Question

Two bulbs 25W – 120V and 200W – 120V are connected in series
across 240V line. It seemed like a good idea at the time but one bulb
burned out almost immediately which one burned out and why?

Answer 1

Answer:

Explanation:

25W = P1 and 200 W = P2 are the ratings on the bulbs.  That means those bulbs are supposed to work at that power ratings.  Voltage ratings of hte bulbs are: 120Volts.  R1 and R2 are their resistances.

 P = V² / R 

   => R1 = V² / P1 = 120² / 25 = 576 Ohms ;  I1 = V/R1 = 120/576 = 0.2083 Amp

                maximum current this bulb can take is  0.2083 Amp

        R2  = V² / P2 = 120² / 200 = 72 Ohms;   I2 = V/R2 = 1.667 Amp

                current rating = 1.667 Amp

When both bulbs are connected in series with 240 Volts line,

       total resistance in the resistance = R1 + R2 = 648 Ohms

    The current in the circuit = I = 240/648 = 0.370  Amperes

Since, 0.370 is more than 0.2083 Amp.  the bulb with smaller power  Watts rating is burnt.

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