# UPPCL JE 2018 Electrical question paper with Explanation 27-Aug-2018

Ques.91. A capacitor charged to 200 V has a 2000 μC of the charger. The value of capacitance will be _____

1. 100 F
2. 100 μF
3. 10 μF
4. 10 F

Charged stored in the capacitor is given as

Q = CV

Where

Q = charge stored = 2000μC = 2000 × 10−6

C = Capacitance =?

V = voltage 200 V

2000 × 10−6 = 200C

C = 2000 × 10−6/200 = 10 μF

Ques.92. A voltage is said to be alternating when it changes in ______

1. Neither magnitude nor direction
2. Magnitude only
3. Direction only
4. Both magnitude and direction

An alternating voltage has nither a constant direction nor a constant magnitude During one-half cycle, alternating voltage flows in one direction, and during the other half cycle, it flows in the reverse direction. The alternating voltage is one whose magnitude changes with time and direction, reverses periodically.

The equation of alternating voltage is given by

V = Vmsinωt

Vm = Maximum voltage

ω = angular frequency

Ques.93. Three identical coils each having a resistance of 10 ohms and an inductance of 0.03 H are connected in delta across a 440-V, 50 Hz, three-phase supply. What will be the power factor?

1. 0.72 lagging
2. 0
3. 1

Given

Inductance L = 0.03 H

Resistance R = 10 Ω

Line voltage VL = 440 V

Inductive Reactance XL = 2πfL

Frequency f = 50 Hz

X= 2 × π × 50 × 0.03

X= 9.42Ω

Impedance per phase ZP

ZP = √(R2 + X2L)

ZP = √(102 + 9.422)

ZP = 13.74

Power factor cosφ = Rph/Zph = 10/13.74 = 0.72

Since the load is inductive in nature, therefore, power factor will be lagging.

0.72 Lagging

Ques.94. A 10μF capacitor in series with a 1 MΩ resistor is connected across a 100-V DC supply. The time constant of the circuit is ______

1. 0.1 s
2. 10 m/s
3. 10 s
4. 100 s

The RC time constant, also called tau, the time constant (in seconds) of an RC circuit, is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e.

τ = RC

Given

R = 1MΩ = 1 × 106

C = 10 mF = 10 × 10−6

τ = 1 × 106 × 10 × 10−6

τ = 10 s

Ques.95. Find the odd one from the following

1. Hardness
2. Ductility
3. Resistivity
4. Tensile strength

One must recognize the unique properties of materials before using them, in order to take full advantage of their benefits. The properties of materials can be classified into physical, chemical, mechanical, and machining properties.

The mechanical property of Metal

The mechanical properties of a material referred to the characteristics shown by the material in a solid-state when a force is exerted on it. Common physical properties include tensile strength, compressive strength, ductility, malleability, toughness, hardness, and stiffness.

Physical Property of Metal

The properties that materials have due to the substance they are made of are called physical properties. Such physical properties will not change under any external forces. For example, the melting point of ice is 0°C, no matter what kind of heat energy is used to melt it.

Common physical properties of materials include density, melting point, boiling point, specific heat capacity, latent heat of fusion, latent heat of vaporization, the coefficient of linear expansion, thermal conductivity, and electrical conductivity and resistivity.

In the given question hardness, ductility, and tensile are the mechanical property while resistivity is the physical property of the metal.

Ques.96. A capacitor consists of two conducting surfaces separated by a/an

1. Semiconductor
2. Alloy
3. Metal
4. Insulator

A capacitor or condenser consists of two conducting surfaces separated by an insulator (dielectric), which can be solid, liquid, gaseous, or a vacuum. The capacitance is measured in farads.

The magnitude of the capacitance depends on the nature of the dielectric and varies directly with the area of the conducting surfaces and inversely with their separation. The capacitance can be altered by changing any of these three factors.

Ques.97. Which of the following properties must be used for the fuse material used in the wire?

1. Low melting point
2. Low conductivity
3. High melting point
4. High resistivity

Fuse is the current interrupting device that breaks or opens the circuit (in which it is inserted) by fusing the elements when the current in the circuit exceeds a certain value.

Fuse is the simplest and cheapest device used for interrupting an electrical circuit under the condition of short-circuiting, or excessive overload, current magnitudes.

A fuse is a safety device having a short length of a thin, tin-plated copper wire having a low melting point, which melts and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depend on the maximum current allowed through the circuit.

An electric fuse works on the heating effect of current. The fuse for protecting our domestic wiring is fitted just above our main switch on the switchboard. A fuse wire is connected in series in the electric circuits.

