1. An air gap is usually inserted in magnetic circuits to
A. increase m.m.f.
B. increase the flux
C. prevent saturation
D. none of the above
Answer: C. prevent saturation
Explanation:
- Air gap is non-magnetic part of a magnetic circuit. It increases the value of magnetizing current and lowers the achievable flux density.
- Thus it prevents the saturation of the magnetic circuit
- Saturation is the state reached when an increase in applied external magnetic field H cannot increase the magnetization of the material further, so the total magnetic flux density B levels off.
- The air gap is mostly used in the magnetic circuit because magnetic saturation causes loss of inductance, increasing current, and power loss in the circuit.
- implementation of the air gap in a magnetic circuit extends the range before magnetic saturation occurs and increases the saturation current of the magnetic inductor.
2. The relative permeability of a ferromagnetic material is
A. less than one
B. more than one
C. more than 10
D. more than 100 or 1000
Answer: D. more than 100 or 1000
Explanation:
Ferromagnetic Substances:
- The substances which are strongly magnetized when placed in an external magnetic field in the same direction to the applied field are called ferromagnetic substances.
- Example: Iron, nickel, cobalt
Properties:
- These are characterized by parallel alignment of magnetic dipoles
- These substances are strongly attracted by a magnet
- It develops strong magnetization in the direction of the applied magnetic field
- By removing the magnetizing filed, it does not lose 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 parallel to the direction of the magnetic field
- Magnetic susceptibility is much greater than 1 i.e. more than 100
- Relative permeability is much greater than 1 i.e. more than 100
- Magnetic permeability is much larger compared to free space
3. The unit of magnetic flux is
A. henry
B. weber
C. ampere-turn/weber
D. ampere/meter
Answer: B. weber
Explanation:
- Magnetic flux: The number of magnetic field lines passing a surface area normally is called magnetic flux. It is denoted by φ.
- Magnetic flux is mathematically equal to the dot product of the magnetic field and area vector through which it is passing.
Magnetic flux (φ) = B. A
Where B is the magnetic field and A is the area vector
- The SI unit of area is m2 and the SI unit of the magnetic field is the tesla (T).
Since magnetic flux (φ) = B A
The SI unit of magnetic flux = SI unit of magnetic field × SI unit of area = tesla. meter2 = T m2
Since 1 Weber = 1 T m2
- Thus the SI unit of magnetic flux is T m2 and which is equal to weber (Wb).
4. Permeability in a magnetic circuit corresponds to in an electric circuit.
A. resistance
B. resistivity
C. conductivity
D. conductance
Answer: D. conductance
Explanation:
- Permeance is the measure of the ease with which flux can be set up in a material.
- In other words, it measures the magnitude of the flux for the number of turns in an electric circuit.
- The permeance is analogous to the conductance in an electrical circuit.
- It is reciprocal of the reluctance (R) of the material in a magnetic circuit.
Resistivity:
- Resistivity is numerically equal to the resistance of a substance having a unit area of cross-section and unit length.
- Resistivity is the intrinsic property of the substance and is independent of the shape and size of the body.
Conductivity:
- Reciprocal of resistivity is known as conductivity.
Conductance:
- Reciprocal of resistance is known as conductance.
- Its unit is Ω-1 or Seimen.
5. Permanent magnets are normally made of
A. alnico alloys
B. aluminum
C. cast iron
D. wrought iron
Answer: A. alnico alloys
Explanation:
Magnet: It is defined as a material that can produce its own magnetic field. There are two types of magnet,
- Permanent magnet
- Temporary magnet
Permanent magnet:
- These magnets do not lose their magnetic property once they are magnetized.
- For example alnico, samarium cobalt, ferrite.
Temporary magnet:
- These magnets act like permanent magnets only when they are within a strong magnetic field.
- It is made up of soft iron. for example electromagnet.
Alnico:
- The alloy which are permanent magnets that are primarily made up of a combination of aluminium, nickel and cobalt but can also include copper, iron and titanium.
