Ques 20. In a balanced 3-phase system, the current coil of a wattmeter is inserted in line 1 and the potential coil across 2 and 3. If the wattmeter reads 100 W, the reactive power drawn by the 3-phase load is
173.2 VAR✔
50 VAR
100 VAR
141. 4 VAR
Reactive power drawn by the 3-phase load is
Q = √3 × reading of wattmeter
Q = √3 × 100
173.21 VAR
Ques 21. An electric iron is rated at 230 V, 400 W, 50 Hz. The voltage rating 230 V refers to
RMS value✔
Peak-to-Peak Value
Average Value
Peak Value
In the AC system, we can’t take the average value because the average value is equal to zero over a period in AC.
So now to calculate different parameters like voltage, current, etc we need the RMS value. This is the reason why we use RMS quantities while dealing with AC.
Ques 22. A sinusoidal periodic waveform is free from DC components, cosine components, and even harmonics. The waveform has
Half wave and odd function symmetry✔
Half wave and even function symmetry
Only odd function symmetry
Only half-wave symmetry
Half-wave symmetry implies that the second half of the wave is exactly opposite to the first half.If a waveform has half-wave symmetry, only odd (odd cosine and odd harmonics will be present and DC offset is zero, as one half must exactly cancel out the other. In other words, all even (even cosine and even sine) harmonics will be zero.
Ques 23. A periodic train of rectangular pulses x(t) with a time period of 25 seconds has a pulse width of 9 seconds as shown in the figure. The RMS value of the waveform is
10 V
√6 V
3.6 V✔
2.16 V
The RMS value of the waveform is
XRMS = Xmax(Ton/T)1/2
6(9/25)1/2
3.6 V
Ques 24. The time constant of the network shown in the figure is
Ques 25. In the series RC circuit, the voltage across C starts increasing, the moment the circuit is switched to V volts DC. The rate of increase of voltage across C at the instant just after the switch is closed (i.e at t = 0t)
RV/C
CV/R
V/RC✔
R/CV
Consider an RC circuit at time t =0
The voltage across the capacitor at any time t
Ques 26. Form factor of an alternating wave is
Form factor = average value/RMS value
Form factor = (RMS value)2/Average value
Form Factor = RMS value/Average value✔
Form factor = RMS value × Average value
In Electrical, the form factor of an alternating current waveform (signal) is the ratio of the RMS (root mean square) value to the average value.
Ques 27. The phase difference between the following voltage and current waves
V = 311 sin(100πt + 30°) volts and i = 17 sin(100πt – 20°)
20°
50°✔
10°
30°
Phase difference Φ = Φ1 – Φ2
=30 -(-20) = 50°
Ques 28. Match the items given in List-I (Material) and those in List-II (Temperature Coefficient of Resistance). Select your answers using codes given in the lists
a → R, b → Q, c → P
a → Q, b → P, c → R
a → P, b → Q, c → R
a → R, b → P, c → Q✔
Temperature coefficient of aluminium is + 0.0034 (positive)
The temperature coefficient of manganin is 0.000002 (negligibly small)
The temperature coefficient of carbon is −0.0005 (negative)
Ques 29. Which of the following material possesses the least resistivity?
Copper✔
Manganin
Aluminum
Iron
The resistivity of the copper is 1.68×10−8
The resistivity of the Manganin is 4.82×10−7
The resistivity of the Aluminium is 2.65×10−8
The resistivity of the Iron is 9.71×10−8
So copper has the least resistivity than Iron, manganin, and aluminum
Ques 30. The resistances of insulations, in general, _____with temperature rise.
Decrease✔
Increase rapidly
Increase slowly
Does not change
In case of an insulator, there is a large energy gap between the two bands. So, if the temperature rise is high, the electrons will go to the upper band. But since it is not crowded due to less number of electrons, so the conductance will increase due to the availability of free electrons in the conduction band. Hence, the resistance decreases with an increase in temperature in an insulator.
