A bulb is rated at 20 V -100 W. What is its resistance? Five such bulbs burn for 4 hours. What is the electrical energy consumed? Calculate the cost if the rate is Rs 4.60 per unit.

Explanation

(a) Calculation of Resistance: Here we know the voltage and power of the bulb. So, the resistance can be calculated by using the formula:
`P = (V^(2))/(R)`
Here, Power, `P = 100` watts
Voltage, `V = 200` volts
And, Resistance, `R = ?` (To be calculated)
Now, putting these value in the above formula, we get:
`100 = ((200)^(2))/(R)`
`100 R = 40000`
And, `R = (40000)/(100)`
= 400 ohms
(b) Calculation of Electrical Energy Consumed. The electrical energy consumed in kilowatt-hours can be calculated by using the formula:
`E = P xx t`
Here, Power, `P = 100` watts
`= (100)/(1000)` kilowatt
`=0.1` kilowatt .......(1)
And, Time, `t = 4` hours ........(2)
So, Energy consumed by 1 bulb `= 0.1 xx 4`
`= 0.4` kilowatt-hours
And, Energy consumed by 5 bulbs `= 0.4 xx 5`
= 2 kilowatt-hours (or 2 kWh)
Thus, the total electrical energy consumed is ''2 kilowatt-hours'' or ''2 units''.
(c) Calculation of Cost of Electrical Energy. We have been given that:
Cost of 1 unit of electricity = Rs. 4.60
So, Cost of 2 units of electricity = Rs `4.60 xx 2`
= Rs 9.20