21. What is the time constant of an inductive circuit?
A. LR
B. R/L
C. 1/LR
D. L/R
Answer: D
The time constant in an inductive circuit is the time taken for the voltage across the inductor to become 63 percent of its initial value. It is given by Time constant = L/R.
22. Calculate the time constant of an inductive circuit having resistance 5 ohm and inductance 10H.
A. 2s
B. 4s
C. 5s
D.10s
Answer: A
We know that: Time constant = L/R
Substituting the values from the given question, we get a time constant = 2s.
23. Calculate the resistance in an inductive circuit whose time constant is 2s and the inductance is 10H.
A. 7ohm
B. 10ohm
C. 2ohm
D. 5ohm
Answer: D
We know that: Time constant = L/R
Substituting the values from the given question, we get R = 5ohm.
24. Calculate the inductance in an inductive circuit whose time constant is 2s and the resistance is 5 ohm.
A. 10H
B. 20H
C. 5H
D. 15H
Answer: A
We know that: Time constant = L/R
Substituting the values from the given question, we get L = 10H.
25. The charging time constant of a circuit consisting of an inductor is the time taken for the current in the inductor to become __________% of the initial current.
A. 33
B. 63
C. 37
D. 36
Answer: B
We know that: i = i0(1 − e − tR/L).
When t = L/R
we have: i = i0(1 − e − 1) = 0.63 × i0.
Hence the time constant is the time taken for the current in an inductive circuit to become 0.63 times its initial current.
26. What is the total applied voltage in an inductive circuit?
A. V = Ri+Ldi/dt
B. V = Ri+di/dt
C. V = i+Ldi/dt
D. V = R+Ldi/dt
Answer: A
The total voltage in an inductive circuit is the sum of the voltage due to the resistor which is Ri and the voltage due to the inductor which is Ldi/dt.
Hence V = Ri+Ldi/dt.
27. What is Helmholtz equation?
A. i = I(eRt/L)
B. i = I(1 − e − Rt/L)
C. i = I(1+e − Rt/L)
D. i = I(e − Rt/L)
Answer: B
Helmholtz equation is an equation that gives the formula for the growth in an inductive circuit. Hence the Helmholtz formula is: i = I(1 − e − Rt/L).
28. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the initial value of the current in the circuit.
A. 5A
B. 10A
C. 0 A
D. 20A
Answer: C
Initially, the inductor behaves as an open circuit for dc current so, i = 0.
29. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the final value of the current in the circuit.
A. 5A
B. 10A
C. 15A
D. 20A
Answer: A
The final value of the current in the circuit is:
I = V/R = 5A.
30. A coil has a resistance of 4 ohms and an inductance of 2H. It is connected to a 20V dc supply. Calculate the value of current 1s after the switch is closed.
A. 5.44A
B. 4.32A
C. 6.56A
D. 2.34A
Answer: B
We know that:
i = I(1 − eRt/L)
I = V/R = 5A
Substituting the remaining values from the given question, we get i = 4.32A.
