JEE Physics Mechanical Properties Of Fluids MCQs Set 08

Practice MCQs for JEE Physics Mechanical Properties Of Fluids

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Multiple Choice Questions

Question 1: The shape of the meniscus for a liquid of zero angle of contact is:
(a) plane
(b) parabolic
(c) hemispherical
(d) cylindrical
Show Answer & Explanation

Answer: (c) hemispherical

Explanation:
1. For a zero angle of contact the surface meets the wall tangentially, forming a hemispherical concave surface

Teacher's Note:
1. Apply the standard formula and check the units.

Question 2: Due to capillary action a liquid will rise in a tube if the angle of contact is:
(a) acute
(b) obtuse
(c) 90 degrees
(d) 180 degrees
Show Answer & Explanation

Answer: (a) acute

Explanation:
1. Liquids rise when the contact angle is acute (wetting) and fall when it is obtuse

Teacher's Note:
1. Apply the standard formula and check the units.

Question 3:
Assertion (A): The angle of contact does not depend upon the inclination of the solid in the liquid.
Reason (R): The angle of contact depends on the cohesive and adhesive forces.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The angle is fixed by the liquid-solid-air combination of forces
2. Inclination does not change those forces

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 4: Two droplets merge with each other to form a larger droplet. In this process:
(a) energy is liberated
(b) energy is absorbed
(c) energy is neither liberated nor absorbed
(d) some mass is converted into energy
Show Answer & Explanation

Answer: (a) energy is liberated

Explanation:
1. The total surface area decreases, so surface energy is released

Teacher's Note:
1. Apply the standard formula and check the units.

Question 5: Two capillary tubes of the same diameter are put vertically one in each of two liquids whose relative densities are 0.8 and 0.6 and surface tensions are 60 dyne/cm and 50 dyne/cm respectively. The ratio of the heights h1/h2 is:
(a) 10/9
(b) 3/10
(c) 10/3
(d) 9/10
Show Answer & Explanation

Answer: (d) 9/10

Explanation:
1. h = 2T cos(theta)/(r rho g), so h1/h2 = (T1/rho1)/(T2/rho2) = (60/0.8)/(50/0.6) = 75/83.3 = 9/10

Teacher's Note:
1. Apply the standard formula and check the units.

Question 6: The property utilised in the manufacture of lead shots is:
(a) specific weight of liquid lead
(b) specific gravity of liquid lead
(c) compressibility of liquid lead
(d) surface tension of liquid lead
Show Answer & Explanation

Answer: (d) surface tension of liquid lead

Explanation:
1. Molten lead dropped from a height forms spheres because of surface tension

Teacher's Note:
1. Apply the standard formula and check the units.

Question 7: The surface tension of a liquid is 5 N/m. If its thin film is made in a ring of area 0.02 m2, then its surface energy will be:
(a) \( 5\times10^{-2} \) J
(b) \( 2.5\times10^{-2} \) J
(c) \( 3\times10^{-1} \) J
(d) \( 2\times10^{-1} \) J
Show Answer & Explanation

Answer: (d) \( 2\times10^{-1} \) J

Explanation:
1. A thin film has two surfaces
2. Energy = T x 2 x area = 5 x 2 x 0.02 = 0.2 J

Teacher's Note:
1. Apply the standard formula and check the units.

Question 8:
Assertion (A): The surface tension of a liquid decreases with an increase in temperature.
Reason (R): The cohesive force between liquid molecules increases with an increase in temperature.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Greater thermal motion weakens the net inward pull, so surface tension decreases and A is true
2. Cohesive force decreases with temperature, so R is false

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 9: If one end of a capillary tube is dipped in water, water rises up to 3 cm. If the surface tension of water is \( 75\times10^{-3} \) N/m, the diameter of the capillary tube is:
(a) 0.1 mm
(b) 0.5 mm
(c) 1 mm
(d) 2 mm
Show Answer & Explanation

Answer: (c) 1 mm

Explanation:
1. r = 2T/(h rho g) = 2 x 0.075/(0.03 x 1000 x 10) = 5e-4 m
2. Diameter = 1 mm

Teacher's Note:
1. Apply the standard formula and check the units.

