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UPRVUNL’21 EE: Daily Practices Quiz 13-July-2021

Each question carries 1 mark.
Negative marking: 1/4 mark
Total Questions: 06
Time: 08 min.

Q1. Two incandescent light bulbs of 50 W and 60 W rating are connected in series across the mains, then
(a) The bulb together consumes 55 W
(b) The bulb together consumes 110 W
(c) The 50 W bulb glows brighter
(d) The 60 W bulb glows brighter

Q2. The average value of the positive half of the sine wave current having peak value I_m will be:
(a) (2I_m)/π
(b) I_m/π
(c) I_m/1.414
(d) I_m/1.11

Q3. What is the value of charging current in short transmission lines?
(a) Less than medium transmission lines
(b) Equal to medium transmission lines
(c) More than medium transmission lines
(d) More than long transmission lines

Q4. For induction motors at low values of slip:
(a) The torque slip is approximately a hyperbola
(b) The torque slip is approximately a straight line
(c) The torque slip curve is at its maximum
(d) The torque slip is approximately a parabola

Q5. Dummy coils are provided in the generator in order to
(a) Reduce eddy current losses
(b) Amplify voltage
(c) Provide mechanical balance for the rotor
(d) Enhance flux density

Q6. Which of the following high resistance materials have the highest operating temperature?
(a) Kanthal
(b) Manganin
(c) Nichrome
(d) Eureka

SOLUTIONS
S1. Ans.(c)
Sol. As we know, P=V^2/R
In this the rated voltage of both bulbs are fixed.
∴, P α 1/R⇒R α 1/P
So, lower rating bulb glows brighter i.e., 50 W bulb.
NOTE:
For parallel connection, higher rating bulb glows brighter.
For series connection, P_T=(P_1 P_2)/(P_1+P_2 )

S2. Ans.(a)
Sol. The average value of the positive half (0-T/2) of the sine wave current having peak value I_m will be: I_avg=(2I_m)/π

S3. Ans.(a)
Sol. Line to Earth capacitance of short transmission line is less than that in medium and long transmission line and is negligible. Because of this the charging current of short transmission line is less than that in medium and long transmission line and is negligible.

S4. Ans.(b)
Sol. For induction motor –
T=(KS E₂²R₂)/(R₂²+(S×_2)²)
at low value of slip, S×_2≪R₂
⇒ T=(KSE₂²R₂)/(R₂²)
⇒ T∝S

S5. Ans.(c)
Sol. Dummy coils are provided in the generator in order to provide mechanical balance to the rotor.

S6. Ans.(a)
Sol. Kanthal is the trademark for a family of iron-chromium-aluminium (FeCrAl) alloys used in a wide range of resistance and high-temperature applications. Kanthal FeCrAl alloys consist of mainly iron, chromium (20–30%) and aluminium (4–7.5 %).
NOTE:
Max. operating temperature of Kanthal is approx. 1200⁰C.
Max. operating temperature of Nichrome is approx. 1150⁰C.

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