# UPPCL JE Electrical question paper with Explanation Set-2 -2018

Ques.121. What is the resistivity of the lead?

1. 5Ωm
2. 22.5 × 108Ωm
3. 20.8 × 108Ωm
4. 10Ωm

Electrical resistivity (also known as resistivity, specific electrical resistance, or volume resistivity) is a fundamental property of a material that shows how strongly that material opposes the flow of electric current.

The resistivity of lead is 20.8 × 108Ωm

Ques.122. What is the time constant of a series R-C circuit?

1. RC
2. R2C
3. RC2
4. R/C

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

The time constant represents the time required for the charge to increase from zero to 63.2% of its maximum equilibrium value. This means that in a period of time equal to one time constant, the charge on the capacitor increases from zero to 0.632Q.

Ques.123. What is the ratio of copper required for 4-wire 2-phase A.C system to a system to an equivalent 2-wire DC System?

1. 1/2cos2φ
2. cos2φ
3. 2cos2φ
4. 2/cos2φ

The volume of copper required in two wire DC system is

VODC = 4P2ρL2 ⁄ W.V2m

Where

P =  power transmitted by the system.

L = Distance over which the power is transmitted is same.

W = Power loss in all the systems are same.

Vm =The maximum voltage (Vm) exists between any conductor and the earth, in all the systems.

Power Transmitted in the 3-phase A.C system is

VOAC = 0.5 × 4P2ρL2 ⁄ W.V2m.Cos2φ

The ratio of copper require is

VODC ⁄ VOAC = 4P2ρL2 ⁄ W.V2m ⁄ 0.5 × 4P2ρL2 ⁄ W.V2m.Cos2φ

VODC ⁄ VOAC =  1/2cos2φ

Ques.124. What is the equation for kW rating calculation in a battery charger using the taper method?

1. kW = (N × Ah × 2.25 × 0.6)/1000
2. kW = (N × Ah × 2.25 × 0.225 × 0.8)/1000
3. kW = (N × Ah × 0.25 × 0.8)/1000
4. None of these

Taper-current chargers are the most commonly found in repair shops. Either constant voltage or constant amperage is applied to the battery through a manually adjusted current selection switch.

Charger current only diminishes as the cell voltage increases. These chargers can cause serious damage to batteries through overcharging if the charge current is adjusted too high. Timers can automatically shut-off the charger to prevent this condition.

kW rating calculation in a battery charger using the taper method

kW = (N × Ah × 2.25 × 0.225 × 0.8)/1000

Ques.125. What is the value of current in series RLC circuit under resonance condition

1. Zero
2. Minimum
3. Maximum
4. None of these

In RLC series circuit the total impedance of the series LCR circuit is given as

Z2 = R2 +  (X1 – X2)2

where X1 is inductive reactance

and X2 is capacitive reactance.

At a particular frequency (resonant frequency), we find that X1=Xbecause resonance of a series RLC circuit occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase. Therefore, the phase angle between voltage and current is zero and the power factor is unity.

Thus, at the resonant frequency, the net reactance is zero because of X1=X2. The circuit impedance Z becomes minimum and is equal to the resistance R. “Since the impedance is minimum, the current will be maximum”.

Hence electrical resonance is said to take place in a series LCR circuit when the circuit allows maximum current for a given frequency of alternating supply at which capacitive reactance becomes equal to the inductive reactance.

Ques.126. What is the S.I unit of conductivity?

1. Ω–1m
2. Ω–1m−1
3. Ω
4. Ωm

Conductivity (or specific conductance)  is a measure of its ability to conduct electricity. The SI unit of conductivity is mho per meter or Ω–1m−1

Ques.127. What is the ranging capacity of the diesel plant?

1. 100-1175 kW
2. 150-750 kW
3. 150-3750 kW
4. 75-3750 kW

Diesel engine power plants are more efficient than the other types of engine plant for the same capacity. The diesel engine plants are more suitable for low- and medium-power outputs.

These plants are commonly employed where fuel prices are low and water availability is limited. The capacity of the diesel power plants is about 75-3750 kW such plants are used as standby plants to hydro and diesel power plants for small output.

These plants do not require a large amount of water for cooling. Diesel engines are widely used in railroad locomotives, road buildings, ship propulsion electric generators for feeding supply to pubs tic industrial and institutional purpose, etc. A wide application of diesel engine is mainly due to the less cost fuel than gasoline products.

Ques.128. What is the equivalent of ferromagnetic Curie temperature for iron in terms of Kelvin?

1. 1418
2. 1095
3. 995
4. 770

In physics and materials science, the Curie temperature (TC), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism.

Curie temperature of Iron is 1043 K. The Curie temperature is an essential temperature for a ferromagnetic material. For example, if a ferromagnetic material has a temperature under its Curie temperature, then the material has a net spontaneous magnetization, which means that the material becomes ferromagnetic, or magnetic.

If a ferromagnetic material has a temperature over its Curie temperature, then the material becomes paramagnetic or does not become a magnet.

Ques.129. What should be the size of the slide wire of the potentiometer to make it to achieve high accuracy?

1. As long as possible
2. As short as possible
3. 1 meter
4. Neither too thin nor too thick

A potentiometer is an instrument designed to measure an unknown voltage by comparing it with a known voltage. This known voltage is supplied by a standard cell or through a reference source. The potentiometer makes use of null or balanced condition for the purpose of measurement.

The potential difference across the length of the potentiometer wire is directly proportional to its Length (or) when a steady current is passed through a uniform wire, potential drop per unit length or potential gradient is constant.

E ∝ L ⇒ E = φL

Where φ is the potential gradient

So In the case of longer wire, the fall of potential per unit length is small. In other words, the potential gradient is small. The lesser the potential gradient, the more accurate is the potentiometer.

Ques.130. What does S in “IP 11S” represent?

1. Machine was tested when not running condition
2. Machine was tested when it was in running
3. Machine was tested ar light load
4. None of these

The IP Code (or International Protection Rating, sometimes also interpreted as Ingress Protection Rating) consists of the letters IP followed by two digits and an optional letter.

Letter Meaning
f Oil resistant
H High voltage device
M Device moving during test
S Device standing still during  test
W Weather conditions

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