Test: Utilization of Electrical Energy- 2 - Electrical Engineering (EE) MCQ

Upon application of an alternating electrical field, a displacement current caused by polarization and a conduction current caused by the presence of free, electrically charged particles appear in the dielectric. The flow of the total current causes the liberation of heat.

The depth of penetration of induced currents into the disc is given by:

Where, ρ is the specific resistance in Ω-cm

f is the frequency in Hz

μ is the permeability of the charge

The depth up to which the current will penetrate is inversely proportional to square of the frequency

In submerged arc furnaces, the power is controlled by

1. By varying the length of the arc by raising or lowering the electrodes thereby varying the resistance of the arc

2. By varying the applied voltage by the use of tappings provided on the primary side of furnace transformer

In direct arc furnace, when supply is given to the electrodes, two arcs are established and current passes through the charge. The arc is in direct contact with the charge and heat is also produced by current flowing through the charge itself. In direct arc furnace current value is high.

Resistance welding is the process of joining two metals together by the heat produced due to the resistance offered to the flow of electric current at the junctions of two metals. The heat produced by the resistance to the flow of current is given by

H = I²R

The heat produced at the joint is proportional to I²R

The main drawbacks of the resistance welding are:

1. Initial cost is very high
2. High maintenance cost
3. The workpiece with heavier thickness cannot be welded, since it requires high input current

Spot welding means the joining of two metal sheets and fusing them together between copper electrode tips at suitably spaced intervals by means of heavy electric current passed through the electrodes.

It is desirable to clean the sheets thoroughly before welding.

The welding current varies widely depending upon the thickness of and composition of plates. It varies from 1000 to 10000 A and voltage between the electrodes is usually less than 2 V.

The length of the arc required for welding depends upon the following factors:

1. The surface coating and the type of electrodes used

2. The position of welding      

3. The amount of current used

Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation

Photovoltaic is a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons by the photovoltaic effect

Photoelectric effect is the emission of electrons or other free carriers when light shines on a material

The illumination of the surface is,

Where, h is the height of source from the surface in m

I is the luminous intensity on candela

The above equation is also known as cosine cube law. This law states that the illumination at any point on a surface is dependent on the cube of cosine of the angle between lines of flux and normal of that point.

The lamps that emit light of the colour produced by discharge takes place through the gas or vapour present in the discharge tube such as neon gas, sodium vapour, mercury vapour, etc.

To start sodium vapour lamp, 380 - 450 V of striking voltage required for 40 - 100 W lamps. These voltages can be obtained from a high reactance transformer or an auto transformer.

Faraday’s first law:

This law states that the mass of substance deposited from an electrolyte is proportional to the quantity of electricity passing through the electrolyte in the given time

m ∝ It

Faraday’s second law:

This law states that when the same quantity of electric current is passed through different electrolytes, the masses of the substances deposited are proportional to their respective chemical equivalents or equivalent weights.

Electrodeposition: It is the process by which one metal is deposited over other metal or non-metal by electrolysis.

Electro metallization: It is the process by which the metal can be deposited on a conduction base for decoration and for protective purposes. Any non-conductive base is made as conductive by depositing graphite layer over it.

Anodizing: The process of deposition of oxide film on a metal surface is known as anodizing and oxidation.

Electrofacing: It is the process of the coating of a metallic surface with a harder metal by electrodeposition

In urban service, there is no free running period. The distance between two stops is less about 1 km. Hence relatively short coasting and longer braking period is required.

Average Speed: It is the mean of the speeds attained by the train from start to stop, i.e. it is defined as the ratio of the distance covered by the train between two stops to the total time of run. It is denoted with V_a

Average speed = distance between stops/actual time of run

V_a = D/T

Where V_a is the average speed of train in kmph

D is the distance between stops in km

T is the actual time in hours

Schedule Speed: The ratio of the distance covered between two stops to the total time of the run including the time for stop is known as schedule speed. It is denoted with V_s

Where T_s is the schedule time in hours

Coefficient of adhesion (μ): It is defined as the ratio of the tractive effort (F_t) to propel the wheel of a locomotive to its adhesive weight (W).

The load fluctuations take place in many of the industrial drives such as rolling mills, electric hammers, presses, and reciprocating pumps, where the load on the motor varies widely within a span of few seconds. The sudden and peak load requires very large current from the supply results high voltage drop in the system or alternately would require very large size of cables. It is very essential to smooth out fluctuating load is known as load equalization.

Tungsten is most commonly used for the filaments in incandescent lamps.

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. Squirrel cage induction motors are used for this purpose.