A U-tube contains mercury (specific gravity 13.6) in one arm and water in the other. If the mercury levels in the two arms differ by 2 cm, find the approximate height h of the water column.

22 cm
25 cm
27 cm
29 cm

Correct answer: 27 cm

Solution

For a U-tube manometer, the pressure at the lowest common interface from the water column equals the pressure from the mercury height difference. So rho_w * h = rho_Hg * (mercury level difference). With mercury difference = 2 cm and specific gravity 13.6: h = 13.6 * 2 = 27.2 cm, approximately 27 cm.

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