# Giancoli Physics Book -- confused at meaning of "test charge" polarity

1. Feb 4, 2016

### kiwibird4

In the giancoli physics textbook for engineers and scientists, it says
"E=F/q where E is the force on a small positive test charge at rest...."
Then about one paragraph later it proceeds to say
"If q is positive, F and E point in the same direction. If q is negative, F and E point in opposite directions".

I am confused. The text just said that the test charge in the E field equation will always be considered a -positive- charge, but then they talk about scenarios where it is negative, contradicting their own definition.
and if a test charge was then (contradicting their definition) negative, and "big" Q is negative, both the e-field and the force would be repulsive, pointing in the same direction whereas they said "if q is negative F and E point in opposite directions" (which would only be true if you assume the direction of the e field -where negative source charge has e field towards it- because you have a positive test charge)

I do not understand what I am missing so any help in clarifying this problem would be helpful.

2. Feb 4, 2016

### SammyS

Staff Emeritus
In the above, the role of the "test charge" is to define and/or determine the electric field, E, at some location. Many books use a subscript of zero when referring to the test charge, i.e., q0. Although it's usual to use a small positive charge for the test charge, the sign is not important. You get the same result for E using either polarity.

In the following, the charge referred to has nothing to do with the above test charge.:

If you know the electric field, E, at some location, then the force, F, on an arbitrary charge, q, placed in that location is given by F = qE. Of course if q is negative, F and E are in opposite direction.