Electromagnetic Theory Questions and Answers – Inductances

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This set of Electromagnetic Theory Multiple Choice Questions & Answers (MCQs) focuses on “Inductance”.

1. Calculate the emf of a coil with turns 100 and flux rate 5 units.
a) 20
b) -20
c) 500
d) -500
View Answer

Answer: d
Explanation: The emf is the product of the turns of the coil and the flux rate. Thus e = -N dφ/dt, where the negative sign indicates that the emf induced is opposing the flux. Thus e = -100 x 5 = -500 units.
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2. The equivalent inductances of two coils 2H and 5H in series aiding flux with mutual inductance of 3H is
a) 10
b) 30
c) 1
d) 13
View Answer

Answer: d
Explanation: The equivalent inductance of two coils in series is given by L = L1 + L2 + 2M, where L1 and L2 are the self inductances and M is the mutual inductance. Thus L = 2 + 5 + 2(3) = 13H.

3. The expression for the inductance in terms of turns, flux and current is given by
a) L = N dφ/di
b) L = -N dφ/di
c) L = Niφ
d) L = Nφ/i
View Answer

Answer: a
Explanation: We know that e = -N dφ/dt and also e = -L di/dt. On equating both we get, L = Ndφ/di is the expression for inductance.
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4. The equivalent inductance of two coils with series opposing flux having inductances 7H and 2H with a mutual inductance of 1H.
a) 10
b) 7
c) 11
d) 13
View Answer

Answer: b
Explanation: The equivalent inductance of two coils in series with opposing flux is L = L1 + L2 – 2M, where L1 and L2 are the self inductances and M is the mutual inductance. Thus L = 7 + 2 – 2(1) = 7H.

5. A coil is said to be loosely coupled with which of the following conditions?
a) K>1
b) K<1
c) K>0.5
d) K<0.5
View Answer

Answer: d
Explanation: k is the coefficient of coupling. It lies between 0 and 1. For loosely coupled coil, the coefficient of coupling will be very less. Thus the condition K<0.5 is true.
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6. With unity coupling, the mutual inductance will be
a) L1 x L2
b) L1/L2
c) √(L1 x L2)
d) L2/L1
View Answer

Answer: c
Explanation: The expression for mutual inductance is given by M = k √(L1 x L2), where k is the coefficient of coupling. For unity coupling, k = 1, then M = √(L1 x L2).

7. The inductance is proportional to the ratio of flux to current. State True/False.
a) True
b) False
View Answer

Answer: a
Explanation: The expression is given by L = Ndφ/di. It can be seen that L is proportional to the ratio of flux to current. Thus the statement is true.
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8. Calculate the mutual inductance of two tightly coupled coils with inductances 49H and 9H.
a) 21
b) 58
c) 40
d) 49/9
View Answer

Answer: a
Explanation: For tightly coupled coils, the coefficient of coupling is unity. Then the mutual inductance will be M = √(L1 x L2)= √(49 x 9) = 21 units.

9. Find the inductance of a coil with turns 50, flux 3 units and a current of 0.5A
a) 150
b) 300
c) 450
d) 75
View Answer

Answer: b
Explanation: The self inductance of a coil is given by L = Nφ/I, where N = 50, φ = 3 and I = 0.5. Thus L = 50 x 3/0.5 = 300 units.
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10. The inductance of a coaxial cable with inner radius a and outer radius b, from a distance d, is given by
a) L = μd ln(b/a)/2π
b) L = 2π μd ln(b/a)
c) L = πd/ln(b/a)
d) L = 0
View Answer

Answer: a
Explanation: The inductance of a coaxial cable with inner radius a and outer radius b, from a distance d, is a standard formula derived from the definition of the inductance. This is given by L = μd ln(b/a)/2π.

Sanfoundry Global Education & Learning Series – Electromagnetic Theory.
To practice all areas of Electromagnetic Theory, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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