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MCQs on Heating And Welding

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01․ In electrical resistance welding material of electrode should have
higher electrical conductivities.
higher thermal conductivities.
sufficient strength to sustain high pressure at elevated temperatures.
all of above.

In resistance welding, heat is generated in localized area which is enough to heat the metal to sufficient temperature, so that the parts can be joined with the application of pressure. Pressure is applied through the electrodes. So to achieve this, the material of the electrode should have higher electrical and thermal conductivities with sufficient strength to sustain high pressure at elevated temperatures.
02․ The metal surfaces for electrical resistance welding must be
rough.
clean.
moistened
coloured

The metal surfaces should be properly cleaned so that they are free from rust, dust, oil and grease. For this purpose metal surfaces may be given pickling treatment i.e. dipping in diluted acid bath and then washing in hot water bath and then in the cold water bath. After that surfaces may be dried through the jet of compressed air. If surfaces are rust free then pickling is not required but surface cleaning can be done through some solvent such as acetone to remove oil and grease.
03․ Radiation from a body is proportional to
T2.
T4.
T5.
T3.

According to Stephens law, \ Where T1 = temperature of the source of heat, and T2 = temperature of the substance to be heated.
04․ Induction heating takes place in
conducting but non-magnetic materials.
conducting materials which may be either magnetic or non-magentic materials.
insulating materials.
conducting and magnetic materials.

In induction heating, the effect of currents induced by electromagnetic action in the charge is employed. The heat developed depends on the power drawn by charge. The power consequently depends upon the voltage and resistance of charge. So for the developed heat to sufficiently melt the charge, the resistance of the charge must be low, which is possible only with metals and voltage must be high, which is obtained by employing higher flux and higher frequency. Therefore magnetic material can be easily treated than non-magnetic materials because of their higher permeability. So induction heating takes place in conducting materials that may be magnetic or non magnetic materials.
05․ The main advantage of dielectric heating is that
heating occurs in the material itself.
heating occurs due to high frequency.
it can be used for drying the explosives.
None of the above.

When an insulting material is subjected to an alternating electric field, the atoms get stresses and due to the inter atomic friction heat is produced. This loss in known as dielectric loss. Hence the main advantages of dielectric heating is that heating occurs in the material itself.
06․ The supply frequency usually employed for high frequency eddy current heating is
1 KHz.
5 KHz.
10 MHz.
10 KHz to 400 KHz.

Eddy current losses do not follow f2 law as frequency is increased higher and higher. At 10 KHz, the total heating may vary directly as the frequency and even drop to one half power at frequencies of order of 500 KHz.
07․ The most modern method for food processing is
Eddy current heating.
Dielectric current.
Induction heating.
Resistance heating.

Dielectric heating has been applied in the food processing industry:
1. Dehydrating fruits, vegetables, eggs etc.
2. Cooking foods without removing the other shells.
3. Pasteurizing milk and beer inside bottles.
4. Defrosting frozen foods in large bakeries and restaurants.
5. Germicidal heating - In dielectric heating process the products do not loose flavor.
08․ In induction heating
magnetic materials can be easily treated in comparison to non-magnetic materials.
heat is produced due to current induced in the charge by electromagnetic action.
the resistance of charge must be low and voltage applied must be high in order to produce sufficient heat.
all of above.

In induction heating, heat is produced due to current induced in the charge by electromagnetic action. To develop heat sufficient to melt the charge, the resistance of the charge must be low and voltage applied must be high because of the power drawn in induction heating is equal of V2/R. In induction heating, magnetic can be easily treated in comparison to non magnetic materials because of their high permeability.
09․ In an electric room heat converter the method of heating used is
Infrared heating.
Induction heating.
Resistance heating.
arc heating.

An electric room heat converter is a heater which operates by air covection currents ciculating through the body of the appliacne and across its heating elements. The resistance heating is provided to it. This heats up the air, causing it to increase it to increase in volume and so become buoyant and rise.
10․ In induction heating, the depth up to which current will penetrate is proportional to
Frequency.
(Frequency)2.
1/√(Frequency).
1/Frequency.

