An immersion heater, in an insulated vessel of negligible heat capacity brings 10 g of water to the boiling point from `16^@C` in 7 min. Then
Q. Power of heat is nearly

Options

A. `8.4xx10^3`
B. 84 W
C. `8.4xx10^3(cal)/(s)`
D. 20 W

Answer

B

Explanation

In 7 min, temperature of 100 g of water is raised by
`(1000-16^@C)=84^@C`. The amount of the heat provided by heater
`Q_W=C_Wm_WDeltaT=((1cal)/(g^@C))(100g)(84^@C)`
`=8.4xx10^3cal=(8.4xx10^3xx4.186)J`
`=3.5xx10^4J`
Power of heater `=(Q_W)/(t_1)`
`=(8.4xx10^3cal)/((7xx60)s)=20(cal)/(s)`
`=(20xx4.18)(J)/(s)=83.6Wapprox84W`