Question

What fluid is in the device shown in Figure 11.28 if the force is $3.16 \times 10^{-3}\textrm{ N}$ and the length of the wire is 2.50 cm? Calculate the surface tension $\gamma$ and find a likely match from Table 11.3.

Question Image

**Figure 11.28**Sliding wire device used for measuring surface tension; the device exerts a force to reduce the film’s surface area. The force needed to hold the wire in place is F = γL = γ(2l) , since there are two liquid surfaces attached to the wire. This force remains nearly constant as the film is stretched, until the film approaches its breaking point.

**Table 11.3**Surface tension of some liquids.

$0.0632 \textrm{ N/m}$

This liquid is

This liquid is

**glycerin**.Solution Video

#### Sign up to view this solution video!

## Calculator Screenshots

Video Transcript

This is College Physics Answers with Shaun Dychko. This device has a sliding wire on this wire frame with a fluid in between here— it is for measuring the surface tension of this fluid— and the force that has to be exerted to keep the wire in place to balance the total force in the surface tension equals the surface tension,

*γ*, times 2 times this length of the surface in contact with the wire. So the force we are told is 3.16 times 10 to the minus 3 newtons and the length of this sliding wire is 2.50 centimeters which I have converted to 2.50 times 10 to the minus 2 meters and we rearrange this formula here to solve for*γ*and we divide both sides by 2*l*and then switch the sides around and*γ*equals force divided by 2 times the length. So that's 3.16 times 10 to the minus 3 newtons divided by 2 times 2.50 times 10 to the minus 2 meters and that's 0.0632 newtons per meter. And if can compare that to our data table for surface tension [11.3], "Glycerin" has a surface tension of close to 0.0632 and this is 0.0631— that's close enough to be a match— this material must be glycerin.