Page 260 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

With welding, the material to be welded forms one electrode and the welding rod the other. To form the arc,
the welding rod is touched to the material to be welded. The heat produced by the current flowing through the
point of contact causes the metal to vaporize, thus causing the surrounding air to ionize. When the rod is pulled
away from the material, the ionized air conducts the current forming an arc. Once the arc is produced, the heat
generated causes the material and the rod to melt. The metal from the rod flows with that from the material to
form a continuous seam
Measurement of heat energy
Unit: Heat

1 Kilocalorie
The kilocalorie (kcal) is the unit of heat in the mks system. It is defined as the amount of heat required to increase
the temperature of one kg of water through 1°C. It has been experimentally proved that, if 4187J of electrical
energy is completely converted to heat, it produces one kilocalorie of heat energy, i.e. 1 kilocalorie = 4187
2 Joule’s law of heating
This law states that the heat generated in a conductor by the flow of an electric current is proportional to the
square of the current value, the resistance of the conductor and the time for which the current flows
The above law can be expressed in the form of an equation as follows
H = I²Rt watt-seconds or joules
Where

I = current in the conductor in amperes
R = resistance of the conductor in Ohms
t = time for which the current flows in seconds
H = quantity of heat developed in joules
If current “I” ampere flows for ‘t’ seconds through a resistance of R ohms, then the electrical energy consumed
(I²Rtjoules) is converted into heat so that we have
H = I²Rt Joules OR watt-seconds
3 Method used for heat transfer

Heat energy is generally transferred from one place or object to another by one of the three methods: conduction,
convection or radiation. In most instances all the three methods are used in varying degrees. The system is
named according to the primary method used.

The conduction method transfers heat from one object to another. One might say the object conducts Keats. An
example of this method is cooking. A pan is placed on the cooking range in contact with the heating element. Heat
is transferred from the element to the pan and from the pan to the ingredients in the panThe convection method
transfers heat through a fluid or air. Hot water, steam, and warm air heating systems use the convection method.

Heat radiation is accomplished when the heat rays are transmitted through space. The rays are absorbed by the
objects they contact, thereby warming the objects. They do not warm the air but Heat radiation is accomplished
when the heat rays are transmitted through space. The rays are absorbed by the objects they contact, thereby
warming the objects. They do not warm the air but warm the objects into which they are absorbed. warm the
objects into which they are absorbed.
4 Applications of heating effect
This principle of electricity is widely used in the operation of many electrical devices, such as electric lamps, fuses,
arc welding, spot welding, domestic heating appliances.

5 Voltage by heat
If two dissimilar pieces of metal, such as iron and copper wires, are twisted together and heated in a flame, a
potential difference or voltage will be developed across the ends of the wires. (Fig 3) Such a device is known
as a THERMOCOUPLE. The thermocouple is used in PYROMETERS, to measure very high temperatures of
furnaces.

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CITS : Power - Electrician & Wireman - Lesson 101-103