Ch 21 Electric Charge
2 Electric Charge. There are two types of charge , positive
and negative
. Particles with same
sign charges repel, while those with opposite sign charges attract. Most objects are composed of many
small charges, but are electrically neutral, i.e. they have net zero charge.
3 Conductors and Insulators. Materials can be characterized by the ability of charges to move about
inside an object made of the material. Additional concepts: insulator, conductor, semiconductor,
superconductor, grounding, discharge, electrons, protons, neutrons, nucleus, conduction electrons,
induced charge.
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Blue Flashes from Wintergreen Lifesaver
Checkpoint 1
4 Coulomb’s Law. This gives the electrostatic force between two charges
force is
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, where
. The magnitude of the
is the electrostatic constant, and
is the permittivity constant. Force direction: like particles (same sign
charges) repel, while unlike particles (opposite sign charges) attract. Superposition of forces: All forces
are additive, so if particle 1 is affected by multiple particles (2, 3, 4,…), the net force is the sum of the
individual forces:
. Electric current:
is the rate of charge flow
through an area. Units: [ ]
,[ ]
If a distribution of charge is spherically symmetric (i.e. looks the same after any rotation), then any
charge outside the sphere is affected as if all the charge were located at the center of the sphere. A
uniformly charged spherical shell attracts or repels a charge outside it as if all the sphere’s charge were
concentrated at its center, but exerts no net force on a charge inside the sphere. Excess charge on a
spherical conductor will be spread out uniformly over the surface of the conductor, allowing the excess
charges to separate as much as possible.
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Checkpoint 2
Sample Problem: Finding the net force due to two other particles
Checkpoint 3
Sample Problem: Equilibrium of two forces on a particle
Sample Problem: Charge sharing by two identical conducting spheres
5 Charge Is Quantized. All particles (including atoms and molecules) are either neutral (
charge


, where
is an integer,
) or have a
, and the electron charge is – .
Checkpoint 4
Sample Problem: Mutual electric repulsion in a nucleus
6 Charge Is Conserved. Under all known forces and processes, the total electric charge is constant.