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Chapter: Electric Energy Generation and Utilisation and Conservation : Illumination

Important Short Questions and Answers: Illumination

Electric Energy Generation and Utilisation and Conservation - Illumination - Important Short Questions and Answers: Illumination



1) Define luminous flux.


It is defined as the total quantity of light energy emitted per second from a luminous body. It is represented by symbol F and is measured in lumens. The conception of luminous flux helps us to specify the output and efficiency of a given light source.


2) What is meant by candle power?


It is defined as the number of lumens given out by the source in a unit solid angle in a given direction n. It is denoted by CP.




3) Define MHCP.


The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power.


4) Define utilization factor.


It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp.


Utilization factor= Total lumens reaching the working plane/Total lumens given out by the lamp


5) What are the laws of illumination?

Law of Inverse Squares:


Illumination at appoint is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.If a source of light emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere. If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius.


Lambert’s cosine law:


The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the surface at that point.


6) What is meant by luminance?


It is defined as the luminous intensity per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.


7) Define space-height ratio.


It is defined as the ratio of horizontal distance between adjacent lamps and height of their mountings.


Space-height ratio= Horizontal distance between two adjacent lamps/Mounting height of lamps above working plane


8) What is polar curve?


In most lamps or sources of light the luminous intensity is not the same in all directions. If the luminous intensity, i.e. the candle power is measured in a horizontal plane about a vertical axis and a curve is plotted between candle power and the angular position,a curve is obtained is called as horizontal polar curve.


The luminous intensity in all the directions can be represented by polar curves. If the luminous intensity in a vertical plane is plotted against the angular position, a curve known as vertical polar curve is obtained.


9) Name the various photometer heads.

1.        Bunsen Head (or) Grease spot photometer

2.        Lummer-Brodhun photometer head

There are two types of Lummer Brodhun heads

a)        Equality of Brightness type photometer head

b)       Contrast type photometer head


10)  What are all the sources of light?


·           According to principle of operation the light sources may be grouped as follows. Arc lamps

·           High temperature lamps

·           Gaseous discharge lamps

·           Fluorescent type lamps


11) What is stroboscopic effect of fluorescent tubes?


With a.c. supply frequency of 50 cycles per second, discharge through the lamp becomes zero, 100 times in a second. Due to the persistence of vision, our eyes do not notice this. If this light falls on moving parts, they may appear to be either running slow or in the reverse direction or even may appear stationary. This effect is called stroboscopic effect.


12) Define beam factor.


The ratio of lumens in the beam of a projector to the lumens given out by lamps is called the beam factor. This factor takes into account the absorption of light by reflector and front glass of the projector lamp. Its values vary from 0.3 to 0.6.


13) Mention the types of lighting schemes.


The distribution of the light emitted by lamps is usually controlled to some extent by means of reflectors and translucent diffusing screens or even lenses. The interior lighting schemes may be classified as

·           Direct lighting

·           Semi-direct lighting

·           Indirect lighting

·           Semi-indirect lighting

·           General lighting


14)  What are the drawbacks of discharge lamps?

Drawbacks of discharge lamps:

·           Take time to attain full brightness.


·           High initial cost and poor power factor.


·           Starting requires trigger-starter.


·           Light output fluctuates at twice the supply frequency.


·           The flicker causes stroboscopic effect.


·           These lamps can be used only in particular position.



15)  What are the requirements of lighting system?


The following factors are required to be considered while designing the lighting scheme.

·           Illumination level

·           Uniformity of illumination

·           Colour of light Shadows Glare

·           Mounting height Spacing of luminaries

·           Colour of surrounding walls.


16) What are the advantages of electric heating?

The main advantages of electric heating over other systems of heating such as coal, oil or gas heating are given below.

·           Economical

·           Cleanliness

·           Absence of flue gases

·           Ease of control or adaptation Automatic protection


·           Upper limit of temperature

·           Special heating features

·           High efficiency of utilisation

·           Better working conditions Safety

·           Heating of non-conducting materials


17)  Classify the methods of electric heating.

Kinds of electric heating

A.      Power frequency heating

a. Resistance heating

i) Direct resistance heating ii) Indirect resistance heating iii) Infrared or Radiant heating

b. Arc heating

i) Direct arc heating ii) Indirect arc heating

B.      High frequency heating

a. Induction heating

i)          Direct induction heating

ii)       Indirect induction heating

b.        Dielectric heating


18)  What is meant by indirect resistance heating?

In this method, the current is passed through a high resistance wire known as heating element. The heat produced due to I2 R loss in the element is transmitted by radiation or convection to the body to be heated.Applications are room heaters, in bimetallic strip used in starters, immersion water heaters and in domestic and commercial cooking and salt bath furnace.


19).What is meant by (1) infra red /radiant heating? (2) Dielectric heating?


(1)When current possess through a resistive element heat energyis produced and the same is dissipated in the form of infrared radiation this is focused upon a body to be heated .e.g. to dry the wet paint on an object.


(2)When a non metallic material is placed between two electrodes at high voltage the dielectric loss is dissipated in the form of heat which is used for heating purposes.



20)  What are the requirements of a good heating material?

1.        High ressistivity

2.        Low temperature coefficient of ressistance

3.        High melting point

4.        Free from oxidation


21)  What are the properties of heating element material?

·           The material of the heating elements should posses the following desirable properties for efficient operation and long life.


·           High resistivity: It should have high specific resistance so that the overall length to produce a certain amount of heat may be smaller.


·           High melting point: It should have high melting point so that high temperatures can be produced without jeopardizing the life of the element.


·           Free from oxidation: It should be able to resist oxidation at high temperatures; otherwise its life will be shortened.


·           Low temperature coefficient: It should have a low temperature coefficient so that resistance remains appreciably constant even with increases of temperature. This helps in accurate control of temperature.


22)  What are the causes of failure of heating elements?

Principle causes are

·           Formation of hot spots

·           General oxidation of the element and intermittency of operation

·           Embrittlement caused by grain growth

·           Contamination of element or corrosion


23) Write short note on infrared heating.


In radiant heating, the elements are of tungsten operating about 2300°C as at this temperature a greater proportion of infra-red radiation is given off.Heating effect on the charge is greater since the temperature of the heating element is greater than in the case of resistance heating. Heat emission intensities up to 7500 watts/sq.m can be obtained leading to heat absorption up to 4300 watts/sq.m. This reduces the time taken by various drying process.


24) What is the basic principle of induction heating?


High frequency eddy current heating produced by eddy currents induced by electromagnetic action in the metal to be heated.It works on the principle of electromagnetic induction as same as a transformer. It has a metal disc surrounded by a copper coil in which a.c supply is flowing. The disc has a finite value of diameter and thickness and is spaced a given distance from the coil and concentric to it. We find that a secondary current is caused to circulate around the outer surface of the disc.


25)  What are the different types of resistance welding?

The different types are as follows


·           Butt welding


·           Spot welding


·           Projection welding


·           Seam welding


·           Percussion welding


26) Compare DC welding and AC welding.


27) What is LASER welding?


LASER (Light Amplification by Stimulated Emission of Radiation) welding is a welding process that uses the heat from a laser beam impinging on the joint. The process is without a shielding gas and pressure.


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