CHARGE IS FUNDAMENTAL
Electric charge is a fundamental property of some
particles of matter, such as electrons and
quarks. The polarity of an electric charge can be either
The electron has one unit of negative charge.
made up of three charged quarks, has one net positive charge.
Charged matter couples to an electromagnetic force field
that pulls together opposite charges and pushes like charges apart.
Electrons are very lightweight and energetic. The electromagnetic
force draws them into orbits around
Protons, meanwhile, clump together with other protons, and also with
neutrons (three quarks whose combined charge is
zero). The protons and neutrons form a nucleus
held together by the
nuclear not the electromagnetic force.
A positive nucleus and its negative electron cloud is called an atom.
Atoms, themselves, bond electromagnetically (chemically) with other atoms, creating
even larger structures.
Tennis balls bounce,
buildings stand, and aspirin thins the blood, all thanks to
math symbol for charge is Q and its unit of
measurement is the coulomb, abbreviated C
(capital C because Coulomb was a person).
1C is the combined charge of about 6¼ billion billion electrons.
An alkaline AA battery supplies about 5,000C of charge over its useful life
while an average bolt of lightning supplies only around 15C of charge!
MOVING CHARGE IS CURRENT
In the periodic table of the elements, the metals have
electrons that are delocalized. That is, they're not
associated with a single atom or chemical bond.
Metals are a 3-D lattice of positive ions (atoms with an
extra proton in the nucleus) sitting in a "sea" of mobile electrons.
Electrons can flow en masse through a metal much like
water flows through a sieve. Flowing charge is called
and materials that support current, like metals, are conductors.
The unit of measurement for electric current is
the ampere, or amp, abbreviated A (capital A because Ampère was a person).
The math symbol for current is I
from the French phrase "Intensité de courant".
1A is the passage of 1C in 1 second (t)
I = Q/t
Well-known metals include copper, tin, nickel, silver and gold.
Materials that don't allow electron flow are called insulators. Insulators include wood, rubber,
ceramics, plastic and glass.
SEPARATED CHARGE IS VOLTAGE
The force behind electrical attraction and repulsion is called the
electromotive force (emf) because, like all forces, it
has the potential to change an object's motion by transferring energy to it.
The unit of measurement for energy (math symbol E) is the
joule, abbreviated J (capital
J because Joule was a person).
Potential energy goes up when oppositely charged particles
are separated, just as it does when an apple is lifted from the earth. The potential
energy comes from the energy spent in separating them.
symbol V) is the energy stored by
separated charges, per charge.
V = E/Q
The unit of measurement for voltage is the volt, abbreviated V
(capital V because Volta was a person).
1V equals 1J of energy per 1C of charge.
Energy exchange (i.e. work) can be done quickly or slowly but
doing it quickly requires more power.
For example, more power is
to run up a hill than to walk even though both ways add equally
to your gravitational potential.
Power (math symbol P) is
defined as the amount of energy exchanged per second.
P = E/t
The unit of measurement for power is the watt, abbreviated W
(capital W because Watt was a person). 1W is the transfer of 1J
in 1 second.
Electrical power can be calculated from voltage and current:
(E/t) = (E/Q) x (Q/t)
we see that
P = VI
So 1W is the power delivered by a 1A current flowing between two points separated by 1V.
An alkaline AA battery can transfer 9 kilojoules of energy over its useful life
while an average bolt of lighting can transfer 1,000,000 kilojoules in 30 microseconds!
In the ordinary world, perfect electrical conductors don't exist.
Even in metals, free electrons bump into positive ions, losing energy in
the form of heat.
This type of impedance is called resistance.
Resistance is used to control currents and
establish voltage levels in electronic circuits. Components called resistors
are engineered to provide the resistance.
Experiments show that the ratio of the voltage across a resistor to
the current through it remains constant. This constant value is defined to be
the resistance (math symbol R) of the
R = V/I
The unit of resistance is the ohm, abbreviated Ω
(capital Omega because Ohm was a person).
1Ω is the resistance between two points that are separated by 1V and conducting a current of 1A.
The above equation is called Ohm's Law. Multiply both sides of
Ohm's Law by I to find that:
V = IR (Voltage equals current times resistance).
Divide both sides of this result by R to find that:
I = V/R (Current equals voltage
divided by resistance).
Ohm's Law lets us calculate the power consumed by a resistance using either
the voltage across the resistance or the amperage passing through it:
P = VI = V(V/R) = V2
P = VI = (IR)I = I2R
Like garden hoses, long and thin electrical conductors have more
resistance than short and fat ones.
In fact, one way to make a resistor is to coil up a long, thin piece of wire.
Wirewound resistors can be precise and also handle large currents.
Another way to make a resistor is to use materials that fall in between a conductor and an insulator,
such as carbon. That's because carbon has relatively few
These vintage, carbon composition resistors are composed of tiny carbon particles bound with clay.
Many modern resistors are made from laser-cut, helical tracks of carbon
or metal film.
Current, voltage, power and resistance are the four pillars of direct
current (DC) circuits. Direct current is the steady flow of electrons in
Alternating current (AC) faces two additional impedance types,
and inductance, whose values depend on the
frequency of the alternation in intensity (and possibly direction) of