Electrical Engineering MCQs – Simple Magnetic Circuits MCQs ( Electrical Engineering ) MCQs
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Latest Simple Magnetic Circuits MCQs ( Electrical Engineering ) Mcqs
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Magnetomotive Force and Magnetic Field Strength
1. What is a permeable substance?
a) Any good conductor
b) Any bad conductor
c) Any strong magnet
d) Any substance through which the magnetic lines of force can pass easily
Answer: d
Explanation: A permeable substance is one through which the magnetic lines of force can pass easily.
2. Materials having good retentivity are?
a) Strong magnets
b) Weak magnets
c) Temporary magnets
d) Permanent magnets
Answer: d
Explanation: Materials with good retentivity are permanent magnets because they can retain magnetism even when no external magnetic field present.
3. Magnetic field exists along which of the following?
a) Moving charges
b) Stationary charges
c) Copper
d) Iron
Answer: a
Explanation: Moving charges have a magnetic field associated with them because they have magnetic flux lines associated with it.
4. Magnetomotive force is equal to__________________
a) current * number of turns
b) current / number of turns
c) current / number of turns per unit length
d) current * number of turns per unit length
Answer: a
Explanation: MMF is ability to produce flux and is equal to product of current flowing and number of turns.
5. Unit of MMF is ______________
a) A/m
b) A-m
c) A
d) unitless
Answer: c
Explanation: MMF is equal to the product of current flowing and number of turns.
unit of Magnetomotive force = unit of current = A or ampere.
6. When a bar magnet is broken into two pieces, which of the following are true?
a) The magnet loses its magnetism
b) The magnet has only north pole left
c) The magnet has only south pole left
d) The magnet turns into two new bar magnets
Answer: d
Explanation: When a bar magnet is broken into two pieces, it forms two different bar magnets. This happens because the broken pieces of the magnet form a separate north and south pole for itself as monopoles do not exist.
7. When an electric current flows into the page, what is the direction of the magnetic field?
a) Clockwise
b) Anti-clockwise
c) Cannot be determined
d) Parallel to the current
Answer: a
Explanation: When the current flows into the page, the magnetic field is clockwise because of the right hand thumb rule, we orient our thumb into the page and our fingers curl in the clockwise direction.
8. When an electric current flows out of the page, what is the direction of the magnetic field?
a) Clockwise
b) Anti-clockwise
c) Cannot be determined
d) Parallel to the current
Answer: b
Explanation: When the current flows out of the page, the magnetic field is anti-clockwise because of the right hand thumb rule, we orient our thumb out of the page and our fingers curl in the anti-clockwise direction.
9. The relation between the direction of current and the direction of magnetic field is?
a) Same direction
b) Opposite direction
c) Perpendicular
d) Unrelated
Answer: c
Explanation: When a conductor carries a certain value of current, the force developed in the conductor, the current in the conductor and the magnetic field in the conductor are mutually perpendicular to each other.
Permeability of Free Space or Magnetic Constant
1. Permeability of free space is also known as _________
a) Magnetic constant
b) Electric constant
c) Electrostatic constant
d) Magnetostatic constant
Answer: a
Explanation: The permeability of free space is also known as the magnetic constant. The permittivity of free space is the electrostatic constant.
2. A substance whose permeability is less than the permeability of free space is?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Not a magnetic substance
Answer: a
Explanation: A diamagnetic material creates a magnetic field opposing that of the external magnetic field and it repels the external magnetic field. Hence its permeability is less than that of free space.
3. Which, among the following, have negative susceptibility?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Not a magnetic substance
Answer: a
Explanation: Magnetic susceptibility is the degree of magnetisation of a material in response to the external magnetic field. Diamagnetic substances repel the magnetic field and hence have negative susceptibility.
4. Which, among the following, have positive susceptibility?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Both paramagnetic and ferromagnetic
Answer: d
Explanation: Magnetic susceptibility is the degree of magnetisation of a material in response to the external magnetic field. Both paramagnetic and ferromagnetic materials have positive susceptibility as they get magnetise when placed in external magnetic field.
5. A substance whose permeability is more than the permeability of free space is?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Both paramagnetic and ferromagnetic
Answer: d
Explanation: Relative permeability=1+Magnetic susceptibility
Since both paramagnetic and ferromagnetic materials have positive susceptibility, their relative permeability is greater than unity i.e. their permeability is more than the permeability of free space.
