It is purely a convention....

the concept of charges was known as early as in 600bc

actually speaking the +ve charge has  outward electric lines of force and -ve charge has inward electric line of force.

u may imagine the +ve charge as a donar and -ve charge as an acceptor{just imagine}.Then u can think that no two donars or no two acceptor will have any attraction because both will have same kind of electric lines of force.the attraction is only possible when one is donar and one is acceptor.

but still its merely a convention since the names +ve & -ve charges are itself conventions

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