Surface Tension and Viscosity

Practice Questions:

Question 1: easy

Two unequal soap bubbles are formed one on each side of a tube closed in the middle by a tap. What happens when the tap is opened to put the two bubbles in communication ?

1. No air passes in any direction as the pressure are same on two sides of the tap

2. Large bubble shrinks and smaller bubble increases in size till they become equal in size

3. Smaller bubble gradually collapses and the bigger one increase in size

4. None of the above

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Question 2: easy

The radii of the two columns in U tube are r1 and r2 (r1 > r2). When a liquid of density \[\rho\] (angle of contact is 0º) is filled in it, the level difference of liquid in two arms is h. The surface tension of liquid is (g = acceleration due to gravity):

1. \[\frac{\rho ghr_{1}r_{2}}{2(r_{1}-r_{2})}\]

2. \[\frac{\rho gh(r_{1}-r_{2})}{2r_{1}r_{2}}\]

3. \[\frac{2 (r_{2}-r_{1})}{\rho ghr_{1}r_{2}}\]

4. \[\frac{\rho gh}{2(r_{2}-r_{1})}\]

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Question 3: easy

If force of cohesion of liquid is three times the force of adhesion between liquid and glass tube molecules, then the shape of meniscus of liquid in this glass tube is:

1. concave upward

2. convex upward

3. plane

4. none of these

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Question 4: moderate

The terminal velocity of a copper ball of radius 2.0 mm falling through a tank of oil at 20°C is 6.5 cms–1. Calculate the viscosity of the oil at 20°C. (Density of oil is \[1.7\times 10^{3}kgm^{-3}\] and density of copper is \[8.9\times 10^{3}kgm^{-3}\]) :

1. \[0.69 kg m^{-1} s^{-1}\]

2. \[0.79 kg m^{-1} s^{-1}\]

3. \[0.29 kg m^{-1} s^{-1}\]

4. \[0.99 kg m^{-1} s^{-1}\]

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Question 5: moderate

A lead sphere of mass m falls in viscous liquid with terminal velocity v0. Another lead sphere of mass M falls through the same viscous liquid with terminal velocity 4v0. the ratio M/m is :

1. 2

2. 4

3. 8

4. 16

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Question 6: easy

Two objects A & B of equal density and radius rA = 1 mm and rB = 2 mm are moving in same medium then find the ratio of their terminal velocity $ \frac{v_{B}}{v_{A}}$ in the medium.

1. 1/4

2. 1/2

3. 4

4. 2

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The terminal velocity $v$ of a spherical object moving through a viscous medium is directly proportional to the square of its radius, expressed as $v \propto r^2$

$v \propto r^2$ , when the density and the medium remain constant.

$\frac{v_B}{v_A} = \left(\frac{r_B}{r_A}\right)^2 = \left(\frac{2}{1}\right)^2 = 4$

Thus, the required ratio \( $\frac{v_B}{v_A}$ is 4 (Option 3).

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