Electrical Polarity
The source and destination of electron flow arbitrarily defined
as Negative (-) as the source of electrons or EMF which flows
through a circuit until it reaches the Positive (+) of the
power supply.
The effects of electron flow direction are not obvious and for
this reason (there is no obvious difference), many, perhaps the
majority of circuits, are designed as if positive electricity
flowed through the circuit downward toward the negative, which
is often common ground or an actual connection to the earth
which has a negative charge. Benjamin Franklin defined the
polarity of electricity, and clouds may be positively charged,
even though it "seems obvious" that lightning falls from the
sky.
In AC circuits, the polarity reverses usually twice the rate of
50 or 60 times, one Hertz or cycle involving a positive half
followed by a negative half. Since low frequencies are larger
in wavelength and less affected by reactance (imaginary-
heatless-resistance), aircraft may use 400 Hz frequency AC so
the reactive circuits especially transformers are much less
massive. AC is more complex than DC but used for power because
it is less affected by real resistance which wastes so much
heat that DC can not follow long wires without losing most of
its power heating up the wires. AC overcomes resistance by
using a transformer to reversibly exchange heat-producing
current for voltage, which can push less current through more
wire and thereby reduce total power loss.