23.2 - Electric fields and Coulomb’s law
You can use Coulomb’s law to calculate the strength of the field around a point charge.
First, calculate the amount of force exerted by that charge on a (positive) test charge
using Coulomb’s law. Then divide by the amount of the test charge, because the
strength of an electric field equals the amount of force divided by the charge used to
measure the force.
We derive the equation for the field caused by a point charge as shown in Equation 1.
We state the definition of the electric field, and then expand the expression for the force,
using Coulomb’s law. The quantity qtest appears in both the numerator and the
denominator of the resulting fraction, so it cancels out. As the equation states, the
strength of the field is proportional to the absolute value of q, the charge causing it,
divided by the square of the distance from this charge.
An example problem is shown on the right, as well: You are asked to calculate the field
strength at a point 0.110 m away from a charge of positive 10.5 C.
Remember: Even after you calculate the strength of the field, your task is not finished,
because an electric field has both magnitude and direction. You must specify the field’s
direction, as we do in the example problem.