Page 64 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Joule’s law can be stated as: The quantity of heat (H) generated in a conductor of Resistance (R), when a current
(I) flows through it for a time (t), is directly proportional to:
The square of current,
The resistance of the conductor,
The time at which the current flows
According to Joule’s Law the heat produced,
H ∞ I²Rt

Where
I is the current,
R is the resistance
T is the resistance

Let V be the potential difference applied between two terminals of a conductor of resistance R,
Then current I = V/R
If the current I is passed through a conductor for time t,
then charge Q = It.
Potential difference is the work done (w) during flow of charge Q is given by:

Work = Potential difference X charge
W = V.Q
Put V = IR and Q = It
Then W = I²Rt

When current flows through a conductor, the work done (I²Rt) is converted into heat.
Thus Heat produced, H = I²Rt joule (current I is in ampere, resistance in ohm and time in second)
Usually heat energy expressed in calorie.
H = I²Rt calorie,
J
where J is Joule’s constant or mechanical equivalent of heat

And J = 4.187 calorie
Hence, heat produced in a conductor in terms of calorie is given by
I²Rt
V² t
H = 4187 calorie H = RX4187 calorie
Wt
V t
1
H = RX4187 calorie H = 4187 calorie
UNIT OF HEAT
Kilocalorie (kcal) is the unit of heat in MKS system

It is defined as the amount of heat required to increase the temperature of one kg of water by 10 C.
4.187 joule = 1 calorie
4187 joules = 1 kilocalorie
1 joule = 0.24 calorie

Temperature
Temperature is the measure of heat. Thermometer is generally used for measuring temperature. Pyrometer is
used to measure high temperature

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CITS : Power - Electrician & Wireman - Lesson 13-19 CITS : Power - Electrician & Wireman - Lesson 13-19