The electric field at point P due to a charged ball is given by
`E_(p)=(1)/( 4pi epsilon_(0))(q)/(r^(2))`
To measure 'E' at point P, A test charge `q_(0)` is placed at point P and measure electric force F upon the test charge. Check whether `(F)/(q_(0))` is equal to ` (1)/(4pi epsilon_(0))(q)/(r^(2))` or not .

Explanation

The formula `E= (1)/(4pi epsilon_(0))(q)/(r^(2))` is valid only when source charge is ficxed. In this problem, source charge is not fixes. When test charge is placed at point P, the ball will be deflected backward. So, `(F)/(q_(0)) lt (1)/( 4pi epsilon_(0)) . (q)/(r^(2))`( It is because effective 'r' is increased )