Some basic facts to remember about electricity and electrical terms:
Think of electrical current as the flow of positive charges.
Because they are positive, the charges are repelled by other positive
charges and flow toward negative charges. As a result, current flows
from positive to negative.
The amount of current is measured in amperes. The symbol for
ampere is A.
A coulumb is a very large number of electrons, about 6420000000000000000.
One ampere represents the flow of one coulumb per second.
Because a coulumb is so large, we usually deal with milliampers (mA)
which are a thousanth of an ampere. You often encounter a micoampere which
is a millionth of an ampere
Ben Franklin defined plus and minus charge before we realized that
electrons usually carry charge in circuits and they are negative.
As a result, a positive current in one direction is actually a flow of
negative electrons in the opposite direction.
Voltages are always measured as differences between one place
and another.
It is conventional to define one part of a circuit as zero volts, or
"ground." Then all other voltages are measured relative
to that voltage.
The voltage difference between two points measures the amount of energy
each charge gains traveling between these two points. One volt difference
will give a columb of charge a joule of energy.
A resistor dissipates the energy a voltage imparts to a current
and resists the current flow.
Resistors are measured in ohms. A resistor of one ohm requires
a volt to push an ampere through it. Resistors with kilohm (Kohm) and megohm
(Mohm) values are common.
It is convenient to think of the voltage difference as the "push"
or "pressure" on charges.
A light emitting diode (LED) generates light when charge flows
through it in one direction. The more current, the brighter it glows. Like
all diodes, LEDs only let current flow through them in one direction.