Question Details

If the work done in blowing a bubble of volume V is W, then the work done in blowing the bubble of volume 2V from the same soap solution will be

Options

A

W/2

B

W √2

C

∛2 W

D

∛4 W

Correct Answer :

∛4 W

Solution :

To find the work done in blowing a soap bubble, we need to understand the relationship between the work done, the surface tension of the soap solution, and the surface area of the bubble.

A soap bubble has two free surfaces (an inner surface and an outer surface) in contact with air. Therefore, the work done W in blowing a bubble of radius R is equal to the surface tension T multiplied by the total increase in surface area:
W=2·T·A
where A=4πR2 is the surface area of a sphere of radius R.
Substituting A into the equation gives:
W=8πR2T

Now, let's relate the radius R to the volume V of the bubble. The volume of a spherical bubble is given by:
V=43πR3
From this, we can express the radius R in terms of the volume V:
R3=3V4π
Taking the cube root on both sides:
R=3V4π13

Now, substitute this expression for R back into the formula for the work done W:
W=8πT3V4π132
W=8πT3V4π23
Since the surface tension T of the same soap solution remains constant, we can see that the work done W is directly proportional to V23:
WV23

Let W1=W be the work done to blow a bubble of volume V1=V.
Let W2 be the work done to blow a bubble of volume V2=2V.
Using the proportionality relation:
W2W1=V2V123
Substitute the values of V1 and V2:
W2W=2VV23
W2W=223
W2=2213W
W2=413W=4W

Therefore, the work done in blowing the bubble of volume 2V is 4W.

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