Week 1: Discrete Charge and the Electrostatic Field
Charge
Objects can carry a (net) chargeqwhen “electrified” various ways. This charge comes in two
flavors, + and -. Like charges exert a long range (action at a distance) repulsive force on one
another. Unlike charges attract. The SI unit of charge is called theCoulomb(C).
Charge Quantization
Charge is discrete and quantized in units ofe/3, wheree= 1. 6 × 10 −^19 C, but we can never
directly observe bare particles with the thirds (quarks). All charges we can directly measure
on independent particles come in units ofe, the charge of the electron or proton.
Approximate Continuous Charge Distributions
When we study charge distributions in actual matter (with many many charged atoms in
even a tiny chunk) we will often be able toapproximatethe average distribution of charge as
beingcontinuous, so that we can use calculus and integration instead of discrete summations
over absurdly large numbers of charges. To facilitate the treatment of continuous charge
distributions next week, we will go ahead and define the followingcharge densities:
ρ =
dq
dV
σ =
dq
dA
λ =
dq
dx
Charge Conservation
Netcharge is a conserved quantity in nature. Later we will learn to writethe conservation
law mathematically in terms of the flux of the current density, but wedon’t yet have the
mathematical tools to do this with.
Mobility of Charge in Matter
Matter comes in three distinct forms:
Insulators
Conductors
Semiconductors
Coulomb’s Law
From performing many careful experiments directly measuring theforces between static charges
and from the consistent observations of many other things such as the electric structure of
