D.2.6—Electric field strength

Define the field

Electric field strength is force per unit positive test charge. For a point source $Q$, it is directed away from positive $Q$ and toward negative $Q$. Its SI unit is $\mathrm{N\,C^{-1}}$.

Worked example — point-charge field

At $r=1.0\,\mathrm{m}$ from $Q=+2.9\times10^{-8}\,\mathrm{C}$, $E=(8.99\times10^9)(2.9\times10^{-8})/(1.0)^2=2.6\times10^2\,\mathrm{N\,C^{-1}}$. Because the source is positive, the field points radially outward.

Add fields as vectors

For more than one source, calculate each electric-field vector at the point and add them. Do not add magnitudes unless all field vectors point in the same direction. The force on a particular charge is then $F=qE$, with its direction reversed from the field if the charge itself is negative.

Common trap

The field direction is defined using a positive test charge, not the sign of the test charge in the question. Also distinguish field strength $E$ from force $F$: changing the test charge changes F but not the source field E.