6. The unit for permeability of free space is?
a) Henry
b) Henry-m
c) Henry/m
d) Henry/m2
Answer: c
Explanation: Using formula of self inductance of solenoid, L=µ0n2A/l
We get unit of µ0 = Henry/m.
7. Which among the following, is a correct expression for µ0.
a) µ0=BH
b) µ0=B/H
c) µ0=H/B
d) µ0=BH2
Answer: b
Explanation: Magnetic permeability is the constant of proportionality between the magnetic flux density and magnetic field strength of a given medium. Hence µ0=B/H.
8. Calculate the magnetic flux density if the magnetic field strength is 2A/m.
a) 4*pi*10-7T
b) 8*pi*10-7T
c) 10*pi*10-7T
d) 12*pi*10-7T
Answer: b
Explanation: We know that:
µ0=B/H
Substituting the value of H from the question, we get B = 8*pi*10-7T.
9. Calculate the magnetic field strength if the magnetic flux density is 4*piT.
a) 10-7/16*pi2A/m
b) 10-7A/m
c) 107A/m
d) 10-7A
Answer: c
Explanation: We know that:
µ0=B/H
Substituting the value of B from the question, we get H = 107A/m.
10. Calculate the magnetic flux density if the magnetic field strength is 3A/m.
a) 4*pi*10-7T
b) 8*pi*10-7T
c) 10*pi*10-7T
d) 12*pi*10-7T
Answer: d
Explanation: We know that:
µ0=B/H
Substituting the value of H from the question, we get B = 12*pi*10-7T.
Relative Permeability
1. What is the unit for relative permeability?
a) H-m
b) H/m
c) H2/m
d) No unit
Answer: d
Explanation: Relative permeability is the ratio of permeability of the material to the permeability of free space. Since it is a ratio, it does not have any units.
2. Which of the following expressions is correct with respect to relative permeability?
a) B = µrµ0/H
b) B = µrµ0H
c) B = µr/µ0H
d) B = µrµ0/H2
Answer: b
Explanation: For a non magnetic material: B=µ0H. Hence for a material having relative permeability µr, B=µrµ0H.
3. A substance whose relative permeability is less than the permeability of free space is?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Not a magnetic substance
Answer: a
Explanation: A diamagnetic material creates a magnetic field opposing that of the external magnetic field and it repels the external magnetic field. Hence its relative permeability is less than that of the free space.
4. A substance whose relative permeability is more than the permeability of free space is?
a) Diamagnetic
b) Paramagnetic
c) Ferromagnetic
d) Both paramagnetic and ferromagnetic
Answer: d
Explanation: Relative permeability=1+Magnetic susceptibility
Since both paramagnetic and ferromagnetic materials have positive susceptibility, their relative permeability is greater than unity i.e. their permeability is more than the permeability of free space.
5. Diamagnetic substances have relative permeability_____________
a) Greater than 1
b) Less than 1
c) Equal to 1
d) Zero
Answer: b
Explanation: A diamagnetic material creates a magnetic field opposing that of the external magnetic field and it repels the external magnetic field. Hence it has relative permeability less than 1.
6. Paramagnetic substances have relative permeability_____________
a) Greater than 1
b) Less than 1
c) Equal to 1
d) Zero
Answer: a
Explanation: A paramagnetic material creates a magnetic field which is weakly attracted to that of the external magnetic field. Hence it has relative permeability greater than 1.
7. As the temperature increases up to the Curie temperature, the relative susceptibility of ferromagnetic materials?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero
Answer: a
Explanation: The susceptibility of ferromagnetic materials increases with the increase in temperature and then it decreases when the temperature goes beyond the Curie temperature.
8. As the temperature increases beyond the Curie temperature, the relative susceptibility of ferromagnetic materials?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero
Answer: b
Explanation: The susceptibility of ferromagnetic materials increases with the increase in temperature and then it decreases when the temperature goes beyond the curie temperature.
Reluctance
1. Reciprocal of reluctance is __________
a) Permeance
b) Susceptibility
c) Resistance
d) Conductance
Answer: a
Explanation: The reciprocal of reactance is permeance. It is the ability of a material to allow the passage of magnetic lines of flux.
2. Reluctance is ________________ to the length of the material.
a) Directly proportional
b) Inversely proportional
c) Not related
d) Reluctance is ________________ to the length of the material.
