Q) water in a electrical kettle connected to a 220v supply took 5 minute to reach its boiling point .how long would it have taken if the supply had been of 200v?
Ans) Heat = Q = m s dT
where dT = change in temperatutre
s = specific heat of water
m = mass of water.
If 100% of electrical energy is converted into heat energy then
Electrical energy = V2 t /R, where t = time
thus Q = m s dT = V2 t /R
now when voltage is 200V then time taken for water to boil is t'
so Q = V12 t1 /R = V22 t2 /R
or V12 t1 = V22 t2
or V12 t1 = V22 t2
or (220)2 (300 sec) = (200)2 t2
or t2 = (220)2 (300 sec) / (200)2
ot t2 = 363 seconds
Moderation Team
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