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A very light, rectangular wire-frame of dimensions 7 cm × 5 cm hangs just above the free surface of a liquid of surface tension T , with its plane parallel to the free surface. The wire-frame is just brought in contact with the liquid surface and then, lifted up. If the force required to lift the wire-frame is 3 . 36 N , then what is the value of T (in N m - 1 )?

Correct answer

7

Step-by-step solution

The downward force of surface tension acting on the wire frame is F surface   tension = T × l , where l is the total length of the wire which is in contact with the liquid. Now since it is a wire frame, the free surface of the liquid will be in contact with the inner as well as the outer perimeter of the wire. Hence, l = 2 × 2 × 7 + 5 = 48   cm T × 48 100 = 3 . 36   N ⇒ T = 7   N   m - 1

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