An electric kettle has two heating coils. when one coil is used, water in the kettle boils in 5 minutes, while when second coil is used, same water boils in 10 minutes. If the two coils, connected in parallel are used simultaneously.the same water will boil in time

Options

A. `20/3 min`
B. 7.5 min
C. `10/3 min`
D. `2.5 min

Answer

C

Explanation

`(V^2)/(R_1) t_1 = (V^2)/((R_1R_2)//(R_1+R_2)) t_(2)^(')`
`(V^2)/(R_1) t_(1) = (V^(2)(R_1+R_2))/(R_1R_2) t_(2)^(')`
`t_(1) = (R_(1)/(R_2)+1)t_(2)^(') = ((t_1)/(t_2)+1) t_(2)^(')`
`(1)/(t_(2)^('))+1/(t_2) +1/(t_1) = 1/5 + 1/10=3/10`
`t_(2)^(') = 10/3 min`.