91Ó°ÊÓ

1. There are four 100 W bulbs in a room.
2. The percentage of conversion of energy into heat is.$73\%$
3. The bulb is lighted for the time, $t=6.9\text{h}$.

An incandescent bulb converts the electrical energy partially to the heat and partially to light. So if the bulb illuminates a room for a while, the temperature of the room will increase.

Formula:

The power radiated by the body in the form of electrical energy,

$P\left(\text{rateofconversionofenergy}\right)=\frac{dE}{dt}$ ,,,(i)

There are four bulbs in the room of 100 W. So, the total power of the bulbs in the room is 400 W.

The portion of conversion of energy into heat is$73\%=0.73$.

Hence, the amount of energy converted to heat is$400\text{W}×0.73=292\text{W}$.

The power is given as the rate of conversion of energy.

$P=292\text{Wor}292\text{J/s}$

i.e. amount of heat generated in$1\text{s}=292\text{J}$.

Therefore, the heat generated in $6.9\text{hror}6.9×3600\text{s}$is given by

$\begin{array}{c}H=292\text{ }\text{J/s}×6.9×3600\text{ s}\\ =7.25×{10}^6\text{J}\\ =7.25\text{MJ}\end{array}$

Hence, the amount of heat produced is.$7.25\text{ M´³}$