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  • (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 current drawn by two bulbs from the line, if the supply voltage is 220 V.

(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 current drawn by two bulbs from the line, if the supply voltage is 220 V.

 

 

 

 
 
 
 
 

Answers (1)

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(a) Joule's law of heating : When a current (I) flows through resistor of resistance (R) for time (t) .Then the amount of heat produced (H) is given by ,

H=I^{2}RT

      =I^{2}RT

(b)

For first bulb P=100 w ,  V= 220 V

                        P=\frac{V^{2}}{R}

Resistance of first bulb P=\frac{V^{2}}{R_{1}}=R_{1}=\frac{\frac{1}{2}}{P}

                                        =\frac{220^{2}}{100}=484\; \Omega

Resistance of second bulb R_{2}=\frac{V^{2}}{P}=\frac{220^{^{2}}}{160}

                                            =\frac{220^{^{2}}}{60}=806.67\; \Omega

The two bulbs are connected in parallel 

TotaL Resistance =\frac{R_{1}R_{2}}{R_{1}+R_{2}}

                            =\frac{484\times 806.67}{484+806.67}=302.5\; \Omega

Current drawn by two bulbs is 

                I=\frac{220}{302.5}=0.73\; A

(b) Both bulbs are connected in parallel , therefore potential difference across each of them will be 220 V because no division of voltage occure in a parallel circuit 

Current drawn by bulb of rating 100 w is given by 

Power= Voltage \times current

 Current = \frac{power}{voltage}=\frac{60}{220}

Current drawn from line =\frac{100}{220}+\frac{60}{220}    

                                        =\frac{160}{220}=0.727

 

 

Posted by

Sumit Saini

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