Answer: a
Explanation: The formula for reluctance is:
S = l/µ0 µr*A.
From the formula, we can see that reluctance is directly proportional to the length of the material.
3. Reluctance is ________________ to the area of cross section the material.
a) Directly proportional
b) Inversely proportional
c) Not related
d) Equal
Answer: b
Explanation: The formula for reluctance is:
S = l/(µ0 µr*A).
From the formula, we can see that reluctance is inversely proportional to the area of cross section of the material.
4. When the length of the material increases, what happens to reluctance?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: a
Explanation: Reluctance is directly proportional to the length of the material hence as length increases, reluctance also increases.
5. When the area of cross section of the material increases, what happens to reluctance?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: b
Explanation: Reluctance is inversely proportional to the area of cross section of the material hence as area increases, reluctance decreases.
6. Unit of reluctance is?
a) AWb
b) A2/Wb
c) Wb/A
d) A/Wb
Answer: d
Explanation: Reluctance is magnetomotive force per unit flux,
So unit of reluctance = unit of MMF / unit of magnetic flux = A/Wb.
7. The electrical equivalent of reluctance is?
a) Resistance
b) Inductance
c) Capacitance
d) Conductance
Answer: a
Explanation: Resistance is the opposition to the flow of charge, similarly reluctance is the opposition to the flow of magnetic flux.
8. As the magnetic field strength increases, reluctance?
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: a
Explanation: Reluctance is directly proportional to the strength of the magnetic field, hence as the strength of magnetic field increases, the reluctance increases.
9. As the magnetic flux density increases, the reluctance _____________
a) Increases
b) Decreases
c) Remains the same
d) Becomes zero
Answer: b
Explanation: Reluctance is inversely proportional to the magnetic flux density, hence as magnetic flux density increases, reluctance decreases.
10. Calculate the reluctance when the magnetomotive force is 10A turns and the flux is 5Wb.
a) 0.5A/Wb
b) 5A/Wb
c) 10A/Wb
d) 2A/Wb
Answer: d
Explanation: We know that:
F=ϕ*S
Substituting the given values from the question:
S=2A/Wb.
Ohm’s Law for a Magnetic Circuit
1. Ohm’s law for magnetic circuits is _________
a) F=ϕS
b) F=ϕ/S
c) F=ϕ2S
d) F=ϕ/S2
Answer: a
Explanation: Ohm’s law for magnetic circuits states that the MMF is directly proportional to the magnetic flux where reluctance is the constant of proportionality.
2. What happens to the MMF when the magnetic flux decreases?
a) Increases
b) Decreases
c) Remains constant
d) Becomes zero
Answer: b
Explanation: Ohm’s law for the magnetic circuit’s states that the MMF is directly proportional to the magnetic flux hence as the magnetic flux decreases, the MMF also decreases.
3. Calculate the MMF when the magnetic flux is 5Wb and the reluctance is 3A/Wb.
a) 10At
b) 10N
c) 15N
d) 15At
Answer: d
Explanation: We know that:
F=ϕS
Substituting the given values from the question, we get MMF = 15At.
4. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Calculate the flux density of the ring.
a) 1.6T
b) 2.6T
c) 3.6T
d) 4.6T
Answer: a
Explanation: ϕ=BA => Flux density B = ϕ/A
Substituting the values, we get B=1.6T.
5. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the reluctance.
a) 1.68 * 10-4A/Wb
b) 1.68 * 104 A/Wb
c) 1.68 * 106 A/Wb
d) 1.68 * 10-6A/Wb
Answer: c
Explanation: Reluctance=l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance=1.68*106 A/Wb.
6. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the magnetomotive force.
a) 1442At
b) 1342At
c) 1432At
d) 1344At
Answer: d
Explanation: Reluctance=l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance S=1.68*106 A/Wb.
F=ϕS
Substituting the given values, we get F = 1344At.
7. A ring having a cross-sectional area of 500 mm2, a circumference of 400 mm and ϕ=800microWb has a coil of 200 turns wound around it. Relative permeability of ring is 380. Calculate the magnetising current.
a) 6.7A
b) 7.7A
c) 7.6
d) 6.1A
Answer: a
Explanation: Reluctance = l/(µ*A) = l/(µrµ0*A)
Substituting the values, we get Reluctance S=1.68*106 A/Wb.
