ELECTRIC Heating is required for both industrial and domestic purposes. In industries, heating is required for the melting of metals, moulding of glass, enamelling of copper, baking of insulator and welding etc. In domestic purposes the heating is required for cooking, water heating, room heating in winter, pressing clothes and many more.
Short notes
Objective questions (Answers are given below)
- As compared to other methods of heating
using gas and coal etc, electric heating is far
superior because of its.
(a) cleanliness
(b) ease of control
(c) higher efficiency
(d) all of the above. - Magnetic materials are heated with the help
of
(a) hysteresis loss (b) electric arc
(c) electric current (d) radiation. - In the indirect resistance heating method, heat
is delivered to the charge
(a) direcly (b) by radiation
(c) by convection (d) both (b) and (c). - The main requirements of a good heating
element used in a resistance furnaces are
(a) high resistivity
(b) high melting-temperature
(c) positive resistance-temperature
coefficient
(d) all of the above. - Electric ovens using heating elements of
……………………… can produce tempera- tures
up to 3000°C
(a) nickel (b) graphite
(c) chromium (d) iron. - The temperature of resistance furnaces can be
controlled by changing the
(a) applied voltage
(b) number of heating elements
(c) circuit configuration
(d) all of the above. - Which of the following heating method is
based on the transformer principle ?
(a) resistance heating
(b) eddy-current heating
(c) induction heating
(d) dielectric heating. - When graphite electrodes are used in arc
furnaces, the temperature obtained is in the
range of ………………………….degree centigrade.
(a) 3000-3500 (b) 2500-3000
(c) 2000-2500 (d) 1500-2000 - Which of the following furnace suffers from
pinch effect?
(a) resistance furnace
(b) core type induction furnace
(c) coreless induction furnace
(d) vertical core type induction furnace. - Which of the following induction furnace has
the lowest power factor?
(a) vertical core type
(b) indirect core type
(c) coreless type
(d) core type. - The coreless induction furnace uses highfrequency
electric supply in order to
obtain high
(a) flux density
(b) eddy-current loss
(c) primary resistance
(d) power factor. - Inflammable articles like plastic and wooden
products etc, can be safely heated by
using……………….. heating.
(a) eddy-current (b) dielectric
(c) induction (d) resistance - Which of the following is an advantages of
heating by electricity?
(a) Quicker operation
(b) Higher efficiency
(c) Absence of flue gases
(d) All of the above - …… has the highest value of thermal
conductivity.
(a) Copper (b) Aluminium
(c) Brass (d) Steel - Which of the following heating methods has
maximum power factor?
(a) Arc heating
(b) Dielectric heating
(c) Induction heating
(d) Resistance heating
- …… method has leading power factor
(a) Resistance heating
(b) Dielectric heating
(c) Arc heating
(d) Induction heating - …… is used for heating non-conducting
meterials.
(a) Eddy current heating
(b) Arc heating
(c) Induction heating
(d) Dielectric heating - Which of the following methods of heating is
not dependent on the frequency of supply?
(a) Induction heating
(b) Dielectric heating
(c) Electric resistance heating
(d) All of the above - When a body reflects entire radiation incident
on it, then it is known as
(a) white body (b) grey body
(c) black body (d) transparent body - For the transmission of heat from one body to
another
(a) temperature of the two bodies must be
different
(b) both bodies must be solids
(c) both bodies must be in contact
(d) at least one of the bodies must have some
source of heating - Heat transfer by condition will not occur when
(a) bodies are kept in vacuum
(b) bodies are immersed in water
(c) bodies are exposed to thermal radiations
(d) temperatures of the two bodies are
identical - A perfect black body is one that
(a) transmits all incident radiations
(b) absorbs all incident radions
(c) reflects all incident radiations
(d) absorbs, reflects and transmits all incident
radiations - Heat is transferred simultaneously by
condition, convection and radiation
(a) inside boiler furnaces
(b) during melting of ice
(c) through the surface of the insulted pipe
carring steam
(d) from refrigerator coils to freezer of a
refrigerator - The process of heat transfer during the re-entry
of satellites and missiles, at very high speeds,
into earth’s atmosphere is known as
(a) ablation
(b) radiation
(c) viscous dissipation
(d) irradiation - Which of the following has the highest value
of thermal conductivity?
(a) Water (b) Steam
(c) Solid ice (d) Melting ice - Induction heating process is based on which
of the following principles?
(a) Thermal ion release principle
(b) Nucleate heating principle
(c) Resistance heating principle
(d) Electro-magnetic induction principle - Which of the following insulating materials is
suitable for low temperature applications?
(a) Asbestos paper
(b) Diatomaceous earth
(c) 80 percent magnesia
(d) Cork - A non-dimensional number generally
associated with natural convection heat transfer
is
(a) Prandtl number
(b) Grash off number
(c) Pecelet number
(d) Nusselt number - The temperature inside a furnace is usually
measured by which of the following?
(a) Optical pyrometer
(b) Mercury thermometer
(c) Alcohol thermometer
(d) Any of the above - Which of the following will happen if the
thickness of refractory wall of furnace is
increased?
(a) Heat loss through furnace wall will
increase
(b) Temperature inside the furnace will fall
(c) Temperature on the outer surface of
furnace walls will drop
(d) Energy consumption will increase - The material of the heating element for a
furnace should have
(a) lower melting point
(b) higher temperature co-efficient
(c) high specific resistance
(d) all of the above - In a resistance furnace the atmosphere is …….
(a) oxidising (b) deoxidising
(c) reducing (d) neutral - By which of the following methods the
temperature inside a resistance furnance can
be varied?
