(a) Write Joule’s law of heating.

(b) Two lamps one rated 100 W 220 V, and the other 60 W 220 V, are connected in parallel to electric mains supply. Find the currents drawn by two bulbs from the line, if the supply voltage is 220 V.




Joule’s law of heating states that the heat produced in a conductor is directly proportional to 

(i) the square of  the current I through it, 

(ii) its resistance R and 

(iii) the time t , for which current is passed.

Therefore, the mathematical expression of Joule’s law of heating is: 

     H = I 2 Rt  


  • H = Produced Heat  
  • I = Current flowing through the device  
  • t = Time of current flow  
  • R = Resistance of the appliance 





Question 7 (b) Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?Given Potential difference = V = 220 V Power of lamp 1 = P1 = 100 W Power of lamp 2 = P2 = 60 W We need to find current through the circuit We know that In parallel combination current gets divided among the resistors But, Net current = Sum of current through individual resistors I = I1 + I2 Finding current We know that, Power = Voltage ร— Current P = V I โˆด I = ๐‘ƒ/๐‘‰ Lamp 1 I1 = ๐‘ƒ_1/๐‘‰ I1 = 100/220 I1 = 10/22 I1 = 5/11 A Lamp 2 I2 = ๐‘ƒ_2/๐‘‰ I2 = 60/220 I2 = 6/22 I2 = 3/11 ANet current I = I1 + I2 I = 5/11 + 3/11 I = 8/11 I = 0.73 A Net current through circuit is 0.73 A

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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo