1.

Bulb is rated at 200 V, 100 W. Calculate its resistance. Five such bulbs are lighted for 4 hours daily. Calculate the units of electrical energy consumed per day. What would be the cost of using these bulbs per day at the rate of Rs. 4.00 per unit?

Answer»

We know that for an electric appliance

P = \(\frac{V^2}{R}\)

Resistance will be given as

R = \(\frac{V^2}{P}\)

Now, here 

P = 100 W 

V = 200 volts

So, R = \(\frac{200^2}{100}\)

R = \(\frac{40000}{100}\) 400Ω

Thus, resistance of the bulb is R = 400 Ω 

Now, electric energy consumed will be 

E = power of each unit × time. 

⇒ E = p × t. 

Here 

P = 100 W 

t = 4 hours 

So, 

E = 100 x 4 = 400 Watt per hour 

Dividing by 1000 to convert into Kilo Watt per hour 

E = \(\frac{400}{1000}=0.4\) kWh 

Electrical energy consumed is 0.4 kWh 

The total cost of electricity = total unit of energy consumed x cost per unit = Cost = 0.4kWh x 4 Rs/kWh = Rs1.6 

Thus, total cost (daily) = Rs. 1.6


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