- It can be easily magnetized in an external magnetic field.
- Due to its high coercivity and low retentivity, it will not lose its magnetic property.
- It has excellent temperature stability.
- Aluminum behaves like a very weak magnet. When exposed to permanent magnets, paramagnetic materials are weakly attracted.
- Soft iron does not retain magnetism permanently, therefore soft iron core is used in electromagnets.
- Copper is not magnetic itself. We can't observe it without very large magnetic fields.
6. Energy stored by a coil is doubled when its current is increased by percent
A. 25
B. 50
C. 41.4
D. 100
Answer: C. 41.4
Explanation:
The energy stored in a coil is given by
E=1/2Li2
Where L is the inductance of the coil
I is the current flows through the coil
Energy is directly proportional to the square of the current.
To get double the energy, the current must be increased by √2 times the previous value.
Therefore, the current must be increased by 41.4%.
7. Relative permeability of vacuum is
A. 1
B. 1 H/m
C. 1/43Ï€
D. 1/4Ï€ x 10-7 H/m
Answer: A. 1
Explanation:
The relative permeability (μr) of a substance is the ratio of magnetic permeability (μ) of the substance to the permeability of free space (μ0)
μr=μ/μ0
Hence, it is a dimensionless quantity and is equal to 1 for vacuum
8. The magnetizing force (H) and magnetic flux density (B) are connected by the relation
A. B = μH
B. H = B / l
C. B = H / l
D. H = B / l
Answer: A. B = μH
Explanation:
- Magnetic field strength or field intensity (H) is the amount of magnetizing force.
- Magnetic flux density (B) is the amount of magnetic force induced on the given body due to the magnetizing force H.
The relation between B and H is,
B = μH
Where, μ is the relative permeability
- Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. It is the degree of magnetization that a material obtains in response to an applied magnetic field.
- It is a constant of proportionality that exists between magnetic flux density and magnetic field intensity.
9. Those magnetic materials are best suited for making armature and transformer cores which have ….. permeability and ….. hysteresis loss.
A. high, high
B. low, high
C. high, low
D. low, low
Answer: C. high, low
Explanation:
Core of a Transformer:
- The core is the part of the transformer which provides a low reluctance path for the flow of magnetic flux.
- Magnetic flux is responsible for inducing the emf in the transformer.
- For providing a low reluctance path, the permeability of core material must be high. Therefore, ferromagnetic materials such as iron and steel must be used.
But silicon steel is preferred over iron because:
- The addition of silicon steel to iron increases the resistivity of the core by five times. Hence the value of the current decreases.
- As current decreases, I2R and eddy current losses decrease.
- The heating of transformer becomes less.
- Magnetostriction decreases.
- Therefore, the best suited magnetic material for the construction of the transformer core is silicon steel.
- To minimize the loss in heat and guarantee better flux linkage the core of transformers must have high permeability and low hysteresis losses
Magnetostriction:
- It is the change in the dimension of the core of the transformer when subjected to a changing magnetic field.
10. The rate of rise of current through an inductive coil is maximum
A. at 63.2% of its maximum steady
B. at the start of the current flow
C. after one time constant
D. near the final maximum value of current
Answer: B. at the start of the current flow
Explanation:
- The rate of rise of current through an inductive coil is maximum at the start of the current flow.
- In an inductive coil the rate of rise of current is maximum Near the final maximum value of current.
Important Points:
Magnetostriction Effect:
- When a magnetic field is applied parallel to the length of a ferromagnetic rod made of a material such as iron or nickel, a small elongation or contraction occurs in its length. This is known as magnetostriction.
- The change in length depends on the intensity of the applied magnetic field and the nature of the ferromagnetic material.
- The change in length is independent of the direction of the field.
- Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field.
- The effect was first identified in 1842 by James Joule when observing a sample of nickel.
- This effect can cause losses due to frictional heating in susceptible ferromagnetic cores.