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JEE Main 2018 · Bulk Modulus & Compressibility

Bulk Modulus & Compressibility question from JEE Main 2018

Q18JEE Main 2018Difficult

Bulk Modulus & Compressibility

A solid sphere of radius r made of a soft material of bulk modulus K is surrounded by a liquid in a cylindrical container. A massless piston of area a floats on the surface of the liquid, covering entire cross section of cylindrical container. When a mass m is placed on the surface of the piston to compress the liquid, the fractional decrement in the radius of the sphere, (drr)\left(\frac{dr}{r}\right) is:

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Answer: (d)

  1. Extra pressure in the liquid (Pascal's law): P=mgaP = \frac{mg}{a}, acting on the sphere from all sides.
  2. ΔVV=PK=mgKa\frac{\Delta V}{V} = \frac{P}{K} = \frac{mg}{Ka}
  3. ΔVV=3drr⇒drr=mg3Ka\frac{\Delta V}{V} = 3\frac{dr}{r} \Rightarrow \frac{dr}{r} = \frac{mg}{3Ka}
  4. So the answer is (d).
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