First slide

Surface tension

Question

A spherical soap bubble has a radius r. The surface tension of the soap film is T. The energy needed to double the diameter of the bubble at the same temperature is:

Moderate

A

$24 {πr}^{2} T$
B

$4 {πr}^{2} T$
C

$2 {πr}^{2} T$
D

$12 {πr}^{2} T$

Solution

$W = 8 πT (R_{2}^{2} - R_{1}^{2}) = 8 πT [{(2 r)}^{2} - r^{2}] = 24 {πr}^{2} T$

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