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Q. 11

Physics for Scientists and Engineers: A Strategic Approach with Modern Physics
Found in: Page 1082

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Short Answer

An oil droplet with excess electrons is observed between two parallel electrodes spaced apart. The droplet hangs motionless if the upper electrode is more positive than the lower electrode. The density of the oil is . What is the radius of the droplet?

A drop will remain suspended between the plates, moving neither up nor down, if the electric field exerts an upward force on a charged drop that exactly balances the downward gravitational force. The forces balance when (m is mass of drop and q is charge on drop).

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Step by Step Solution

Step 1:Given

It was given to us that there is oil droplet of excess electrons between parallel plates which are apart.


Density of oil=

droplet consists of 15 excess electron means it will charge of 15 electrons

Step 2:Finding the radius 

Using formula:

Now using:

Equating forces acting on droplet:

Hence, radius of droplet is

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