
What's
Electricity?


Electric
Charge 
Charge is a fundamental property of
some elementary (subatomic) particles of matter. Charge determines the strength of the
particle's coupling to
electromagnetic fields.
In a way, this is similar to the mass property
that couples particles to gravitational fields.
There are two "polarities" of electric charge,
‘positive’ and ‘negative’, and
every charged particle is, itself, the source of an electromagnetic (EM)
field.
Physical forces of attraction and
repulsion arise from coupled EM fields.
Charged Particles
A variety of elementary particles possess electric charge, including electrons and
quarks.
The electron has one elementary, negative
charge ( symbol −e) , whose
value is a fundamental physical constant.
The proton is a composite particle
comprising 3 fractionallycharged
elementary particles called quarks. Two of the
proton's quarks have a charge of +2⁄3 e and one has
a charge of −1⁄3 e.
Combined, the three quarks give the proton one elementary,
positive charge ( symbol +e
or just e ).
● The math symbol for electric charge is q
or Q , from the 18th century phrase ‘quantity of electricity’.
● The unit of electric charge is the coulomb,
abbreviated C — capital C because Coulomb was a person.
< >
► CharlesAugustin de Coulomb (17361806) –
a French officer, engineer and physicist – discovered that the
electrostatic force between two charges is proportional to the product of the
two charges but inversely proportional to the square of the distance between them
( Coulomb's Law ).
Elementary Charge
Around 1910, Robert Millikan became the first person to experimentally
obtain a value, in coulombs, for the tiny, elementary charge of the electron.
Millikan's value was extremely
close to today's International System of Units (SI) definition of the
elementary charge e:
1 e ≡ 1.602 176 634 × 10^{ 19} C
By taking the inverse of this number, we get the number of elementary
charges in a coulomb :
1 C = 6.241 509 074 460 × 10^{18} e
≅ 6¼ billion billion electrons
An alkaline AA battery delivers about 5,000 C of charge during its
useful life. An average bolt of lightning delivers just 15 C
but it does so in a mere 30 microseconds!
The Atom
Surrounding every electric charge is a space (a
field) acting to move the charge away from likepolarity charges
and toward oppositepolarity charges.
Negativelycharged electrons, being lightweight and energetic, are electrically
drawn
toward the more massive and
more stationary, positivelycharged protons.
Meanwhile, protons clump together with other protons and with
neutrons (particles comprising
three quarks totaling zero charge). The
clumps of matter are held
together by the strong nuclear force,
which is much stronger than the electric force.
The positivelycharged clump of particles (called a nucleus)
along with all the attracted, negativelycharged electrons is called an atom.
The gathered electrons repel one another, forming a negative cloud
around the nucleus.
The diameter of a nucleus is between 1.6 and 15 femtometers,
abbreviated f m. One fm is also called a fermi in
honor of nuclear physicist Enrico Fermi.
1 f m (or fermi) = 1 quadrillionth (10^{ 15 }) of a meter
The Atomic Nucleus, Surrounded by a Cloud of Electrons
Why Don't Electrons
Stick To The Nucleus?
In 1924, Louis de Broglie, a French physics graduate student, delivered
a thesis to Paris University containing his important findings attributing wavelengths to electrons.
It was already known that light waves could behave as particles, called photons, and de Broglie reasoned that nature was symmetric.
Why shouldn't particles behave as waves?
In 1929, after the wave nature of electrons
was demonstrated experimentally, the Nobel Prize for Physics was
conferred on de Broglie for his discovery.
A particle's de Broglie wavelength (symbol lambda λ)
is inversely proportional to its momentum :


[1]

Where:
λ = the particle's wavelength, in meters
p = the particle's momentum (mass × velocity), in kg⋅m ⁄ s
ℎ = Planck's constant ≡ 6.626 070 15 × 10^{ 34} kg⋅m^{2 }⁄ s
● NOTE : Equation [1] also converts light wavelength to its
photon momentum. Although photons have no mass, they have momentum thanks to quantum mechanics.
De Broglie wavelengths are usually expressed in nanometer (nm) or Angstrom
(Å) subunits :
1 nm = 10^{ 9 } m
1 Å = 10^{ 10 } m
● An electron traveling at a reasonable speed has a wavelength of about
0.01 nm, thousands of times longer than the 1.6 to 15 f m diameter of an atomic nucleus.
● So, because the electron is
comparatively spread out over space, it can't even exist closer
than a couple wavelengths to a nucleus, much less stick to one.
The Elements
Hydrogen, the lightest atom, has one proton and one electron.
Heavier atoms have over a hundred of each. Each atomic size is one element
in the periodic table of chemical elements.
Atoms can electrically bond with other atoms, sharing their electrons to form
larger molecules, compounds,
and other fancy stuff.
Tennis balls bounce, buildings stand, and aspirin thins the blood,
all thanks to electric charge.

