Quiz: Electrical Engineering

Exam: DFCCIL

Topic: Magnetic circuit

Each question carries 1 mark.

Negative marking: 1/4 mark

Time: 10 Minute

Q1. An emf of 16 volts is induced in a coil of inductance 4 H. The rate of change of current must be

(a) 4 A/s

(b) 16 A/s

(c) 32 A/s

(d) 64 A/s

Q2. In case of all flux from the current in coil 1 links with coil 2, the coefficient of coupling will be

(a) Zero

(b) 0.5

(c) 1.0

(d) 2.0

Q3. The magnetic field due to an infinite linear current carrying conductor is

(a) H = μi/2πr A/m.

(b) H = i/2πr A/m.

(c) H = μi/(2r ) A/m

(d) H = i/r A/m

Q4. Area of hysteresis loop represents

(a) copper loss

(b) eddy current loss

(c) dielectric loss

(d) Energy loss to magnetize and demagnetize the core

Q5. Two coils have inductances of 8 mH and 18 mH and a co-efficient of coupling of 0.5. If

the two coils are connected in series aiding, the total inductance will be

(a) 32 mH

(b) 38 mH

(c) 40 mH

(d) 48 mH

Q6. The energy in joules stored in the magnetic field of 0.15 H inductance with a 180mA

current will be

(a) 2.43

(b) 2.43×10^(-3)

(c) 2.43×10^(-6)

(d) 2.43×10^(-9)

Q7. What is the inductive reactance if the Q of a coil is 60, and the winding resistance is 5 Ω?

(a) 12 Ω

(b) 300 Ω

(c) 30 Ω

(d) 0.083 Ω

Q8. Inductance is:

(a) directly proportional to the length of the coil

(b) directly proportional to the number of turns on the coil

(c) inversely proportional to the cross-sectional area of the coil

(d) inversely proportional to the permeability

Q9. What is the total inductance, assuming no mutual inductance?

(a) 0 mH

(b) 6 mH

(c) 12 mH

(d) 0.73 mH

Q10. Mutual inductance between two magnetically coupled coils depends on

(a) Permeability of the core material

(b) Number of turns of the coils

(c) The cross-sectional area of their common core

(d) All of the above

SOLUTIONS

S1. Ans.(a)

Sol. e=L dI/dt

dI/dt=(induced emf)/inductance=16/4=4 A/s

S2. Ans.(c)

Sol. If all flux from the current in coil 1 links with coil 2, then that is referred as an ideal coupling. For an ideal coupling, the coefficient coupling is 1. Any other coupling the coupling co-efficient is always less than 1.

S3. Ans.(b)

Sol. Magnetic field, H=i/2πr A/m

S4. Ans.(d)

Sol. Energy loss due to magnetize and demagnetize the core.

S5. Ans.(b)

Sol. L_1=8 mH L_2=18 mH

K= 0.5

Total inductance

=L1+L2+2M

where; M=K√(L1*L2 )

Total inductance=8+18+2 X 0.5√144

=26+12=38 mH

S6. Ans.(b)

Sol. Energy stored in magnetic field

=1/2 LI²

=1/2×0.15×(0.18)^2

=2.43×10^(-3) J

S7. Ans.(b)

Sol. Q=XC/R=XC/5=60

∴Inductive reactance=XC=60×5=300 Ω

S8. Ans.(b)

Sol. inductance of a coil (L)=(N^2µA)/l

S9. Ans.(c)

Sol. for series connection of inductance: Leqv=L1+L2+L3=1+5+6=12 mH

S10. Ans.(d)

Sol. The expression of the mutual inductance between two magnetically coupled coils is as follows;

Where, μr = Relative permeability of core N = The number of the turns of the coil A = Cross sectional area of their common core Hence, the mutual inductance depends on all the above factors.