Electric fields are created by charges.

• Electric fields add. The field due to several point charges is the sum of the fields due to each charge.

• Electric fields are vectors. Summing electric fields is vector addition.

• Two equal but opposite charges form an electric dipole.

• Electric fields can be represented by electric field vectors or electric field lines.

《 LOOKING BACK Section 22.5 The electric field of a point charge

For macroscopic charged objects, like rods or disks, we can think of the charge as having a continuous distribution.

• A charged object is characterized by its charge density—the charge per length, area, or volume.

• We’ll divide objects into small point charge–like pieces $\text{Δ}Q$.

• The summation of their electric fields will become an integral.

• We’ll calculate the electric fields of charged rods, loops, disks, and planes.

We will develop and use four important electric field models.

Two parallel conducting plates with equal but opposite charges form a parallel-plate capacitor. You’ll learn that the electric field between the plates is a uniform electric field, the same at every point. Capacitors are also important elements of circuits, as you’ll see in Chapter 26.

Electric fields exert forces on charges.

• Charged particles accelerate. Acceleration depends on the charge-to-mass ratio.

• A charged particle in a uniform field follows a parabolic trajectory.

• A dipole in an electric field feels a torque that aligns the dipole with the field.

《 LOOKING BACK Section 4.2 Projectiles