Mixing Droplets

If 1000 droplets each of charge q q and radius r r are mixed to form a big drop, then the potential of big drop, as compared to small droplet, will be:

None of these. 1000 times 500 times Depends on size of droplet. 100 times 1 time

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2 solutions

Akhil Bansal
Nov 17, 2015

Assuming drops to be a spherical in shape.
Let charge on smaller drop be q 1 \color{#3D99F6}{q_1} and charge on bigger drop be q 2 \color{#D61F06}{q_2} .
On mixing all the small drops, q 2 = 1000 q 1 \color{#D61F06}{q_2} = 1000\color{#3D99F6}{q_1} .
Volume of 1000 drops (radius r 1 \color{#3D99F6}{r_1} ) = Volume of bigger drop (radius r 2 \color{#D61F06}{r_2} )
1000 × 4 3 π r 1 3 = 4 3 π r 2 3 \Rightarrow 1000 \times \dfrac{4}{3}\pi \color{#3D99F6}{r_1}^3 = \dfrac{4}{3}\pi \color{#D61F06}{r_2}^3 10 r 1 = r 2 \Rightarrow 10 \color{#3D99F6}{r_1} = \color{#D61F06}{r_2}

Therefore, radius of bigger drop is 10 times the radius of smaller drops.

As, V 1 = k q 1 r 1 ( V 1 = potential of smaller drop ) \Rightarrow V_1 = \dfrac{k \color{#3D99F6}{q_1}}{\color{#3D99F6}{r_1}} \quad \quad \quad \quad \quad \quad \quad (V_1 = \text{potential of smaller drop})

V 2 = k q 2 r 2 = 1000 k q 1 10 r 1 = 100 V 1 ( V 2 = potential of bigger drop ) \Rightarrow V_2 = \dfrac{k \color{#D61F06}{q_2}}{\color{#D61F06}{r_2}} = \dfrac{1000k\color{#3D99F6}{q_1}}{10\color{#3D99F6}{r_1}} = 100V_1 \quad \quad (V_2 = \text{potential of bigger drop})

Its a popular question and answer is= n^(2/3)* v. Putting n and v we get the answer.

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