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This question already has an answer here:

Why a test charge is always positive. WHy cant we assume the test charge to be negative? Just that if we assume it to be negative test charge, will the electric field turns to be negative?

Additional Details reply: does it mean that if we put a negative charge as a test charge near a positive charge source, it shows that the positive charge actually exerts a field pointing inward? before, if we put a positive charge near a positive source, it means field radially outward,how if we change the test charge to a negative charge?

Thanks in advance!

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marked as duplicate by sammy gerbil, Bill N, John Rennie homework-and-exercises Jul 9 '17 at 4:24

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  • $\begingroup$ This is a matter of convention. If you choose it to be negative the directions of the field vectos will be reversed. $\endgroup$ – Alpha001 Jul 8 '17 at 12:40
  • $\begingroup$ Is electric field assumed to be positive by convention? $\endgroup$ – user419155 Jul 8 '17 at 14:07
  • $\begingroup$ No. But you can change your convention (i.e. for the pictures of field lines . For exampel of a point charge). $\endgroup$ – Alpha001 Jul 8 '17 at 15:36
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The direction of the electric field depends on the source charge, not the test charge. However, the direction of the force does depend on the signs of the test charge and the source charge. Since $$ {\vec F}={q_{test}}\vec E $$ using a positive $q$ means direction of $\vec E$ is simply the direction of $\vec F$.

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we can put a charge with known electric quantity positive or negative in the field and observe its reaction and with coulomb`s law we can know the property of source charge.The field of source charge is not determined by test charge,for the same source charge,test charge with the same electric property tends to be rejected and that with opposite electric property tends to be attracted.

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