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An electric kettle has two coils of same...

An electric kettle has two coils of same power . When one coil is switched on , it takes `15 min` to boil water , and when the second coil is switched on , it takes `30 min`. How long will it take to boil water when both the coils are used in `i`. Series and `ii`. parallel?

A

45

B

60

C

75

D

90

Text Solution

Verified by Experts

Let the resistace of winding 1 and 2 are `R_1 and R_2` respectively
`therefore` When winding 1 is present `p=v^2/R_1 therefore m_(v ater)L_f=V^2/R_1 times 15`
For `2^(nd)` winding `P=V^2/R_2 therefore m_(vater)L_f=V^2/R_1 times 15`………(ii)
When the winding are joined in series,
`R_(eq)=(R_1+R_2) therefore P=V^2/((R_1+R_2)) therefore m_(vater) L_f=(V^2/(R_1+R_2))t`.......(iii)
from eqs (i), (ii) and (iii), t=45 min
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