Chapter 23: Electric Fields
- Electric charge: Concept, types, attraction/repulsion; Units
for charge (C); Charge conservation.
- Types of materials: Conductors vs insulators, and microscopic
interpretation; Charging and grounding.
- Coulomb's law: Electric force between two point charges,
F = k q1q2/r 2,
with electrostatic constant k = 8.99 · 109 N·m2/C2 =
1/(4πε0)
.
- Results: Shell theorems for spherical charge distributions.
- Fundamental charge: Charge quantization; Value of the
charge of an electron or proton,
e =
1.60 × 10–19 C .
- Definition of electric field: If a charge q feels
an electric force F at a point P, the electric
field there is
E = F/q .
Interpretation
as force that a 1-C charge would feel there. Units for
electric fields
(N/C).
- Electric field lines: The general concept; How to draw them
for simple charge arrangements; How they provide information on the
direction and magnitude of the electric field. Other properties: No
crossings; They start at positive charges and end at negative ones,
if charges are present.
- Electric field due to point charges: From Coulomb's law, each charge produces an electric field,
E = k q/r 2 =
1/(4πε0) q/r 2 ,
with electric permittivity of vacuum ε0 =
8.85 × 10–12 C2/N·m2 .
- [Electric field due to an electric dipole: Concept of dipole,
electric dipole moment p, with p = qd, and
E = 1/(2πε0) p/z 3 on
the axis of the dipole .]
- Electric field due to a line or surface charge distribution: How to set up
the appropriate integral
E = k ∫ (dq/r3) r .
- Electric charges in electric fields: If the field E is
known at a place, then the force felt by a charge q placed
there is F = qE.
The normal values of electric field near the surface of the Earth are
around 100 N/C, and in most practical situations
the gravitational forces felt by particles such as electrons and protons
can be ignored if there are electric forces acting on them.
- [Electric dipoles in electric fields: Torque τ =
p × E,
and what happens to the dipole, qualitatively.]
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