Joule's Law of Electric Heating
Joule’s law of electric heating :
The amount of work required to maintain a current of I amperes through a resistance of R ohm for t second is
W.D. = I2 Rt joules
= VIt joules ………………………………. (∵ R = V/I)
= Wt joules ………………………………. (∵ W = VI)
= V2t/R joules ………………………………. (∵ I = V/R)
This work is converted into heat and is dissipated away. The amount
of heat produced is
where J = 4,186 joules/kcal = 4,200 joules / kcal (approx)
∴ H = I2Rt/4,200 kcal = Vlt/4,200 kcal
= Wt/4,200 kcal = V2t/4,200 R kcal
Thermal Efficiency
It is defined as the ratio of the heat actually utilized to the total heat produced electrically. Consider the case of the electric kettle used for boiling water. Out of the total heat produced (i) some goes to heat the apparatus itself i.e. kettle (ii) some is lost by radiation and convection etc. and (iii) the rest is utilized for heating the water. Out of these, the heat utilized for useful purpose is that in (iii). Hence, thermal efficiency of this electric apparatus is the ratio of the heat utilized for heating the water to the total heat produced.
Hence, the relation between heat produced electrically and heat absorbed usefully becomes
Read article – Units of Resistivity
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