Question Details

If work W is done in blowing a bubble of radius R from a soap solution, then the work done in blowing a bubble of radius 2R from the same solution is

Options

A

W/2

B

2W

C

4W

D

2W/3

Correct Answer :

4W

Solution :

The correct option is 4W.

Step-by-step Explanation:
When a soap bubble is blown in air, it has two free surfaces (an inner surface and an outer surface) in contact with the air.
The work done W in blowing a soap bubble of radius R is equal to the increase in its surface energy.
The formula for the work done in blowing a soap bubble of radius R with surface tension T is given by:

W=2×T×A

where A is the surface area of a sphere of radius R, which is 4πR2.
Substituting the area into the equation:

W=2×T×(4πR2)=8πTR2

From this expression, we can see that since the surface tension T of the same soap solution remains constant, the work done W is directly proportional to the square of the radius R of the bubble:

WR2

Let W1=W be the work done for a bubble of radius R1=R.
Let W2 be the work done for blowing a bubble of radius R2=2R.
Taking the ratio of the work done in both cases:

W2W1=(R2R1)2

Substituting the given values:

W2W=(2RR)2

W2W=22=4

W2=4W

Therefore, the work done in blowing a bubble of radius 2R from the same solution is 4W.

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