IIT JEE , AIEEE Forum - Physics , Electricity

niki61

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?

nishant_88

let R be resistance of kettle, m be mass of water, c be sp. heat capacity and d@ be change in temp
Let the time taken be t
In first case,
Energy spent=220²/R*5*60
Also, heat energy supplied=mcd@
thus, mcd@=220²/R*5*60.......1
Similarly in second case
mcd@=200²/R*t*60.......2
on equating equations 1 and 2,
we get t=6.05 minutes or 6 minutes 3 seconds

edison

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 = V²t /R, where t = time

thus Q = m s dT = V²t /R

now when voltage is 200V then time taken for water to boil is t'

so Q = V₁²t₁ /R = V₂²t₂ /R

or V₁²t₁ = V₂²t₂

or (220)²(300 sec) = (200)²t₂

or t₂= (220)²(300 sec) / (200)²

ot t₂ = 363 seconds

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