11. Among the following, which is the right formula for inductance?
A. L = emf × t/I
B. L = emf/t × I
C. L = emf × I/t
D. L = emf × t × I
Answer: A
The average emf induced is proportional to the current per unit time, the constant of proportionality being L.
Hence emf = LI/t. Making L the subject of the formula, we get
L = emf × t/I.
12. Among the following, which is the right formula for the inductance of N turns?
A. L = et/Ni
B. L = N × I × e × t
C. L = Ni/et
D. L = N/iet
Answer: A
We know that:
emf = NLi/t
Inductance of N turns = L = et/N.
13. For a coil having a magnetic circuit of constant reluctance, the flux is ___________ to the current.
A. Directly proportional
B. Inversely proportional
C. Not related
D. Very large compared to
Answer: A
For a coil having a magnetic circuit of constant reluctance, the flux is directly proportional to the current.
14. For a coil having a magnetic circuit of constant reluctance, if the flux increases, what happens to the current?
A. Increases
B. Decreases
C. Remains constant
D. Becomes zero
Answer: A
For a coil having a magnetic circuit of constant reluctance, the flux is directly proportional to the current. Hence as the flux increases, the current also increases.
15. The unit for inductance is _________
A. Ohm
B. Henry
C. A/m
D. A/s
Answer: B
The unit of induction is named after a famous scientist Joseph Henry who independently discovered electromagnetic induction.
16. If either the inductance or the rate of change of current is doubled, the induced e.m.f?
A. Remains constant
B. Becomes zero
C. Doubles
D. Becomes half
Answer: C
If either the inductance or the rate of change of current is doubled, the induced e.m.f. becomes double because of emf = LI/t.
17. If the current changes from 5A to 3A in 2 seconds and the inductance is 10H, calculate the emf.
A. 5V
B. 10V
C. 15V
D. 20V
Answer: B
We know that:
emf = L(i2 − i1)/t
Substituting the values from the question, we get emf = 10V.
18. If the current changes from 5A to 3A in x sec and inductance is 10H. The emf is 10V, calculate the value of x.
A. 2s
B. 3s
C. 4s
D. 5s
Answer: A
We know that:
emf = L(i2 − i1)/t
Substituting the values from the question, we get x = 2s.
19. If the current changes from 3A to 5A in 2s and the emf is 10V. Calculate the inductance.
A. 10H
B. 20H
C. 30H
D. 40H
Answer: A
We know that:
emf = L(i2 − i1)/t
Substituting the values from the question,
10 = L(5 − 3)/2
L = 10 H
20. As the number of turns in the coil increases, what happens to the inductance of the coil?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: A
Inductance is directly proportional to the square of the number of turns in the coil, hence as the number of turns increases, inductance also increases.
21. What happens to the inductance when the magnetic field strength decreases?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: B
Inductance is directly proportional to the magnetic field strength in the coil, hence as the magnetic field strength decreases, inductance decreases.
22. What happens to the inductance when the current in the coil becomes double its original value?
A. Becomes half
B. Becomes four times
C. Becomes double
D. Remains same
Answer: D
ϕ is directly proportional to i.
Φ = Li
where
L is the constant of proportionality
So, when the current gets double ϕ also becomes double keeping L the same.
23. When the coil is wrapped around a ferromagnetic core, why is it difficult to determine the inductance?
A. The variation of flux is no longer proportional to the variation of current
B. Current does not exist in the coil
C. Flux does not exist in the coil
D. The value of the current is too large to measure
Answer: A
When a coil is wrapped around a ferromagnetic core, it is difficult to determine the inductance because the variation of flux is no longer proportional to the variation of the current.
24. What happens to the inductance as the area of the cross-section of the coil increases?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: A
Inductance is given as
L = µ0 × N2 × A/l,
hence as the area of cross section A increases, the inductance also increases.
25. What happens to the inductance as the length of the magnetic circuit increases?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: B
Inductance L is given as
L = µ0 × N2 × A/l
hence as the length of the magnetic circuit l increases, the inductance decreases.
26. If the current changes from 20A to 10A in 5 seconds and the value of inductance is 1H, calculate the emf induced.
A. 8 V
B. 6 V
C. 4 V
D. 2 V
Answer: D
We know that emf is given as:
emf = L(i2 − i1)/t
Substituting the values from the question we get emf = 2V.
27. When a ferromagnetic core is inserted into an inductor, what happens to the flux linkage?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: A
When a ferromagnetic core is introduced into an inductor, its flux increases because the number of magnetic field lines increases due to the introduction of the magnetic field within the coil.
28. What happens to the current when a ferromagnetic material is introduced within an inductor?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: C
When a ferromagnetic is introduced within an inductor, the current remains fairly constant. This is because the current does not depend on the magnetic field.
29. What is the relation between the flux and the magnetizing current when a ferromagnetic core is introduced within the inductor?
A. Directly proportional
B. Inversely proportional
C. Not proportional
D. Current is double of flux
Answer: C
When a ferromagnetic core is introduced within an inductor the flux changes rapidly, whereas the current changes at the same pace. Hence the two are not proportional.
30. What happens to the effective inductance when a ferromagnetic core is introduced?
A. Increases
B. Decreases
C. Remains the same
D. Becomes zero
Answer: A
The effective inductance increases when a ferromagnetic core is introduced within an inductor because the flux increases. Inductance varies directly with the flux hence it increases.
