Question

An electric kettle has two coils.When one coil is switched on it takes 15 minutes and the other takes 30 minutes to boil certain mass of water.The ratio of times takebn by them,when connected in series and in parallel to boil the samemass of water is

Answer 1

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Solution :

Heat produced, H=V2tIRH=V2tIR

=>R∞t=>R∞t

Given R1=6ΩR1=6Ω

and R2=3ΩR2=3Ω

∴R1R2=63∴R1R2=63

=2Ω=2Ω

∴R1=2R2∴R1=2R2

When the coils are connected in series , the net resistance , R1=R1+R2=R2+2R2=3R2R1=R1+R2=R2+2R2=3R2

∴R1R2∴R1R2=R2+2R2=R2+2R2

=3R2=3R2

∴R1R2=t13∴R1R2=t13

t1=3R1R2t1=3R1R2

=3×3R2R2=3×3R2R2

=9m=9m

Answer : 9 m

Answer 2

see the attachment...

hope this helps you