15.3. Electrostatic Fields http://www.ck12.org
FIGURE 15.16
Michael Faraday.That is,~E=~Fq.
The electric field is a vector quantity (also called a vector field) since force is a vector quantity and charge is a scalar
quantity. We will not always use the vector notation throughout our discussion.
It is helpful to express the analogous quantity for the gravitational case as~g=~Fm. A small test massmplaced two
Earth radii from the center of the earth experiences a gravitational field only one-fourth as strong as the field at the
Earth’s surface.
Check Your Understanding
- Coulomb’s law gives the force between two point charges. Use Coulomb’s law to find the equation for the electric
field due to a point charge.
Answer:E=Fq′=
kqqr 2 ′
q′ =k
q
r^2 →E=kq
r^2- What are the units ofE?
Answer: SinceE=Fq′the units ofEcan be expressed asNC.
Electric Field Lines
The force that a small positive test charge experiences at various positions in an electric field can be represented by
the common vector representation of an arrow indicating the instantaneous magnitude and direction of the electric
force. The magnitude is shown by the length of the arrow. The electric field which produces the force could be
represented as a series of arrows in space proportional in length to the force vectors(E∝F). This would be rather
tedious. A simpler way of representing the electric field is with electric lines of force, orelectric field lines.
Electric Field Lines for Isolated Positive and Negative Charges
The force experienced by a test charge in the presence of a positive charge is directed away from the positive
charge, since like charges repel. The electric field lines would therefore be drawn pointing away from the positive
charge. Similarly, the force experienced by the test charge in the presence of a negative charge is directed toward the
negative charge, since unlike charges attract. The electric field lines would therefore be drawn pointing away from