Three bulbs with resistance 4 ohm are connected across a voltage source of 110 V. What is the power consumption.

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justaguide | College Teacher | (Level 2) Distinguished Educator

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The resistance of each bulb is given as 4 ohm. It is not given whether the bulbs are connected across the voltage source in parallel or series. Both the cases have been considered here.

When 2 resistors r1 and r2 are connected in series, the equivalent resistance is equal to R = r1 + r2. If all the three bulbs were connected in series, the equivalent resistance would be 4 + 4 + 4 = 12 ohm. The power consumption is equal to V^2/R = 110^2/12 = 3025/3 W.

On the other hand if the bulbs were connected in parallel to the power source, the resultant resistance would be 1/R = 1/4 + 1/4 + 1/4= 3/4 or R = 4/3. This gives the power consumption as 110^2/(4/3) = 9075 W.

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