Question

What is difference between source charge and test charge?

Answer 1

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$\mathfrak\red{Point\: charge :}$Any charge whether positive or negative, whose electric field is to be found at a particular distance(point) is called point charge.

$\mathfrak\blue{Test \:charge :}$  Any charge whose magnitude is very small, in fact negligible, as compared to that of the point charge, and which does not affect the electric field of the point charge, whose magnitude is to be found out, is called test charge.