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.

A bulb is rated at 20 V -100 W. What is its resistance? Five such bulb

1.	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.
Answer» <P> Solution :(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