The main fuse in domestic wiring consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it. This is connected in the live wire. The other part of the fuse is a removable fuse grip G which is also made of porcelain. The fuse grip has a fuse wire fixed in it.

When fuse grip is inserted in the fuse holder as shown in Figure, then the circuit of our domestic wiring is completed. So, under normal circumstances when the current is within the limit, then the fuse wire is intact and electric current is available in our wiring.

When a short circuit takes place, or when overloading takes place, the current becomes large and the fuse wire too much. Since the melting point of fuse, wire is much lower than copper wires, the fuse wire melts and breaks the circuit as shown in Figure. When the fuse wire breaks, the electricity supply is automatically switched off before any damage can be done to the rest of the wiring (or the electric appliances being used).

We will now give some important points about the fuse wire to be used in electrical circuits. First of all, we should know why we use a thin wire as a fuse wire and not a thick wire. We use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires.

Due to its high resistance, the heating effect of current will be much more in the fuse wire than anywhere else in the circuit. This will melt the fuse wire whereas other wirings will remain safe. We should not use a thick wire as a fuse wire because it will have low resistance and hence it will not get heated to its melting point easily.

The fuse wire is usually made from tin-plated copper wire having a low melting point so that it may melt easily. A pure copper wire cannot be used as a fuse wire because it has a high melting point due to which it will not be easy when a short circuit takes place.

Fuse wire is made with an alloy of lead and tin having a low melting point and low resistance (although the resistance of fuse wire is higher than that of electrical appliances).

If due to any malfunction or fault, excessive current begins to flow through the circuit, the fuse wire immediately melts due to the heat generated by the flowing current. The circuit is broken and the excess current, which may damage equipment is prevented from flowing.

It is used for Overload and for short-circuit protection in high voltage (upto 66 kV) and for low Voltage (upto 120 V – 240 V) installations/circuits.

Characteristics of a fuse are:-

1. It should have a low melting point.
2. It should have low ohmic losses.
3. It should have high conductivity. ( or low resistivity)
4. It should be economical.
5. It should be free from detraction.

Ques.98. The capacitance of a capacitor formed by two parallel plates, each 200 cm2 in area, separated by a 4-mm-thick dielectric is 0.0004 μF. If a voltage of 20000 V is applied across it, then the total change on the plate will be _____

1. 8 mC
2. 8 C
3. 8 nC
4. 8 μC

Charged stored in the capacitor is given as

Q = CV

Where

Q = charge stored = ?

C = Capacitance = 0.0004μF = 4 × 10−4 μF

V = voltage = 20000  V = 2 × 104

Q = 4 × 10−4 × 2 × 10= 8 μC

Ques.99. In a three-phase system, the order in which the voltage attain their maximum positive value is called _____

1. RMS voltage
2. Peak to peak voltage
3. Phase sequence
4. Powe factor

Phase Sequence:- The sequence in which the voltages in three phases reach their maximum positive values is called phase-sequence. Generally, the phase sequence is R-Y-B.

A balanced three-phase voltage supply consists of three individual sinusoidal voltages that are all equal in magnitude and frequency but are out-of-phase with each other by exactly 120o electrical degrees.

The phase voltages are all equal in magnitude but only differ in their phase angle. The three windings of the coils are connected together at points, a1, b1 and c1 to produce a common neutral connection for the three individual phases.

Then if the a2 is taken as the reference phase each individual phase voltage can be defined with respect to the common neutral as.

Then Red phase  VRN = Vm sinθ
Yellow Phase VYN = Vm sin(θ – 120°)
Blue Phase VBN = Vm sin(θ – 240°)
or VBN = Vm sin (θ + 240°)

If the red phase voltage, VRN is taken as the reference voltage so the voltage in the yellow phase lags VRN by 120°, and the voltage in the blue phase lags VYN also by 120°. But we can also say the blue phase voltage, VBN leads the red phase voltage, VRN by 120°.

As the three individual sinusoidal voltages have a fixed relationship between each other of 120o they are said to be “balanced” therefore, in a set of balanced three-phase voltages their phasor sum will always be zero as Va + Vb + Vc = 0

In a 3-phase, delta-connected system, no wire exists for the return current i.e there is no neutral point, so the phasor sum of all line currents is forced to be zero regardless of a balanced or unbalanced load.

With an unbalanced 3-phase load, however, this imposed-zero on the return current produces unbalanced line-to-line voltages.

Ques.100. The time constant value in an R-C circuit is given by:-

1. R/L
2. RL
3. R/C
4. RC

The RC time constant, also called tau, the time constant (in seconds) of an RC circuit, is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads), i.e.

τ = RC

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