Question

Bulb is rated as $200V,100W$. Calculate its resistance. Five such bulbs are lighted for $4$ hours daily. Calculated the units of electrical energy consumed per day. What would be the cost of using bulbs per day at the rate of $Rs.400$ per unit?

Answer 1

We know that for an electric appliance
$P=\frac{V^2}{R}$
Resistance will be given by
$R=\frac{V^2}{P}$
Now, here
$P=100W$
$V=200$ volts
So, $R=\frac{{200}^2}{100}$
$R=\frac{40000}{100}=400\Omega$
Thus, resistance of the bulb is $R=400\Omega$
Now, electric energy consumed will be
$E=$ power of each unit $\times$ time.
$\Rightarrow Energy=P\times t$
Here
$P=100W$
$t==4hours=4\times 60\times 60sec=14400sec$
So,
$Energy=100\times 14400J=1440000J=1.44\times {10}^{6}J$
Electrical energy consumed by each bulb in Unit

$=\frac{1.44\times {10}^{6}J}{3.6\times {10}^{6}J}=0.4kWh(\because 1kWh=3.6\times {10}^{6}J)$
The total cost of electricity = total unit of energy consumed $X$ cost per unit =

Total Cost $=5\times 0.4\times 400=Rs.800$
Thus, total cost of using all bulbs per day is $=Rs.800$