An electric refrigerator rated $400 \ W$ operates $8 \ h$ per day. What is the cost of the energy to operate it for $30$ days at ₹ $3.00$ per $kWh$?

$(a)$ What material is used in making the filament of an electric bulb?
$(b)$ Name the characteristics which make it suitable for this.

The rating of an electric heater is $1100 \ W, 220 \ V$. Calculate its resistance when it operates at $220 \ V$. Also,calculate the energy consumed in $kWh$ in the month of November,if the heater is used daily for four hours at the rated voltage.

Which of the following is $NOT$ a unit of electrical energy?

$(a)$ State the commercial unit of electric energy and find its relation with its $SI$ unit.
$(b)$ The current through a resistor is made three times its initial value. Calculate how it will affect the heat produced in the resistor.
$(c)$ Find the increase in the amount of heat generated in a conductor if another conductor of double resistance is connected in the circuit,keeping all other factors unchanged.

If $R_{1}$ and $R_{2}$ are the resistances of filaments of a $400 \ W$ and a $200 \ W$ lamp,designed to operate on the same voltage,then