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Multiple choice question for engineering

Set 1

1. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the time constant.
a) 1s
b) 2s
c) 3s
d) 4s

View Answer

Answer: d [Reason:] The time constant is the product of the resistance and capacitance in a series RC circuit. Therefore, time constant= 8*10-6*4*106=4s.

2. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the initial charging current.
a) 100 microA
b) 500 microA
c) 400 microA
d) 1000microA

View Answer

Answer: c [Reason:] In a series RC circuit, the initial charging current is: I=V/R= 200/(0.5*106s)= 400*10-6A= 400 microA.

3. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the time taken for the potential difference across the capacitor to gow to 160V.
a) 6.44s
b) 7.77s
c) 2.33s
d) 3.22s

View Answer

Answer: a [Reason:] From the previous explanations, we know that the initial current is 400mA and the time constant is 4s. Substituting the values of capacitor voltage, initial voltage, initial current and time constant in the equation: Vc=V(1-e-t/RC), we get t=6.44s.

4. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the voltage in the capacitor 4s after the power is supplied.
a) 123.4V
b) 126.4V
c) 124.5V
d) 132.5V

View Answer

Answer: b [Reason:] We can get the value of the potential difference across the capacitor in 4s, from the following equation: Vc=V(1-e-t /RC). Substituting the values in the given equation, we get Vc= 126.4V.

5. An 8microF capacitor is connected in series with a 0.5 megaohm resistor. The DC voltage supply is 200V. Calculate the current in the capacitor 4s after the power is supplied.
a) 150 microA
b) 149 microA
c) 148 microA
d) 147 microA

View Answer

Answer: d [Reason:] In the given question, the time constant is equal to the time taken= 4s. Hence the value of current will be 63% of its initial value= I=0.63*400= 147 microA.

6. The discharging time constant of a circuit consisting of a capacitor is the time taken for the charge in the capacitor to become __________% of the initial charge.
a) 33
b) 63
c) 37
d) 36

View Answer

Answer: c [Reason:] We know that: Q=Q0(e-t /RC). When RC=t, we have: Q=Q0(e-1)= 0.37*Q0. Hence the time constant is the time taken for the charge in a capacitive circuit to become 0.37 times its initial charge.

7. The discharging time constant of a circuit consisting of a capacitor is the time taken for the charge in the capacitor to become __________% of the initial charge.
a) 33
b) 63
c) 37
d) 36

View Answer

Answer: c [Reason:] We know that: Q=Q0(e-t /RC). When RC=t, we have: Q=Q0(e-1)= 0.37*Q0. Hence the time constant is the time taken for the charge in a capacitive circuit to become 0.37 times its initial charge.

8. A circuit has a resistance of 2 ohm connected in series with a capacitance of 6F. Calculate the discharging time constant.
a) 3
b) 1
c) 12
d) 8

View Answer

Answer: c [Reason:] The discharging time constant in a circuit consisting of a capacitor and resistor in series is the product of the resistance and capacitance= 2*6=12.

9. What is the value of current in a discharging capacitive circuit if the initial current is 2A at time t=RC.
a) 0.74A
b) 1.26A
c) 3.67A
d) 2.89A

View Answer

Answer: a [Reason:] At time t=RC, that is the time constant, we know that the value of current at that time interval is equal to 37% of the initial charge in the discharging circuit. Hence, I=2*0.37= 0.74A.

10. While discharging, what happens to the current in the capacitive circuit?
a) Decreases linearly
b) Increases linearly
c) Decreases exponentially
d) Increases exponentially

View Answer

Answer: c [Reason:] The equation for the value of current in a discharging capacitive circuit is: I=I0*e-t /RC. From this equation, we can see that the current is exponentially decreasing since e is raised to a negative power.

Set 2

1. 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

View Answer

Answer: a [Reason:] 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.

2. Among the following, which is the right formula for inductance?
a) L=NΦ/t
b) L=NΦt
c) L=N/Φt
d) L=NΦ2t

View Answer

Answer: a [Reason:] We know that: emf=L(i2-i1)/t Inductance = L=NΦ/t.

3. 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

View Answer

Answer: a [Reason:] For a coil having a magnetic circuit of constant reluctance, the flux is directly proportional to the current.

4. 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

View Answer

Answer: a [Reason:] 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.

5. The unit for inductance is ___________
a) Ohm
b) Henry
c) A/m
d) A/s

View Answer

Answer: b [Reason:] The unit of induction is named after a famous scientist Joseph Henry who independently discovered electromagnetic induction.

6. 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

View Answer

Answer: c [Reason:] If either the inductance or the rate of change of current is doubled, the induced e.m.f. becomes double because, emf=LI/t.