Question 10: If the surface tension of a liquid is T and its surface area is increased by A, the surface energy of that surface will be increased by:
(a) AT
(b) A/T
(c) \( A^2T \)
(d) \( A^2T^2 \)
Show Answer & Explanation

Answer: (a) AT

Explanation:
1. Work done = T x change in area = AT

Teacher's Note:
1. Apply the standard formula and check the units.

Question 11: Two soap bubbles of radii 4 cm and 5 cm are touching each other over a common surface S1S2. The radius of the common surface is:
(a) 4 cm
(b) 20 cm
(c) 5 cm
(d) 4.5 cm
Show Answer & Explanation

Answer: (b) 20 cm

Explanation:
1. r = r1 r2/(r2 - r1) = 4 x 5/(5 - 4) = 20 cm

Teacher's Note:
1. Apply the standard formula and check the units.

Question 12: The radius of a soap bubble is r. The surface tension of the soap solution is T. Keeping the temperature constant, the radius of the soap bubble is doubled. The energy necessary for this is:
(a) \( 24\pi r^2T \)
(b) \( 8\pi r^2T \)
(c) \( 12\pi r^2T \)
(d) \( 16\pi r^2T \)
Show Answer & Explanation

Answer: (a) \( 24\pi r^2T \)

Explanation:
1. A bubble has two surfaces
2. Change in energy = 2T x 4 pi ((2r)^2 - r^2) = 24 pi r^2 T

Teacher's Note:
1. Apply the standard formula and check the units.

Question 13:
Assertion (A): Smaller drops of liquid resist deforming forces better than larger drops.
Reason (R): The excess pressure inside a drop is directly proportional to its surface area.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Excess pressure 2T/r is larger in smaller drops, so A is true
2. It is inversely proportional to the radius, not directly proportional to area, so R is false

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 14: A liquid does not wet the sides of a solid if the angle of contact is:
(a) zero
(b) obtuse (more than 90 degrees)
(c) acute (less than 90 degrees)
(d) 45 degrees
Show Answer & Explanation

Answer: (b) obtuse (more than 90 degrees)

Explanation:
1. A liquid does not wet the solid when the contact angle is obtuse

Teacher's Note:
1. Apply the standard formula and check the units.

Question 15: The excess pressure inside a soap bubble over the outer pressure is:
(a) \( 2T/r \)
(b) \( 4T/r \)
(c) \( T/2r \)
(d) \( T/r \)
Show Answer & Explanation

Answer: (b) \( 4T/r \)

Explanation:
1. A soap bubble has two surfaces, so the excess pressure is 4T/r

Teacher's Note:
1. Apply the standard formula and check the units.

Question 16: In a capillary tube experiment, a vertical 30 cm long capillary tube is dipped in water. The water rises to a height of 10 cm due to capillary action. If this experiment is conducted in a freely falling elevator, the length of the water column becomes:
(a) 10 cm
(b) 20 cm
(c) 30 cm
(d) zero
Show Answer & Explanation

Answer: (c) 30 cm

Explanation:
1. In free fall the effective g is zero, so the water fills the whole tube of length 30 cm

Teacher's Note:
1. Apply the standard formula and check the units.

Question 17: The radius of a capillary is \( 2\times10^{-3} \) m. A liquid of weight \( 6.2\times10^{-4} \) N may remain in the capillary. The surface tension of the liquid is:
(a) \( 5\times10^{-3} \) N/m
(b) \( 5\times10^{-2} \) N/m
(c) 5 N/m
(d) 50 N/m
Show Answer & Explanation

Answer: (b) \( 5\times10^{-2} \) N/m

Explanation:
1. Weight = 2 pi r T
2. T = 6.2e-4/(2 pi x 2e-3) = 0.049, about 5e-2 N/m

Teacher's Note:
1. Apply the standard formula and check the units.

