Question

Two bulbs B1 and B2 with ratings (10 W, 220 V) and (20 W, 220 V) respectively are connected in parallel across a 440 V input supply. Then what will happen to the bulbs

Answer 1

Answer:

Two bulbs B1 and B2 with ratings (10 W, 220 V) and (20 W, 220 V) respectively are connected in parallel across a 440 V input supply. Then Both bulbs will fuse

Explanation:

Bulb on which voltage is greater than the rated voltage would fuse.

Sol. : Since both bulbs are connected in parallel, the voltage across every bulb will be 440 V. Thus, both bulbs will fuse.

Answer 2

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Both the bulbs has a voltage rating is 220 V, thus voltage remains constant in both the cases.  

The bulb that has a lower power rating will have higher resistance, whereas both the bulbs are connected in series.

Hence, both the resistances will combine into one.

Resistance= Voltage²/ Power

R-25= 220²/25=1936 Ohms.

R-100= 220²/100 = 484 Ohms.

Since, both the bulb are in series, total resistance = 1936+484 =2420 Ohm

Voltage=Current×Resistance

For 440 V, current is:  

440 V/2420 Ohm= 0.1818 Amp

Voltage across 25 W bulb will be = Current × resistance of 25W bulb

= 0.1818 × 1936 = 352 Volts.

Voltage across 100 W bulb will be  =Current × resistance of 100 W bulb

= 0.1818 × 484 = 88 Volts

In a series connection, the equipment with higher resistance have to bear more voltage.

Thus, the 25W bulb will fuse first.

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