TS EAMCET · Physics · Thermodynamics

A metal sheet $4 m$ on a side and of negligible thickness is attached to a balance and inserted into container fluid. The balance to which metal sheet is attached reads $0.50 N$ and the contact angle is found to be zero. A small amount of oil is then spread over the metal sheet. The contact angle now becomes $180 °$ and the balance now reads $0.49 N .$ The surface tension of the fluid is

A $6.25 \times 10^{- 3} N m^{- 1}$
B $1.25 \times 10^{- 1} N m^{- 1}$
C $4.25 \times 10^{- 2} N m^{- 1}$
D $0.1 N m^{- 1}$

Verified Solution

Answer & Solution

Correct Answer

(A) $6.25 \times 10^{- 3} N m^{- 1}$

Step-by-step Solution

Detailed explanation

In the first case, when the contact angle is zero, the liquid will apply a force on the metal sheet in the downwards direction. Net force equation can be written as 0.50=mg+T4×2 ....1 When the oil is applied on the metal sheet, the contact angle becomes 180° and in…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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