Question 18:
Assertion (A): A thin stainless steel needle can lie floating on a still water surface.
Reason (R): Any object floats when the buoyancy force balances the weight of the object.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Both statements are true
2. The needle floats because of surface tension, not buoyancy, so R is not the explanation

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 19: A capillary tube of radius r can support a liquid of weight \( 6.28\times10^{-4} \) N. If the surface tension of the liquid is \( 5\times10^{-2} \) N/m, the radius of the capillary must be:
(a) \( 2\times10^{-3} \) m
(b) \( 2\times10^{-4} \) m
(c) \( 1.5\times10^{-3} \) m
(d) \( 12.5\times10^{-4} \) m
Show Answer & Explanation

Answer: (a) \( 2\times10^{-3} \) m

Explanation:
1. r = W/(2 pi T) = 6.28e-4/(2 pi x 5e-2) = 2e-3 m

Teacher's Note:
1. Apply the standard formula and check the units.

Question 20: Water rises in a capillary tube such that the upward force of surface tension balances the weight of water, which is \( 75\times10^{-4} \) N. If the surface tension of water is \( 6\times10^{-2} \) N/m, the internal circumference of the capillary must be:
(a) \( 12.5\times10^{-2} \) m
(b) \( 6.5\times10^{-2} \) m
(c) \( 0.5\times10^{-2} \) m
(d) \( 1.25\times10^{-2} \) m
Show Answer & Explanation

Answer: (a) \( 12.5\times10^{-2} \) m

Explanation:
1. T x circumference = W
2. Circumference = 75e-4/6e-2 = 0.125 m

Teacher's Note:
1. Apply the standard formula and check the units.

Question 21: Two small drops of mercury, each of radius R, coalesce to form a single large drop. The ratio of the total surface energies before and after the change is:
(a) 1 : 2^(1/3)
(b) 2^(1/3) : 1
(c) 2 : 1
(d) 1 : 2
Show Answer & Explanation

Answer: (b) 2^(1/3) : 1

Explanation:
1. New radius = 2^(1/3) R
2. Energy before = 2 x 4 pi R^2 T and after = 4 pi R^2 2^(2/3) T
3. Ratio = 2 : 2^(2/3) = 2^(1/3) : 1

Teacher's Note:
1. Apply the standard formula and check the units.

Question 22: Inside a drop, the excess pressure is maximum in a drop of diameter:
(a) \( 0.200\ \mu m \)
(b) \( 20.0\ \mu m \)
(c) \( 200\ \mu m \)
(d) \( 2.0\ \mu m \)
Show Answer & Explanation

Answer: (a) \( 0.200\ \mu m \)

Explanation:
1. Excess pressure 2T/r is largest for the smallest drop

Teacher's Note:
1. Apply the standard formula and check the units.

Question 23:
Assertion (A): Surface tension is a property of only liquids.
Reason (R): Only liquids have free surfaces among fluids.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Gases fill their container and have no free surface
2. Surface tension needs a free surface

Teacher's Note:
1. Decide whether each statement is true, then whether R explains A.

Question 24: The diameter of one drop of water is 0.2 cm. The work done in breaking one drop into 1000 equal droplets will be (surface tension of water = \( 7\times10^{-2} \) N/m):
(a) \( 7.9\times10^{-6} \) J
(b) \( 5.92\times10^{-6} \) J
(c) \( 2.92\times10^{-6} \) J
(d) \( 1.92\times10^{-6} \) J
Show Answer & Explanation

Answer: (a) \( 7.9\times10^{-6} \) J

Explanation:
1. Radius of the big drop = 1e-3 m and of a droplet = 1e-4 m
2. Increase in area = 4 pi(1000 x 1e-8 - 1e-6) = 1.13e-4 m2
3. Work = T x area = 7e-2 x 1.13e-4 = 7.9e-6 J

Teacher's Note:
1. Apply the standard formula and check the units.

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Practice MCQs for JEE Physics Mechanical Properties Of Fluids

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