(a) By disconnecting some of the heating
elements
(b) By varying the operating voltage
(c) By verying the current through heating
elements
(d) By any of the above method
- In induction heating …… is abnormally high.
(a) phase angle (b) frequency
(c) current (d) voltage - By the use of which of the following high
frequency power supplyv for induction
furnaces can be obtained?
(a) Coreless transformers
(b) Current transformers
(c) Motor-generator set
(d) Multi-phase transformer - Induction furnaces are employed for which of
the following?
(a) Heat treatment of castings
(b) Heating of insulators
(c) Melting aluminium
(d) None of the above - In an electric room heat convector the method
of heating used is
(a) arc heating
(b) resistance heating
(c) induction heating
(d) dielectric heating - In a domestic cake baking oven the
temperature is controlled by
(a) voltage variation
(b) thermostat
(c) auto-transformer
(d) series-parallel operation - In an electric press mica is used
(a) as an insulator
(b) as a device for power factor improvement
(c) for dielectric heating
(d) for induction heating - Induction heating takes place in which of the
following?
(a) Insulating materials
(b) Conducting materials which are magnetic
(c) Conducting materialswhich are nonmagnetic
(d) Conducting materials which may or may
not be magnetic - For heating element high resistivity material
is chosen to
(a) reduce the length of heating element
(b) increase the life of the heating element
(c) reduce the effect of oxidation
(d) produce large amount of heat - In resistance heating highest working
temperature is obtained from heating elements
made of ……..
(a) nickel copper (b) nichrome
(c) silicon carbide (d) silver - For intermittent work which of the following
furnaces is suitable?
(a) Indirect arc furnace
(b) Core less furnace
(c) Either of the above
(d) None of the above - Due to which of the following reasons it is
desirable to have short arc length?
(a) To achieve better heating
(b) To increase the life of roof refractory
(c) To have better stirring action
(d) To reduce problem of oxidation
(e) All of the above - In the indirect resistance heating method,
maximum heat-transfer takes place by
(a) radiation (b) convection
(c) conduction (d) any of the above - Property of low temperature co-efficient of
heating element is desired due to which of the
following reasons?
(a) To avoid initial rush of current
(b) To avoid change in kW rating with
temperature
(c) Both (a) and (b)
(d) Either (a) or (b) - Which of the following methods is used to
control temperature in resistance furnaces?
(a) Variation of resistance
(b) Variation of voltage
(c) Periodical switching on and off of the
supply
(d) All of the above methods - It is desirable to operate the arc furnace at
power factor of
(a) zero (b) 0.707 lagging
(c) unity (d) 0.707 leading - Radiations from a black body are proportional
to
(a) T (b) T2
(c) T3 (d) T4 - In arc furnace the function of choke is
(a) to stabilize the arc
(b) to improve power factor
(c) to reduce severity of the surge
(d) none of the above - Ajax Wyatt furnace is started when
(a) it is filled below core level
(b) it is filled above core level
(c) it is fully empty
(d) none of the above - In electric press, mica is used because it is …….
conductor of heat but/and ……. conductor of
electricity.
(a) bad, good (b) bad, bad
(c) good, bad (d) good, good - Resistance variation method of temperature
control is done by connecting resistance
elements in
(a) series
(b) parallel
(c) series-parallel connections
(d) star-delta connections
(e) all of the above ways
- Hysteresis loss and eddy current loss are used
in
(a) induction heating of steel
(b) dielectric heating
(c) induction heating of brass
(d) resistance heating - In heating the ferromagnetic material by
induction heating, heat is produced due to
(a) induced current flow through the charge
(b) hysteresis loss taking place below curie
temperature
(c) due to hysteresis loss as well as eddy
current loss taking place in the charge
(d) one of the above factors - Radiant heating is used for which of the
following?
(a) Annealing of metals
(b) Melting of ferrous metals
(c) Heating of liquids in electric kettle
(d) Drying of paints and varnishes - Which of the following devices is necessarily
required for automatic temperature control in
a furnace?
(a) Thermostat
(b) Thermocouple
(c) auto-transformer
(d) Heating elements of variable resistance
material - For radiant heating around 2250oC, the heating
elements are made of
(a) copper alloy (b) carbon
(c) tungsten alloy (d) stainless steel alloy - Which of the following is an advantage of eddy
current heating?
(a) The amount of heat generated can be
controlled accurately
(b) Heat at very high rate can be generated
(c) The area of the surface over which heat is
produced can be accurately controlled
(d) All of the above - The electrode of a direct arc furnace is made
of
(a) tungsten
(b) graphite
(c) silver
(d) copper - Direct arc furnaces have which of the following
power factors?
(a) Unity
(b) Low, lagging
(c) Low, leading
(d) Any of the above - In direct arc furnace, which of the following
has high value?
(a) Current
(b) Voltage
(c) Power factor
(d) All of the above
- (d) 2. (a) 3. (d) 4. (d) 5. (b) 6. (d) 7. (c) 11.(a) 9. (b) 10. (d) 11. (b) 12. (b) 13. (d) 14. (a) 15.(d) 16. (b) 17. (d) 18. (c) 19. (a) 20. (a) 21. (d) 22.(b) 23. (a) 24. (a) 25. (c) 26. (d) 27. (b) 28. (b) 31.(a) 30. (c) 31. (c) 32. (a) 33. (d) 34. (b) 35. (c) 36.(a) 37. (b) 38. (b) 39. (a) 40. (d) 41. (a) 42. (c) 43.(a) 44. (e) 45. (a) 46. (c) 47. (d) 48. (b) 49. (d) 50.(a) 51. (d) 52. (c) 53. (e) 54. (a) 55. (c) 56. (d) 57.(b) 58. (c) 59. (d) 60. (b) 61. (b) 62. (a)
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