Ques.61. The resistances of insulations, in general, _____with temperature rise (SSC-2012)
Decrease
Increase rapidly
Increase slowly
Does not change
Answer.1. Decrease
Explanation:-
In the 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.
Ques.62. A 10 μF and a 20μF capacitor are in series. The combination is supplied at 150 V from a sinusoidal voltage source. The voltage across the 20μF capacitor is then (SSC-2012)
75 V
125 V
100 V
50 V
Answer.4. 50V
Explanation:-
The potential difference across the capacitor is inversely proportional to its capacitance in series combination
V ∝ 1/C
The voltage across 20 μf capacitor is
V20μf = 150 x C1/(C1 + C2)
= 1500/(10+20)
= 50 V
Ques.63. The conditions at which the following potential divider is independent of frequency may be (SSC-2012)
R1/R2 = C1/C2
R1/R2 = C2/C1
R1C1 << 1, R2C2 <<1
R1 + R2 + 1/C1 + 1/C2
ii and iv are true
1 and iii are true
i is true only
ii is true only
Answer.ii. R1/R2 = C2/C1
Explanation:-
For independent frequency-time constants, RC network must be equal i.e the circuit has an output which is independent of frequency if the low and high-frequency response is identical.
Vo = R1/(R1 + R2) = C2/(C1 + C2)
= R1C1 = R2C2
R1/R2 = C2/C1
Ques.64. Which of the following is the best conductor of electricity? (SSC-2012)
Warm water
Salt Water
Cold Water
Distilled water
Answer.2. Salt Water
Explanation:-
The conductivity of water depends on the concentration of dissolved ions in solution. Salt molecules are made of sodium ions and chloride ions. (An ion is an atom that has an electrical charge because it has either gained or lost an electron.) When we put salt in water, the water molecules pull the sodium and chlorine ions apart so they are floating freely which is responsible for the conductance of electricity.
So, pure/distilled water is an extremely bad conductor, while impure water with ions in it is a good conductor
Ques.65. The wave shape of current flowing through an inductor is (SSC-2012)
The wave shape of voltage drop (v) across the inductor is
Answer. a
Explanation
i = 2.t/T
Voltage across an inductor at any moment in time t is V = Ldi/dt
Ques.66. SI unit of electrical energy is (SSC-2012)
Watt-second
Joule
KWh
Volt-ampere-Second
Answer.2. Joule
Explanation:-
The electrical energy consumed in a circuit is defined as the total work done in maintaining the current in an electrical circuit for a given time. S.I. unit of electrical energy is Joule whereas the commercial unit of electrical energy is kilowatt-hour (kWh).
Ques.67. Two wires A and B of the same material but of different lengths L and 2L have the radius r and 2r respectively. The ratio of specific resistance will be (SSC-2012)
1:4
1:8
1:1
1:2
Answer.3. 1:1
Explanation:-
Specific resistance defines as the resistance offered by the material per unit length and unit area. Therefore, it is independent of the length and area of the material. Hence, for the same material, the specific resistance of the wire with any length and area always be the same. Hence, the ratio always is unity.
Ques.68. A 20 microfarad capacitor is connected across an ideal voltage source. The current in the capacitor (SSC-2012)
Will be very high at first, then exponentially decay
Will be very high at first, then exponentially decay and at steady state will become zero
Will be zero at first, then exponentially rise
None of these is true
Answer.2. Will be very high at first, then exponentially decay and at steady state will become zero
Explanation:-
The voltage across a capacitor cannot change instantaneously as some time is required for the electric charge to build upon or leave the capacitor plates. As the capacitor begins to charge the circuit current rises very rapidly.
Now the capacitor is charged, the rate of change of voltage slows and charge slows as the charging current falls. The curve describing the charging of the capacitor follows a recognizable mathematical law describing an exponential curve until the current is practically zero and the voltage across the capacitor is at its maximum.
Ques.69. In figure D in an ideal diode. If the RMS value of the input voltage is 50V, then the RMS current through 100 Ω is (SSC-2012)
0.5/√2A
0.25A
0.5A
0.5√2 A
Answer.1. 0.5/√2A
Explanation:-
In the given figure the circuit consists of the single diode, therefore, the Output will be rectified as half wave, hence RMS current of half wave rectifier is
Irms = Im/2
RMS voltage = 50 V
RMS Value of output (Vrms) of Half voltage rectifier
Vrms = Vm/√2
Vm = Vrms x √2 = 50 x √2
Im = Vm/R
= 50√2/100 = 1/√2
Irms = Im/2
= 1/2√2 = 0.5/√2
Ques.70. In an R-L series circuit R = 20 Ω, L = 0.056 H and the supply frequency in f = 50 Hz. The magnitude of the impedance of the circuit is (SSC-2012)
