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  4. Two unit negative charges are placed on a straight line. A positive charge q is placed exactly at the mid-point between these unit charges. If the system of three charges is in equilibrium the value of q (in C) is:

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Q. Two unit negative charges are placed on a straight line. A positive charge q is placed exactly at the mid-point between these unit charges. If the system of three charges is in equilibrium the value of q (in C) is:

EAMCETEAMCET 2006Electric Charges and Fields

Solution:

For equilibrium of charge q.
$ \frac{1}{4\pi {{\varepsilon }_{0}}}\frac{-O\times q}{{{(r/2)}^{2}}}+\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{(-Q)(-Q)}{{{r}^{2}}}=0 $ $ \Rightarrow $ $ 4q=Q $ $ \Rightarrow $ $ a=\frac{Q}{4}=\frac{1}{4} $ $ (\because \,Q=1\,C) $ $ =0.25\,C $

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