7. 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

View Answer

Answer: b [Reason:] We know that: emf=L(i2-i1)/t Substituting the values from the question, we get emf=10V.

8. 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

View Answer

Answer: a [Reason:] We know that: emf=L(i2-i1)/t Substituting the values from the question, we get x=2s.

9. 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

View Answer

Answer: a [Reason:] We know that: emf=L(i2-i1)/t Substituting the values from the question we get L=10H.

10. If the current changes from 20A to 10A in 5 seconds and the value of inductance is 1H, calculate the emf induced.
a) 8V
b) 6V
c) 4V
d) 2V

View Answer

Answer: d [Reason:] We know that: emf=L(i2-i1)/t Substituting the values from the question we get emf=2V.

Set 3

1. Inductor does not allow sudden changes in?
a) Voltage
b) Current
c) Resistance
d) Inductance

View Answer

Answer: b [Reason:] The inductor does not allow sudden changes in current because if current changes in the inductor occur in zero time, the voltage becomes zero which is not possible.

2. In case of Inductive circuit, Frequency is ______________ to the current.
a) Directly proportional
b) Inversely proportional
c) Unrelated
d) Much greater than

View Answer

Answer: b [Reason:] Inductance is inversely proportional to current since, as the inductance increases, current decreases.

3. If the resistance in a circuit is 2 ohm and the inductive resistance is 20 ohm, calculate the power factor.
a) 10
b) 20
c) 30
d) 40

View Answer

Answer: a [Reason:] We know that: cos(phi)=R/XL From the given question, we find that the power factor is 10.

4. If the power factor is 10 and the resistance is 2 ohm, calculate the inductive reactance.
a) 10 ohm
b) 20 ohm
c) 30 ohm
d) 40 ohm

View Answer

Answer: b [Reason:] We know that: cos(phi)=R/XL From the given question, we find that the inductive reactance is 20 ohm.

5. An induced emf is said to be ___________
a) Inductive
b) Capacitive
c) Resistive
d) Cannot be determined

View Answer

Answer: a [Reason:] Any circuit in which a change of current is accompanied by a change of flux, and therefore by an induced emf, is said to be inductive.

6. In a pure inductive circuit, the power factor is?
a) Maximum
b) Minimum
c) 0
d) Infinity

View Answer

Answer: c [Reason:] In a pure inductive circuit, current is lagging by 90 degrees from the voltage. The power factor is the cosine of the angle in between the voltage and the current. If the angle between the voltage and current is 90, then cos90=0. Hence, the power factor is zero.

7. 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

View Answer

Answer: a [Reason:] 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.

8. Among the following, which is the right formula for inductance?
a) L=NΦ/t
b) L=NΦt
c) L=N/Φt
d) L=NΦ2t

View Answer

Answer: a [Reason:] The average emf induced in a coil is L=NΦ/t.

9. If the current in a coil having a constant inductance of L henrys grows at a uniform rate, what is the value of the average current?
a) I
b) I/2
c) I/4
d) 0

View Answer

Answer: b [Reason:] The average current is the average of the current which flows in the inductor. Hence it is I/2.

10. Find the average current in an inductor if the total current in the inductor is 30A.
a) 10A
b) 26A
c) 15A
d) 5A

View Answer

Answer: c [Reason:] Average current= I/2. Substituting the value of I from the equation, average current= 13A.

Set 4

1. In case of Inductive circuit, Frequency is ______________ to the inductance.
a) Directly proportional
b) Inversely proportional
c) Unrelated
d) Much greater than

View Answer

Answer: a [Reason:] The formula for frequency in an inductive circuit is: XL=2*pi*f*L. Therefore: XL is directly proportional to f.

2. If the current and voltage are 90 degree out of phase, the power factor will be?
a) 0
b) Infinity
c) 1
d) Insufficient information provided

View Answer

Answer: a [Reason:] The power factor is the cosine of the angle in between the voltage and the current. If the angle between the voltage and current is 90, then cos90=0. Hence, the power factor is zero.

3. If the power factor is 10 and the value of inductive reactance is 20 ohm, calculate the resistance in the circuit.
a) 1 ohm
b) 2 ohm
c) 3 ohm
d) 4 ohm

View Answer

Answer: b [Reason:] We know that: cos(phi)=R/XL From the given question, we find that the resistance in the circuit is 2 ohm.

4. What is the unit for inductive reactance?
a) Henry
b) Ohm
c) Farad
d) Volts

View Answer

Answer: b [Reason:] Inductive reactance is nothing but the impedance. Impedance is the AC equivalent of resistance, hence the unit for inductive reactance is ohm.

