residual magnetism.
Important Notes:
- Retentivity measures the ability to retain magnetism.
- Permanent magnets have high retentivity.
✔ Answer: C) Retentivity
Question 488
On the B-H hysteresis curve, residual magnetism is represented when:
Options:
A) H is maximum
B) H = 0
C) B = 0
D) Current is maximum
Answer: B) H = 0
Step-by-Step Solution:
After reducing the magnetizing force to zero, the magnetic material still retains some flux density.
This remaining flux density is called residual magnetism.
Important Notes:
- Residual magnetism exists at H = 0.
- Represented by the intercept on the B-axis.
✔ Answer: B) H = 0
Question 489
Residual Magnetism is also known as:
Options:
A) Coercivity
B) Reluctivity
C) Remanence
D) Permeance
Answer: C) Remanence
Step-by-Step Solution:
Residual magnetism is commonly referred to as Remanence because it represents the magnetic flux remaining after the external field is removed.
Important Notes:
- Residual Magnetism = Remanence.
- Important property of permanent magnets.
✔ Answer: C) Remanence
Question 490
Residual magnetism in a magnetic material is removed by applying:
Options:
A) Higher positive magnetic field
B) Alternating voltage only
C) Reverse magnetizing force
D) Mechanical force
Answer: C) Reverse magnetizing force
Step-by-Step Solution:
Residual magnetism is removed by applying a magnetic field in the opposite direction.
The reverse magnetic field required to reduce the residual magnetism to zero is called the Coercive Force.
Important Notes:
- Reverse magnetic field removes remanence.
- Related to coercivity.
✔ Answer: C) Reverse magnetizing force
Question 491
The reverse magnetizing force required to eliminate residual magnetism is called:
Options:
A) Retentivity
B) Permeability
C) Coercive Force
D) Saturation
Answer: C) Coercive Force
Step-by-Step Solution:
The reverse magnetic field required to reduce the residual flux density to zero is known as the Coercive Force.
It is represented by the intercept on the negative H-axis of the hysteresis loop.
Important Notes:
- Symbol: Hc
- Unit: A/m
✔ Answer: C) Coercive Force
Question 492
Residual magnetism is commonly observed in:
Options:
A) Transformers only
B) Generators only
C) Motors only
D) Transformers, generators and motors
Answer: D) Transformers, generators and motors
Step-by-Step Solution:
Residual magnetism naturally exists in the magnetic cores of transformers, generators, and motors because of the hysteresis property of ferromagnetic materials.
Important Notes:
Applications where residual magnetism is observed:
- Transformers
- Generators
- Motors
✔ Answer: D) Transformers, generators and motors
Question 493
The total magnetic moment per unit volume after saturation is called:
Options:
A) Initial Remanence
B) Saturation Remanence
C) Coercivity
D) Permeability
Answer: B) Saturation Remanence
Step-by-Step Solution:
Saturation Remanence (SIRM) is the total magnetic moment per unit volume remaining after a material has been magnetized to saturation.
It is generally denoted by Mr.
Important Notes:
- Symbol: Mr
- Also called Saturation Isothermal Remanence.
✔ Answer: B) Saturation Remanence
Question 494
Saturation Remanence is generally denoted by:
Options:
A) Hc
B) Br
C) Mr
D) μr
Answer: C) Mr
Step-by-Step Solution:
Saturation Remanence represents the magnetic moment per unit volume after saturation.
Its standard notation is Mr.
Important Notes:
- Mr → Saturation Remanence
- Hc → Coercive Force
- μr → Relative Permeability
✔ Answer: C) Mr
Question 495
Isothermal Residual Magnetism is generally represented by:
Options:
A) Br
B) Mr(H)
C) Hc
D) μ
Answer: B) Mr(H)
Step-by-Step Solution:
Isothermal Residual Magnetism (IRM) is denoted by Mr(H).
It is mainly used to measure the residual magnetism of small magnetic particles.
Important Notes:
- IRM = Isothermal Remanence.
- Symbol: Mr(H).
✔ Answer: B) Mr(H)
Question 496
Hysteresis loss is defined as the energy wasted due to:
Options:
A) Copper resistance
B) Eddy currents
C) Internal friction of magnetic molecules during magnetization reversal
D) Mechanical friction
Answer: C) Internal friction of magnetic molecules during magnetization reversal
Step-by-Step Solution:
When a magnetic material is magnetized and then demagnetized repeatedly, its magnetic domains oppose the reversal of magnetization due to internal molecular friction.
The work done to overcome this internal friction is converted into heat, resulting in hysteresis loss.
Therefore, hysteresis loss is caused by the internal friction of magnetic molecules during magnetization reversal.
Important Notes:
- Hysteresis loss occurs due to repeated magnetization and demagnetization.
- The lost energy appears as heat.
- It is one of the core (iron) losses.
✔ Answer: C) Internal friction of magnetic molecules during magnetization reversal
Question 497
Hysteresis loss mainly occurs in:
Options:
A) Copper windings
B) Magnetic parts of electrical machines
C) Air gaps
D) Insulating materials
Answer: B) Magnetic parts of electrical machines
Step-by-Step Solution:
Hysteresis loss occurs wherever magnetic flux reverses continuously.
Hence, it mainly occurs in the magnetic cores of transformers, motors, generators, and other electrical machines.
Important Notes:
Occurs in:
- Transformer cores
- Motor cores
- Generator cores
✔ Answer: B) Magnetic parts of electrical machines
Question 498
Hysteresis loss is converted into:
Options:
A) Mechanical energy
B) Light energy
C) Heat energy
D) Magnetic energy
Answer: C) Heat energy
Step-by-Step Solution:
The work done in overcoming the internal friction of magnetic domains is dissipated as heat.
Therefore, hysteresis loss increases the temperature of the magnetic core.
Important Notes:
- Hysteresis loss causes heating.
- Reduces machine efficiency.
✔ Answer: C) Heat energy
Question 499
Repeated reversal of magnetization in a magnetic material results in:
Options:
A) Copper loss
B) Hysteresis loss
C) Dielectric loss
D) Corona loss
Answer: B) Hysteresis loss
Step-by-Step Solution:
Every reversal of magnetic flux requires energy to overcome the internal friction of magnetic domains.
This energy is lost as hysteresis loss.
Important Notes:
- Occurs only in magnetic materials.
- Depends on the hysteresis loop.
✔ Answer: B) Hysteresis loss
Question 500
To minimize hysteresis loss, electrical machines should use magnetic materials having:
Options:
A) Wide hysteresis loop
B) Narrow hysteresis loop
C) High coercivity
D) High retentivity
Answer: B) Narrow hysteresis loop
Step-by-Step Solution:
A narrow hysteresis loop indicates less energy loss during one cycle of magnetization.
Therefore, soft magnetic materials having narrow hysteresis loops are preferred for electrical machines.
Important Notes:
- Narrow loop → Low hysteresis loss.
- Used in transformers and motors.
✔ Answer: B) Narrow hysteresis loop