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A soap bubble is being blown at the open end of a tube of radius 1 cm. The film at the mouth of the tube makes an angle of 60 deg with the tube wall, and the surface tension of the soap solution is 0.05 N/m. Find the gauge (excess over atmospheric) pressure inside the tube.
- 5 Pa
- 1 Pa
- 10 Pa
- 20 Pa
Correct answer: 10 Pa
Solution
The bubble is a spherical cap on the tube mouth. The tube radius is r = R*cos(60 deg), so R = r/cos(60 deg) = 2r. The excess pressure is then 4T/R.
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