Question

Charges of $40\mu C,-40\mu C$ and$10\mu C$ are placed in air at the corners A, B and C respectively of an equilateral triangle of side $2cm$. The resultant force on the charge at C is :

• A. $9000N$
  
• B. $900N$
• C. $4500N$
• D. $450N$

Answer 1

The correct option is A

$9000N$

Let $40\mu C$ charge be q and $-10\mu C$ be $q^{{}^{\prime }}$.
Force between $q$ and $q^{{}^{\prime }}$ $F=Kq{q}^{{}^{\prime }}/r^2$
Force between $-q$ and $q^{{}^{\prime }}$ $F=-Kq{q}^{{}^{\prime }}/r^2$
Force along $y-axis$ get cancelled and force along $x-axis$ is given by
$F_x=2\times Kq{q}^{{}^{\prime }}/r^{2}cos{60}^0=9000N$