Eduprobe
Question:
A thin liquid film formed between a U-shaped wire and light slider supports a weight of 1.5 × 10⁻⁶ N (see figure). If the length of the slider is 30 cm and its weight is negligible, then the surface tension of the liquid film is:
0.0125 Nm⁻¹
0.1 Nm⁻¹
0.05 Nm⁻¹
0.025 Nm⁻¹
Solution:
Here, W = 1.5 × 10⁻⁶ N, l = 30 cm = 30 × 10⁻² m
The force due to surface tension will balance the weight. i.e., F = W
A liquid film has two free surfaces.
2Tl = W or T = W/2l
T = 1.5 × 10⁻⁶ / (2 × 30 × 10⁻²) = 0.025 Nm⁻¹
A slider will support the weight when the force of surface tension acting upwards on the slider (2Sl) balances the downward force due to weight (W).