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I am new to field of electricity so my knowledge of it is very basic. I am learning about electric fields and was wondering for a while why small test charges are used to test electric field instead of regular big charges. I think i know the reason for this now. But then why do we see example of question in physics (like second picture above) where we have to draw electric field lines with multiple big, regular charges and test charge in various location. Wouldn’t the presence of big charges next to each other affect the electric field line we are trying to draw and move each other, thus defeating purpose of using small test charge to draw the electric field lines in first place. Thank You.

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    $\begingroup$ The bigger the charge the more it affects your actual field. The idea of a test charge is used to define the E = F/q which appeared not to depend on the test charge value q, thereby being a characteristic only of a field. $\endgroup$
    – Ice-Nine
    Jan 18 '17 at 22:52
  • $\begingroup$ You will come across this if/when you cover superposition of charges, and the calculations involved in their net effect. Although this is already an example. , I suppose. $\endgroup$
    – user140606
    Jan 18 '17 at 22:57
  • $\begingroup$ I know that principle of superposition of charges is involved in the second picture. But i was wondering how is the drawing of electric fields involving multiple regular charges (like you see in second picture) accurate. Wouldn't the line (like the way they look in drawing) be diffeent since the force between the regular charges causes displacement of one or both of the charges? $\endgroup$
    – TLo
    Jan 18 '17 at 23:17

The reason we use small magnitude test charges is that, if it were large then the test charge would change the field, like what you see in the pictures you uploaded. The test charge is very small such that the effect due to its presence is negligible.

Now when we consider a system of charges, two or more charges like in the pictures you uploaded, we assume that the charges are very firmly held such that no force would move them. But yes, it would be meaningless of drawing the field lines of a system of charges if the charges were not firmly held. The charges would either attract or repel as per the coulomb's law if it were not firmly held. After all there is reason it is called 'electrostatics' meaning study of charges at rest and not electrodynamics which is study of charges at movement.


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