A $+ 3{\text{ C}}$ charge and a $- 4{\text{ C}}$ charge are a distance $x$ apart. P is a distance $x$ from the $+ 3{\text{ C}}$ charge on the straight line joining the charges.

What is the magnitude of the electric field strength at P?

A.     $\frac{1}{{\pi {\varepsilon _0}{x^2}}}$

B.     $\frac{1}{{2\pi {\varepsilon _0}{x^2}}}$

C.     $\frac{1}{{4\pi {\varepsilon _0}{x^2}}}$

D.     $\frac{1}{{7\pi {\varepsilon _0}{x^2}}}$

B

Although it is convenient to use k in the equation for coulombic attraction, candidates need to be able to unpack this as $\frac{1}{{4\pi {\varepsilon _0}}}$.