JEE Advanced · 2010 · 11 Apr 2010 · Paper 2 · Fluid Mechanics

When liquid medicine of density

ρ

is to be put in the eye, it is done with the help of a dropper. As the bulb on the top of the dropper is pressed, a drop forms at the opening of the dropper. We wish to estimate the size of the drop. We first assume that the drop formed at the opening is spherical because that requires a minimum increase in its surface energy. To determine the size, we calculate the net vertical force due to the surface tension

T

when the radius of the drop is

R

. When this force becomes smaller than the weight of the drop, the drop gets detached from the dropper.

If

r = 5 \times 1 0^{- 4} m, ρ = 1 0^{3} kgm^{- 3}, g = 10 ms^{- 2}, T = 0.11 Nm^{- 1}

, the radius of the drop when it detaches from the dropper is approximately

(A) $1.4 \times 1 0^{- 3} m$ ✓ Correct
(B) $3.3 \times 1 0^{- 3} m$
(C) $2.0 \times 1 0^{- 3} m$
(D) $4.1 \times 1 0^{- 3} m$

Answer: (A)

Solution

At detachment, the surface tension force equals the weight:

\frac{2 π r ^{2} T}{R} = m g = \frac{4}{3} π R^{3} ρ g

Solving for

R

\frac{2 π r ^{2} T}{R} = \frac{4}{3} π R^{3} ρ g

R^{4} = \frac{3 r ^{2} T}{2 ρ g}

R = (\frac{3 r ^{2} T}{2 ρ g})^{1/4}

Substituting values:

R = (\frac{3 \times ( 5 \times 1 0 ^{- 4} ) ^{2} \times 0.11}{2 \times 1 0 ^{3} \times 10})^{1/4} \approx 1.4 \times 1 0^{- 3} m

The answer is **(A)

1.4 \times 1 0^{- 3}

m**.

JEE Advanced · 2010 · 11 Apr 2010 · Paper 2 · Single correct option

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JEE Advanced · 2010 · 11 Apr 2010 · Paper 2 · Fluid Mechanics

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