F=ϕS Substituting the given values, we get F = 1344At.
I=F/N Substituting the values from the question, we get I=6.7A.
8. Can we apply Kirchhoff’s law to magnetic circuits?
a) Yes
b) No
c) Depends on the circuit
d) Insufficient information provided
Answer: a
Explanation: Magnetic circuits have an equivalent to the potential difference of electric circuits. This is the magnetic potential difference which allows us to apply Kirchhoff’s laws to magnetic circuit analysis.
9. What is MMF?
a) Magnetic Machine Force
b) Magnetomotive Force
c) Magnetic Motion Force
d) Magnetomotion Force
Answer: b
Explanation: MMF stands for magnetomotive force. Actually, it is not a force. It is analogous to potential in electric field.
10. The equivalent of the current I in magnetic ohm’s law is?
a) Flux
b) Reluctance
c) MMF
d) Resistance
Answer: a
Explanation: The equivalent of current in magnetic ohm’s law is flux as:
V=IR is equivalent to F=ϕS.
Determination of the B/H Characteristic
1. The B/H characteristics can be determined using _______
a) Ammeter
b) Fluxmeter
c) Voltmeter
d) Multimeter
Answer: b
Explanation: The fluxmeter is an electronic display instrument used to measure the magnetic flux of permanent magnets hence it can be used to determine B/H characteristics.
2. The B/H curve can be used to determine?
a) Iron loss
b) Hysteresis loss
c) Voltage loss
d) Eddy current loss
Answer: b
Explanation: Hysteresis loss is basically a heat loss due to the reversal of magnetisation of the transformer core whenever it is subjected to a changing magnetic field. It can be determined using the B/H curve.
3. The B/H ratio is not constant for ________
a) Diamagnetic materials
b) Ferromagnetic materials
c) Paramagnetic materials
d) Non-magnetic materials
Answer: b
Explanation: As the magnetizing field increases, the relative permeability increases, reaches a maximum, and then decreases. Due to varying permeability, B/H ratio is not constant for ferromagnetic materials.
4. When using a fluxmeter, if the flux changes from Φ to -Φ, what happens to the current?
a) Becomes zero
b) Becomes infinity
c) Remains the same
d) Reverses
Answer: d
Explanation: When the flux changes from Φ to -Φ, the current direction will change as the direction of flux is changing.
5. Why is the coil of a ballistic galvanometer wound on a non- metallic former?
a) To minimise damping when high resistance is connected in series
b) To maximise damping when high resistance is connected in series
c) To minimise damping when high resistance is connected in parallel
d) To maximise damping when high resistance is connected in parallel
Answer: a
Explanation: The coil of ma ballistic galvanometer is wound on a nonmagnetic former in order to minimise damping when high resistance is connected in series.
6. The ballistic galvanometer is usually lightly damped so that ________
a) It can oscillate
b) It will remain stable
c) Amplitude of the first swing is very large
d) Amplitude of the first swing is very small
Answer: c
Explanation: The ballistic galvanometer is usually lightly damped so that the amplitude of its first swing is very large.
7. PMMC instruments can be used as a fluxmeters by _______
a) Using a low resistance shunt
b) Removing the control spring
c) Making the control springs having a large moment of inertia
d) Using a high resistance in series
Answer: b
Explanation: A PMMC or a permanent moving magnet coil instrument can be used as a fluxmeter by removing the control spring.
8. Hysteresis loss is determined from _______
a) B/H curve
b) H/B curve
c) BH curve
d) B2H curve
Answer: c
Explanation: Hysteresis loss is basically a heat loss due to the reversal of magnetisation of the transformer core whenever it is subjected to a changing magnetic field. It can be determined using the B/H curve.
9. What is a PMMC instrument?
a) Permanent moving magnet coil instrument
b) Permanent machine magnet coil instrument
c) Permanent moving machine coil instrument
d) Premature moving magnet coil instrument
Answer: a
Explanation: A PMMC instrument is a permanent moving magnet coil instrument. It uses two magnets to create a stationary magnetic field.
10. B/H curve shows the relationship between?
a) Magnetic field strength and magnetic flux
b) Magnetic field strength and magnetic flux density
c) Current and magnetic flux density
d) Voltage and magnetic flux density
Answer: b
Explanation: The B/H curve shows the relation between magnetic field strength and magnetic flux density.