But, please don’t go nuts... . The formulas for force on a charge in an electric field (F = qE ) and a
charge’s electrical potential energy (PE = qV ) are your first recourse, your fundamental tools of
electrostatics. On the AP exam, most electric fields are NOT produced by point charges! Only use the
equations in this section when you have convinced yourself that a point charge is creating the electric
field or the voltage in question.
First, the value of the electric field at some distance away from a point charge:
Q is the charge of your point charge, ε 0 is the permittivity of free space (on the table of information), and
r is the distance away from the point charge.^2 The field produced by a positive charge points away from
the charge; the field produced by a negative charge points toward the charge. When finding an electric
field with this equation, do NOT plug in the sign of the charge or use negative signs at all .
Second, the electric potential at some distance away from a point charge:
When using this equation, you must include a + or − sign on the charge creating the potential. (See Figure
18.5 .)
Figure 18.5 Electric field produced by point charges.And third, the force that one point charge exerts on another point charge: