Charge is a fundamental property of some sub-atomic
particles and comes in two types: negative and positive. For example, electrons have
a full dose of negative charge and protons have a full dose of positive charge.
Charges produce electromagnetic fields which cause opposite charges to
move towards one other, and same-signed charges to move apart.
The standard unit of charge is the coulomb, which is the combined
charge of about 6.24 quintillion (1018 or a billion billion)
Electrons are tiny, fast-moving and abundant, and tend to gather around more massive, proton-rich
nuclei thanks to electromagnetism. One nucleus and its
cloud of electrons is called an atom.
Atoms, in turn, bond electromagnetically to other atoms, forming even more stable electrical
fields or landscapes. Tennis balls bounce,
buildings stand, and aspirin thins the blood thanks to these fields.
CONDUCTORS, CURRENT, and INSULATORS
Some atoms have weakly bound, unpaired electrons on their outer fringes and 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 domino-style. 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.
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 "
R" and the equation
R = V/I is called Ohm's Law even though it was
discovered a century before Ohm popularized it.