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in Physics by (71.9k points)

A circuit consists of four 100-W lamps connected in parallel across a 230-V supply. Inadvertently, a voltmeter has been connected in series with the lamps. The resistance of the voltmeter is 1500 Ω and that of the lamps under the conditions stated is six times their value then burning normally. What will be the reading of the voltmeter?

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Best answer

The circuit is shown in Fig. 1.18. 

The wattage of a lamp is given by : 

W = I2 R = V2/R 

∴ 100 = 2302/R 

∴ R = 529 Ω 

Resistance of each lamp under stated condition is = 6 × 529 = 3174 Ω 

Equivalent resistance of these four lamps connected in parallel = 3174/4 = 793.5 Ω 

This resistance is connected in series with the voltmeter of 1500 Ω resistance. 

∴total circuit resistance = 1500 + 793.5 = 2293.5 Ω 

∴ circuit current = 230/2293.5 A 

According to Ohm’s law, voltage drop across the voltmeter = 1500 × 230/2293.5 = 150 V (approx)

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