Ques 71. A magnet is kept in the medium of air surrounded by an iron ring. The magnetic lines of force from the magnet will be
Very small in the ring
Crowded in the ring✔
Passing out of the ring
Evenly distributed within the ring
If soft iron surrounds a magnet, being in contact with its poles, that magnet may receive a much higher charge because Magnetic fields are distorted by ferromagnetic materials in their vicinity. If a piece of Iron is placed near a magnet, most of the flux lines will be crowded in the iron ring. It is because iron is a good conductor of magnetic flux.
Ques 72. Which semiconductor device behaves like two SCRs?
Triac✔
MOSFET
JFET
UJT
The triac is similar in operation to two transistor connceted in reverse parallel but using a common gate. The triac can conduct in both direction.
Ques 73. Three resistors, each of ‘R’ Ω are connected in star. What is the value of equivalent delta connected resistors
3 RΩ✔
R/2Ω
2RΩ
R/3Ω
The value of equivalent delta connected resistor
R1 + R2 + R1R2/R3
since all the resistor are of RΩ
R + R + R2/R = 3RΩ
Ques 74. Superposition theorem can be applied only to
Bilateral networks
Linear networks
Non-Linear Networks
Linear Bilateral Networks✔
The superposition theorem for electrical circuits states that for a linear system the response(voltage or current) in any branch of a bilateral linear circuit having more than one independent source equals the algebraic sum of the responses caused by each independent source acting alone, where all the other independent sources are replaced by their internal impedances.
Ques 75. Moving coil (PMMC) and moving iron instruments can be distinguished by observing its
Size of Terminal
Pointer
Range
Scale✔
The scale of the moving iron instruments is not uniform and is cramped at the lower end.
Ques 76. In a fluorescent tube circuit, the function of choke is primarily to:
Improve the brightness of the tube
Initiate the discharge
Reduce the Flicker
Reduce the starting current✔
There are mainly two functions of the choke coil
Limit the current.
Produce high voltage across tube light.
Limit the current
In a gas discharge, such as a fluorescent lamp, current causes resistance to decrease. This is because as more electrons and ions flow through a particular area, they bump into more atoms, which frees up electrons, creating more charged particles.
In this way, the current will climb on its own in a gas discharge, as long as there is an adequate voltage (and household AC current has a lot of voltage).
If the current in a fluorescent light isn’t controlled, it can blow out the various electrical components.
Produces High voltage Across Tubelight
Choke is nothing but the coil/ballast ( inductor) which is used to induce the high voltage across it. as we know that inductor has the property to induce high voltage for a brief period of time, this high voltage is required to ionize the gases in the starter.
Ques 77. The magnetic field energy in an inductor change from the maximum value to the minimum value in 5 msec when connected to an A.C. source. The frequency of the source is
500 Hz
20 Hz
50 Hz✔
200 Hz
Magnetic energy stored in an inductor = LI2/2.
The energy stored is maximum when the current is maximum and energy stored is minimum when the current is minimum (i.e. zero). ln an a.c. circuit, current goes from the maximum value to zero in T/4 sec. where T is the time period of a.c.
T/4 = 5
T = 20 ms = 20 x 10-3
Frequency = 1/T = 1/20 x 10-3 = 50 Hz
Ques 78. The distribution losses that the utility suffers while transferring power from generating station to the consumer is accounted under
Maintenance cost
Fixed charges
Running Charges✔
Cost of fuel
The cost of generation and transmission of electrical energy is divided into two parts
Capital charge (Fixed charge)
Running Charge
The fixed charges include the depreciation, the interest on the capital cost of the conductor and maintenance cost.
The cost electrical, energy wasted due to losses during operation constitutes running charges.
The capital cost and cost of energy wasted in the line are based on the size of the conductor. If the conductor size is big then due to its lesser resistance, the running cost (cost of energy due to I2R losses) will be lower while the conductor may be expensive. For a smaller size conductor, its cost is less but the running cost will be more as it will have more I2R losses.
The cost of energy loss is inversely proportional to the conductor cross-section while the fixed charges are directly proportional to an area of the cross-section of the conductor.
Ques 79. The magnetic potential difference in a magnetic circuit is given by
BlH
HJL
BL
Hl✔
The flux-producing ability of the coil is proportional to the number of turns N and the current I. The product NI is called the magnetomotive force (MMF) and determines the amount of flux developed in the magnetic circuit.
MMF = NI ampere-turns
The reluctance of the magnetic circuit is given as
S = 1/μA
Ques 80. Two electric bulbs have tungsten filament of the same thickness. If one of them gives 60 W and the other gives 100 W, then
60W and 100 W lamp filaments have equal length
60 W lamp filament has shorter length
100 W lamp filament has the longer length
60 W lamp filament has longer length✔
Let’s suppose the voltage is same for both the bulb
Power P = V2/R
P = V2A/ρl………………. (since specific resistance R= ρ (L/A).
L =V2A/ρP
Since both, the filament has the same thickness. So ‘A’ voltage will be same
l ∝ 1/P
Hence Filament of low power will have the longer length.
Note:- If P ↑ R↓ L ↓
For more watt bulb, less resistance will be there so, length of that bulb will be lower.
