# Objective Questions on Inductor | Page – 4

1. The unit of inductive susceptance is

The unit of inductive susceptance is Simense. Susceptance (symbolized with B) is the opposite of reactance. Inductive susceptance becomes exactly like the formula for capacitive reactance, except that it of course uses inductorrather than capacitance.
$B_L = \frac{1}{ 2 \pi f L}$

2. An inductor supplied with 100V with a frequency of 10 kHz and passes a electric current of 15.92 mA. The value of inductor is

Here the supply voltage is 100 V and electric current through the inductor is 15.92 mA
∴ impedance of the inductor is
$Z_L = \frac{100}{15.92 \times 10^{-3}} \Omega\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot(1)$
Expression for value of inductive impedance is given by ZL = 2πfL
Here, frequency f = 10 KHz, hence,
$Z_L = 2\pi 10000L\Omega\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot(2)$
Now, comparing, equation (1) and (2) we get,
$\frac{100}{15.92 \times 10^{-3}}=2\pi \times 10000L\Rightarrow L = 10^{-1}=100\;mH$

3. Which of the following case represents the largest mmf ?

The mmf of any magnetic circuit is measured as a product of number of turns in the coil and electric current passing through that coil. Hence if I electric current flows through N number of turns in a coil the mmf will be NI.

4. A 100 mH inductor is connected across a supply for 50 V AC. For which of the following frequency the circuit will have least rms current?

The impedance of a inductor is directly proportional to its supply frequency. RMS value of the electric current through the inductor is supply voltage/impedance. So, it can be concluded that electric current in inductor is inversely proportional to its supply frequency.

5. A 200 mH inductor is connected across a supply for 100 V AC. For which of the following frequency the circuit will have highest rms current?

The impedance of a inductor is directly proportional to its supply frequency. RMS value of the electric current through the inductor is supply voltage/impedance. So, it can be concluded that electric current in inductor is inversely proportional to its supply frequency.

6. When ac flows through a pure inductor then the current

When ac flows through an inductance, the electric current lags the emf by 90°.

7. The reactance of an inductor of 1 / π Henry at 50 Hz is

Inductive reactance of an inductor is
$X_L = 2\pi fL \;\Omega$
$Here,\; f = 50 Hz,\; L =\frac{ 1}{\pi}\; Henry\;\Rightarrow X_L = 2\pi 50\frac{1}{\pi} \Omega = 100 \Omega$

8. The unit of inductor is Henry. It can also be represented as

We know the induced voltage in an inductive circuit of inductor L, is
$V = L\frac{di}{dt}$
Where, L is proportional constant known as inductance
From above expression we get,
$L = V\frac{dt}{di}$
So, from above equation, it can be concluded that, unit of inductor henry is volt - sec /A

9. The energy stored in an inductor of inductor L Henry is represented as,

The voltage in an inductor is given as
$v=L\frac{di}{dt}\;and\;Power=vi\;and\;Energy=\int vidt=\int Li\frac{di}{dt}dt=\frac{1}{2}Li^2$

10. The instantaneous power in an inductor is proportional to the

$V_L\;=\;L\frac{di}{dt}\;and\;\;Power\;=\;Vi\;=\;Li\frac{di}{dt}$

11. The voltage induced in an inductor of L Henry is represented as,

$Voltage\; across\; inductor,\; V_L\;=\;L\frac{di}{dt}$

12. Statement 1 :- Inductor doesn’t accept sudden changes in current.
Statement 2 :- Inductor doesn’t accepts sudden changes in voltage.

In order to accept sudden changes in electric current it requires infinite energy, infinite power and infinite voltage those are not desirable. So inductor doesn’t accept sudden changes in current.

13. Which energy is stored in inductor and capacitor?

The property of inductor is which stores magnetic field energy and electric field energy stored by capacitor.

14. A circuit is having inductor, switch (initial at open) and it is connected to supply. After some time switch is closed then at time t = 0 + how inductor behaves ?

In case of inductor electric current through it does not change instantaneously. If the initial conditions are zero, at the time of closing the switch for connecting an inductor to an energy source, the inductor will behave like an open circuit i.e., no electric current will flow at t = 0 +.

15. The strength of electric current in 1 henry inductor changes at a rate of 2 A / sec. Find the voltage across it & determine the magnetic of energy stored in the inductor after 2 secs?

$Here,\;L = 1 H,\;\frac{di}{dt} = 2\; A/secs,$
$Voltage\; across\; the\; inductor,\; V = L\frac{di}{dt} = 1 \times 2 = 2 V,$
$The\; energy\; store\; (W) = \frac{1}{2}Li^2 = \frac{1}{2}\times 2 \times 2^2 = 4\; Joules$

16. The switch is closed at time t=0, then the voltage across the inductor is given by

At the time t = 0+, inductor will acts as open circuit. Hence voltage across the inductor is V0.

17. Two inductances are in series their equivalent will be

The equivalent induactance when 2 inductances are connected in series, is equal to their sum which is always like resistances.

18. Property of pure inductor is

Pure inductor only stores energy but does not dissipate it.

19. The max value of mutual inductor of two inductively coupled coils with self inductor L1 = 49 mH & L2 = 81 mH

$M=\sqrt{L_1L_2}=\sqrt{40\times 81}=63\; mH$

20. A coil has an inductive reactance of 4 ohm and a resistance of 3 ohm the admittance of the coil is

$Impedance,\; Z = 3 + j4 = \sqrt{9+16} = 5$
$Y = g -ib,\; g =\frac{ R}{Z^2} =\frac{ 3 }{ 5^2} = 0.12 \;ohm\;and\;b = \frac{X_L}{Z^2} =\frac{ 4}{ 5^2} = 0.16 \;ohm$
Correct answer is 0.12 – i 0.16.