An electric kettle used to prepare tea, takes 2 minutes to boil 4 cups of water (1 cup contains 200 cc of water) if the room temperature is `25^@ C`, (a) If the cost of power consumption is `Re 1.00` per unit `(1 unit = 1000 watt-hour)`, calculate the cost of boiling 4 cups of water. (b) What will be the correspoinding cost if the room temprature drops to `5^@ C`?

Answer

A, B

Explanation

Volume of water boiled `
`=4xx200 CC =800 CC `
`theta_1 =25^@C`
` `
`theta_2 = 100^@C`
`theta_2 - theta_1 = 75^@C`
` mass of boil = 800xx1 =800gm =0.8 kg Q(heat req.) `
`= ms theta = .8xx 4200xx75`
` =252000J 100- watt-hour `
`=1000 xx3600 watt sec. Cost of boiling 4 cup water `
`=(1)/(1000xx3600) xx252000`
` =Rs. 0.7 i.e. 7 paise. (b) `
`Q = mstheta = 0.8xx4200xx95J`
` No. of units = 0.0886 = 0.09 Money consumed = 0.09 Rs = 9 paise`.