Q.
A U - shaped wire is dipped in a soap solution and removed. The thin soap film formed between the wire and light slider supports a weight of 1.5×10−2N . The length of the slider is 30cm . What is the surface tension of the film?
A soap film has two free surfaces, so the total length of the film to be supported, l=2×30cm=0.60m
Let the surface tension of the film
If the total on the slider due to surface tension,
then f=T×2l=T×0.6N W=1.5×10−2N
In equilibrium position, the force f on the slider due to surface tension must be balanced by the weight (w) supported by the slider
i.e., f=w=mg T×0.6=1.5×10−2 T=0.61.5×10−2 T=2.5×10−2Nm−1