Illustration Electric field lines - force on a test charge

Electric field lines are a useful model that can be used to represent the electric force on a test charge in an electric field. By definition, they start at positive charges and end on negative charges.

For example, if a negative test charge is placed at some location in the E-field of an electric dipole, then that test charge experiences a repulsive force \(\boldsymbol{F}_1\) from the negative charge and an attractive force \(\boldsymbol{F}_2\) from the positive charge. The resulting force \(\boldsymbol{F}\) points tangentially along the field line, but moves against the field lines (in the opposite direction).

A positive test charge, on the other hand, would experience an attractive force from the negative charge and a repulsive force from the positive charge of the dipole. The resulting force on a positive test charge also points tangentially along the field line, but this time the test charge travels along the field lines.