Ques.11. If the steel disk in a crankshaft position sensor has stopped with the tab in the magnet’s air gap, the induced voltage
Increases
Decreases
Will remain constant
Answer.3. Zero
When the crankshaft turns, the tabs move through the magnetic field. Steel has a much lower reluctance than does air, so the magnetic flux increases when a tab is in the air gap. This change in the magnetic flux causes an induced voltage to appear across the coil.
But when the crankshaft position sensor has stopped with the tab in the magnet’s air gap there will be no change in magnetic field hence no emf is induced therefore the induced emf is zero.
Induced Voltage
e = BlV
B = flux density,
L = length,
V = velocity = 0
e = 0
Ques.12. The magnetism left in the iron after the exciting field has been removed is known as
Susceptance
Residual magnetism
P
Reluctance
Answer.2. Residual magnetism
Residual (remnant) magnetism’ is defined as “the magnetization left behind in the acquired ferromagnetic material, after the withdrawal of the external magnetic field’.
Ques.13. Which of the following is not a unit of flux?
Maxwell
Tesla
Weber
None of the above
Answer.2. Tesla
Tesla is the unit of magnetic induction or magnetic flux density. 1 tesla [T] = 1 weber/meter².
Ques.14. The retentivity (a property) of material is useful for the construction of
Permanent magnets
Transformers
Non-magnetic substances
Electromagnets
Answer.1. Permanent magnets
Magnets that retain their magnetic properties are called permanent magnets. Magnets that retain only a small portion of their magnetic properties are called temporary magnets.
Ques.15. The materials having low retentivity are suitable for making
Weak magnets
Temporary magnets
Permanent magnets
None of the above
Answer.2. Temporary magnets
Magnets that retain their magnetic properties are called permanent magnets. Magnets that retain only a small portion of their magnetic properties are called temporary magnets.
Ques.16. A coil of wire is placed in a changing magnetic field. If the number of turns in the coil is decreased, the voltage induced across the coil will
Decrease
Increase
Remain constant
Be excessive
Answer.1. Decrease
Voltage induced in the coil is given by
E = N (dφ/dt)
So the induced voltage is directly proportional to the number of turns therefore if the number of turns is decreased then the induced voltage also decreased.
Also, the use of a core material of high permeability will increase an electromagnet’s strength.
Ques.17. A keeper is used to
Provide a closed path for flux
Restore lost flux
Amplify flux
Change the direction of magnetic lines
Answer.1. Provide a closed path for flux
To keep a magnet strong, place a keeper between its poles when it is not in use. A keeper is a piece of soft iron. The poles magnetize the keeper, which in turn keeps the poles’ domains pointing in the same direction.
Ques.18. Two infinitely long parallel conductors in a vacuum and separated 1 meter between centers with one-ampere current flowing in each of the two infinitely long parallel conductors current produce on each other a force of
2 × 10−5
2 × 107
3 × 10−5
2 × 10−7
Answer.4. 2 × 10−7
The force between two infinite long parallel conductors is given as
F = (μoI1I2l)/2πd
Where
μo = Vacuum Permittivity = 4π × 10−7
I1 & I2 = current is conductor 1 and 2
l = Length
d = distance
Now
F = (4π × 10−7I1I2l)/2πd
F = 2 × 10−7I1I2l/d
If I1 = I2 =,d = 1
then
F = 2 × 10−7N/m
Hence, we can define one ampere current as that current which when flowing in each of the two infinitely long parallel conductors situated in a vacuum and separated 1 metre between centres, produces on each conductor a force of 2 x 10−7 Newton per meter length.
Ques.19. The unit of flux is the same as that of
Reluctance
Resistance
Permeance
Pole strength
Answer.4. Pole strength
The unit of pole strength is found to have the same dimensions as the unit of magnetic flux i.e weber. The unit of magnetic pole strength .
The unit of flux is weber.
Magnetic Pole strength can be defined as force acting per magnetic field, which is m = F/B.
The magnetic pole strength (sometimes called the ‘magnetic field intensity’) at a point in a magnetic field is defined as being the force per unit N pole at that point. Thus, magnetic field strength is defined in such a way that, if the end of a magnetic needle with pole of strength p were to be placed at a point in a magnetic field, the force experienced by the pole of the needle
H = Pole strength/Area
Hence pole strength can also be defined as the force per unit field intensity (H) and its S.I unit is weber.
Ques.20. A permanent magnet
Attracts all paramagnetic substances and repels others
Attracts some substances and repels others
Attracts only ferromagnetic substances
Attracts ferromagnetic substances and repels all others
Answer.2. Attracts some substances and repels others
Permanent magnets attract all magnetic material and some substance because some materials have either has small magnetic moments to begin with, or they have atoms whose magnetic moments do not easily change direction.
