Question

If two bulbs B₁25W and B₂ 100W, both rated at 200 V are connected in series across a 440 volts supply, then

• A. 100W bulb will fuse
• B. 25W bulb will fuse
• C. None of the bulb will fuse
• D. Both the bulbs will fuse

Answer 1

The correct option is B

25W bulb will fuse

We know,

P= VI

P₁= VI₁

25 = 200I₁

I₁ = 1/8

Implies that bulb1 can tolerate the current of 1/8 ampere. Similarly, using same method we can see that bulb2 will tolerate current of 1/2 amperes.

Let their resistances be R₁ and R₂

R₁ = V²/P₁

R₁ = V²/25

R₂ = V²/100

When connected in series (with a 400v source)

Net resistance

V²/25 + V²/100 = V²/20

Now current I' through the bulbs

I' = 400 x 20/V²

Put V = 200

We get I' = 1/5

But we know that bulb1 can tolerate only 1/8 ampere of current hence it'll fuse