5. If the current in a coil having a constant inductance of L henrys grows at a uniform rate, what is the value of the average current?
a) I
b) I/2
c) I/4
d) 0

View Answer

Answer: b [Reason:] The average current is the average of the current which flows in the inductor. Hence it is I/2.

6. Find the average current in an inductor if the total current in the inductor is 26A.
a) 10A
b) 26A
c) 13A
d) 5A

View Answer

Answer: c [Reason:] Average current= I/2. Substituting the value of I from the equation, average current= 13A.

7. Calculate the emf induced in an inductor if the inductance is 10H and the current is 2A in 4s.
a) 2.5V
b) 1.5V
c) 3.5V
d) 5V

View Answer

Answer: a [Reason:] The expression for emf in an inductive circuit is: emf= LI/2t Substituting the values from the given question, we get emf= 2.5V.

8. Calculate the current in an inductor if the energy stored is 160J and the inductance is 20H.
a) 1A
b) 2A
c) 3A
d) 4A

View Answer

Answer: d [Reason:] The expression for energy in an inductor is: W= LI2/2t Substituting the values from the given question, we get I=4A.

9. Find the time taken for the current in an inductor to change to 2A from 0A if the power in the inductor is 5W. The value of inductance is 10H.
a) 1s
b) 2s
c) 3s
d) 4s

View Answer

Answer: d [Reason:] The expression for power in an inductive circuit is: P= LI2/2 Substituting the values from the given question, we get t=4s.

10. An induced emf is said to be?
a) Inductive
b) Capacitive
c) Resistive
d) Cannot be determined

View Answer

Answer: a [Reason:] Any circuit in which a change of current is accompanied by a change of flux, and therefore by an induced emf, is said to be inductive.

Set 5

1. The instantaneous voltage is a product of the resistance and the _____________ current in a resistive circuit.
a) Instantaneous
b) Average
c) RMS
d) Peak

View Answer

Answer: a [Reason:] The instantaneous voltage is a product of the instantaneous current and the resistance in the circuit.

2. Find the value of the instantaneous voltage if the resistance is 2 ohm and the instantaneous current in the circuit is 5A.
a) 5V
b) 2V
c) 10V
d) 2.5V

View Answer

Answer: c [Reason:] We know that, v=iR, substituting the given values from the question, we get v=10V.

3. The power for a purely resistive circuit is zero when?
a) Current is zero
b) Voltage is zero
c) Both current and voltage are zero
d) Either current or voltage is zero

View Answer

Answer: d [Reason:] The power in a resistive circuit is the product of the voltage, current and the cosine of the phase angle. Hence if either voltage or current is zero, the power is zero.

4. If the maximum voltage in the circuit is 10V and the resistance is 5 ohm, calculate the maximum current in the circuit.
a) 1A
b) 2A
c) 3A
d) 4A

View Answer

Answer: b [Reason:] We know that: Im=Vm/R Substituting the given values from the question, we get Im=2A.

5. Calculate the resistance in the circuit if the rms voltage is 20V and the rms current is 2A.
a) 2 ohm
b) 5 ohm
c) 10 ohm
d) 20 ohm

View Answer

Answer: c [Reason:] We know that: R=V/I Substituting the given values from the question, we get R=10 ohm.

6. The correct expression for the instantaneous current in a resistive circuit is?
a) i=Vm(sint)/R
b) i=Vm(cost)/R
c) i=V(sint)/R
d) i=V(cost)/R

View Answer

Answer: a [Reason:] The instantaneous voltage can be written in terms of the maximum voltage in the following manner: v=Vm(sint) Since i=v/R, we can write, i=Vm(sint)/R.

7. Can ohm’s law be applied in an ac circuit?
a) Yes
b) No
c) Depends on the rms current
d) Depends on the rms voltage

View Answer

Answer: a [Reason:] Ohm’s law can be applied in ac as well as dc circuits. It can be applied in ac circuits because the condition V=IR holds true even in ac circuits.

8. What is the current found by finding the current in n equidistant regions and dividing by n?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: b [Reason:] The average value of current is the sum of all the currents divided by the number of currents.

9. What is the effective value of current?
a) RMS current
b) Average current
c) Instantaneous current
d) Total current

View Answer

Answer: a [Reason:] Effective current is also known as the effective current. RMS stands for Root Mean Square. This value of current is obtained by squaring all the current values, finding the average and then finding the square root.

10. Find the average value of current when the current that are equidistant are 4A, 5A and 6A.
a) 5A
b) 6A
c) 15A
d) 10A

View Answer

Answer: a [Reason:] The average value of current is the sum of all the currents divided by the number of currents. Therefore average current= (5+4+6)/3=5A.

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