The depth of penetration of induced current is given as \ Where, ρ = Specific resistance of molten change in ohm - cm, f = frequency in Hz µ = permeablity of the change. The depth is ∝ (1/√f).
11․ _______________ is not an applictaion of dielectric heat.
Diathermy.
Soldering.
Food processing.
Gluing of wood.

Soldering at high temperatures or brazzing where silver, copper, their, alloys arc used as solders is another important industruial application. Two pieces of metal arc firred properly and suitable flux is applied to the joint and the soldering alloy is placed near the joint. By induction heating heat develops very rapidly melting the solder which runs into the joint sealing it properly.
12․ Low frequency supply is necessary for direct core type induction furnaces because
With normal frequeny supply the electromagnetic forces causes severe stirring action in the molten metal.
Magnetic coupling between the primary and secondary circuit is poor.
Both A and B.
None of the above.

Low frequency supply is necessary for direct core type induction furnace. If normal frequeny supply is employed for operation of such furnaces the electromagnetic forces cause severe stirring action in the molten metal. Hence low frequency supply is also necessary from this point of view. If magnetic coupling between the primary and secondary circuit is poor, therefore leakage reactance is high and power factor is low.
13․ For intermittent work which of the following furnace is suitable?
Core less furnace.
Indirect arc furnace.
Either of above.
Nither of above.

In indirect arc furnace, the arc is produced between two electrodes and heat is transferred to the charge by radiation only. Direct arc furnace is more suitable in situation where the charge has to be varied frequently or where heating intermitternt.
14․ The dielectric loss in a dielectric is proportional to
Voltage impressed on the dielectric.
The square of the voltage impressed on the dielectric.
Square root of the voltage impressed on the dielectric.
None of above.

Dielectric loss in a dielectric is given as \ From above equation, P ∝ V2. Hence, we can say that dielectric loss proportional to the square of the voltage impressed on the dielectric.
15․ The power factor will be leading in case of
Induction heating.
Resistance heating.
Dielectric heating.
Electric arc heating.

In dielectric heating, the current is drawn by the capacitor. When an AC supply voltage is applied across the capacitor plates, the current does lead the supply voltage by exactly 90° and there is always an in-phase component of the current. Since the dielectric heating is involved the capacitor , So the power factor will be leading in the case of dielectric heating. The value of power factor for a particular non-conducting material is constant.
16․ For dielectric heating the range of frequency normally employed is
10 KHz to 100 KHz.
100 KHz to 10 MHz.
1 MHz to 10 MHz.
10 MHz to 40 MHz.

In dielectric heating, for producing sufficient heat frequency is used between 10 MHz and 30 MHz. Dielectric heating depend upon the value of frequency. Hence to achieve more heat, high frequency is used.
17․ The metod of heating used for non conducting materials is
radiant heating.
dielectric heating.
resistance heating.
induction heating.

When non conducting parts such as wood, plastics, bones are subjected to dielectric heating, an alternative electrostatic field dielectric loss occurs. In dielectric heating use of these losses is made.
18․ The advantage(s) of eddy current heating is/are
easy temperature control, little wastage of heat.
heat can made to penetrate into metal surface to any desired depth.
All.
the area of surface over which the heat is produced can be accurately controlled.

The advantages of eddy current heating are
  1. It is quick, clean and convenient method.
  2. The control of temperature is very easy.
  3. The heat can made to penetrate into the metal surface to any desired depth.
  4. The area of surface over which heat is produced can be accurately controlled.
  5. It can easily take place in vacuum or any other special atmosphere. There is little wastage of heat, as heat is produced in the body to be heated up directly.
19․ The method appropriate for heating of non ferrous materials is
resistance heating.
radiant heating.
eddy current heating.
indirect arc heating.

Due to indirect arc heating, a furnace becomes suitable for comparatively low melting point. So these furnaces are used for melting non ferrous metal.
20․ Hysteresis loss and eddy current loss arc used in
Dielectric heating.
Induction heating of steel.
Induction heating of brass.
Resistance heating.

Hysteresis loss and eddy current loss arc used in induction heating of ferromagnetic materials.
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