CHARGE

Charge is a fundamental physical property of some subatomic particles.
It comes in two polarities: negative and positive.
Each charge is surrounded by an electromagnetic field that attracts
opposite charge and repels samesigned charge.
The electron has a full dose of negative charge and the proton has a full dose of positive charge.
Electrons are tiny, fastmoving, and abundant particles that tend to
surround more massive, positively charged nuclei. One nucleus and its
cloud of electrons is called an atom.
Atoms, themselves, bond electromagnetically with other atoms to form even more stable
electrical fields. Tennis balls bounce,
buildings stand, and aspirin thins the blood thanks to these fields.
The standard unit of electrical charge is the coulomb, which is equal
to the combined
charge of about 6.24 quintillion (10^{18} or a billion billion)
electrons.

CONDUCTORS, CURRENT, and INSULATORS

Some atoms have weakly bound, unpaired electrons on their outer fringes
that can flow en masse in materials called conductors. Metals like tin, copper, and gold have lots of free electrons
and are good conductors.
The flow of charged particles is called electric current.
The standard unit of current is the ampere (or amp for short) and is abbreviated "A"
(capital A because Ampère was a person). One ampere is the
flow of one coulomb per second.
Materials whose electrons are all tightly bound are called insulators.
These materials, like wood, rubber and glass, are poor conductors of electric current.

RESISTORS and VOLTAGE

Resistors are engineered from materials like carbon that fall in
between conductors and insulators. They obstruct
current in a partial, controllable way.
Resistors have no pool
of free electrons that can be shoved downstream dominostyle. Some
electrons drift through the atomic lattice but mostly they're captured,
yielding more heat than current.
A pressure (potential difference) called a voltage rises across a
resistor like water pressure rises across a clogged drain. The
standard unit of voltage is the
volt, abbreviated "V"
(capital V because Volta was a person).
One volt is the potential difference between two points of a circuit
carrying one ampere of current and losing one watt of power.

OHM'S LAW

If we divide the voltage across a resistor by the current through it
(volts/amps), we get a number that's constant for that resistor.
So if we double the voltage, the amperage also doubles.
This constant is called the resistance of the resistor.
The standard unit of measure is the ohm, abbreviated "Ω" (capital
Omega because Ohm was a person).
The math symbols for voltage, current, and resistance are "V ", "I ", and
"R " and the equation R = V/I is called Ohm's Law even though it was
discovered a century before Ohm popularized it.

