Question

13. A 220 V, 40 W rated bulb and a 220 V, 60 W rated
bulb are connected in series. Which of the
following bulb will glow more brightly?
a. 60 W
b. 40 W
c. Both 'a' and 'b' d. None​

Answer 1

Explanation:

We now that,

P = V^2 /R

R = V^2 /P

Now, as all the bulbs are given the same voltage so resistance of the 40W bulb is the greatest.

As the bulbs are connected in series so the current through the bulbs are the same P = i2R

So the power if the 40W bulb is the greatest and the power dissipated gives the glow of the bulb.

so answer is b

Answer 2

Answer:

B . 40 W

Explanation:

Rating of bulbs hasn't been specified which is very important. Let's assume each bulb is rated at 120V,50Hz supply. Now what it means is that when 40W (or 60W) bulb will be connected across rated voltage i.e.120V ac supply then power dissipation across it will be 40W (or 60W). Rating is important because say if 40W bulb rated at 120V supply is connected across 220V supply then it will not work (might burn or blow up), meaning thereby a device can not work at a voltage much higher than its rated voltage.

One more information that you get from rating is the resistance of each bulb.

P=(V*V)/R

Since both bulbs are rated at same voltage, we can say that resistance of each bulb is inversely proportional to its rated power. Therefore

R (40W bulb) > R (60W bulb)

We should know that the bulb across which power dissipation is more will glow brighter. Now

1. When connected in series: In a series connection, current flowing across each element is same. So when 40W bulb and 60W bulb are connected in series, same current will flow through them. To find which bulb will glow brighter we need to find the power dissipation across each of them. From the relation

P=(I*I) R

since current is same we can say that power dissipation will be higher for the bulb with higher resistance i.e. 40W bulb.

Hence 40W bulb will glow brighter in series connection.

2. When connected in parallel: In a parallel connection, voltage across each element is same. So when 40W bulb and 60W bulb are connected in parallel, voltage across them will be same (100 V in the given case). To find which bulb will glow brighter we need to find the power dissipation across each of them. From the relation

P=(V*V)/R

since voltage is same we can say that power dissipation will be higher for the bulb with lower resistance i.e. 60W bulb.

Hence 60W bulb will glow brighter in parallel connection.

NOW HOW TO REMEMBER THIS-

At our homes, loads (such as bulbs) are connected in parallel and you always see that higher rated bulb glows more brightly i.e 100W bulb glows more brightly than 60W bulb or 40W bulb.

So always remember if bulbs are connected in parallel, the bulb with higher rated power will glow brighter and if they are connected in series, the bulb with lower rated power will glow brighter.

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