Current — Moving Charge

In the periodic table, the metal elements bond into structures
wherein many of the atoms' outer electrons
are delocalized. That is.
they aren't tied to any particular atomic nucleus or chemical bond.
The metallic structure resembles a fixed lattice of positive ions (atoms
lacking an electron) sitting in a ‘sea’ of mobile electrons
that can move en masse through the lattice like
water through a sieve.
Materials that support the flow of electronic charges are called conductors and
they include copper, tin, nickel, silver, and gold.
Materials that don't support such flow are called insulators
and they include wood, rubber, ceramics, plastic, and glass.
The rate of flow of electric
charge is called electric current,
symbol ‘ I ’
from the French phrase “ intensité de courant ”.
Current is the amount of charge
(q) moving past a certain point in space
per second (t):


[2]

AC / DC
Direct current (DC) is a
flow of electrons in one direction. The
intensity of the current may fluctuate but its direction doesn't change.
Alternating current (AC)
is electron motion that alternates in its direction. In AC, electrons
vibrate forward and back around a center position.
This vibration propagates a
longitudinal electric wave of
charge compression and
rarefaction that can travel through electrical conductors and components.
This is similar to how a vibrating speaker cone creates a longitudinal
acoustic wave of vibrating air particles and pressure.
Since an alternating current is always increasing, decreasing and
changing direction, an averaging method called
rootmeansquare
(RMS) is used to ascribe a fixed value to the current's energy.
AC current, voltage and power are ordinarily expressed using RMS values.
Units of Current and Charge
The unit of electric current is
the ampere, abbreviated capital A
in honor of French mathematician and physicist
AndréMarie Ampère (17751836), who is considered to be
the father of electrodynamics.
The ampere is the base unit of
electricity in the International System of Units (SI).
◼ There are 7 base units
in the SI: length (meter), time (second), mass (kilogram),
electric current (ampere), temperature (kelvin), amount of substance
(mole), and luminous intensity (candela). All other SI units can be
expressed in terms of these 7.
The SI defines an ampere to be a flow of 6.241
509 074 × 10^{18}elementary
charges per second. As stated earlier, that's the number of elementary
charges in one coulomb.
In the SI, charge ( Q ) is a unit derived from
the two base units, current ( I ) and time
( t ):


[3]

So, one coulomb ( 1 C )
is defined as the amount of electric charge delivered by one ampere in
one second :
1 C ≅ (6.24 × 10^{18} e) × (1) = 6.24 × 10^{18} e
● A coulomb is sometimes called an amperesecond.

Voltage — Separated Charge

Voltage is an energy. The unit of energy (symbol ‘E’) is the joule
, abbreviated capital ‘ J ’ in honor of James Prescott Joule, a 19th century English physicist,
mathematician and brewer who related heat to mechanical energy, laying the foundation for the law of
energy conservation.
Potential energy arises from
separated charge just as it does from a rock separated
from the
ground. The rise in energy comes from the energy spent in separating the
charge or lifting the rock.
Voltage (V)
is a measure of the electric energy per coulomb of charge (q) :


[4]

The unit of voltage is the volt,
abbreviated capital ‘ V ’
in honor of Alessandro Volta, an Italian physicist and chemist who
invented the primary battery in 1799.
One volt
is one joule of energy per coulomb of charge.
Voltage is sometimes called an electromotive force (emf )
because it can change an object's motion by transferring energy to it.
Despite its name, however, emf is not a force but rather a potential
source of energy, like a stretched rubber band.

Power

The transfer of energy (i.e., work) can be accomplished quickly or slowly,
though doing it quickly takes more power.
For example, it takes more power
to run up a hill than to walk up, even though both ways
gain you the same amount of gravitational energy.
Power (symbol P) measures
energy transfer per time. In other words, It's the speed of energy
transfer :


[5]

The unit of power is the watt, abbreviated
W, for James Watt (17361819), a
Scottish inventor, mechanical engineer and chemist whose improvements
to steam engine technology drove the Industrial Revolution.
One watt is the power needed to transfer one joule of energy in one second.
Equation [5] also says that :
E = P × t
In fact, your electric service is ordinarily billed in units of
kilowatthours ( P × t )
instead of joules.
One kilowatthour ( kWh ) of energy is 1,000 watts of power
over a 1 hour (3,600 second) time span, which comes to 3.6 megajoules :
1 kW h = 1,000 W x 3,600 s = 3,600,000 J = 3.6 M J
An alkaline AA battery delivers about 9,000 joules ( 9 kJ ) of energy in its
useful life. An average bolt of lighting delivers 1,000,000 kJ in
just 30 microseconds!
Power = Volts × Amps
Take a look at the following algebraic identity, where the two q' s cancel each other out :


[6]

Since the above three ratios are the definitions of
P, V,
and I ( see equations
[5],
[4], and
[2] ),
we see that the power P of
an electric current I driven by a
potential energy V is :


[7]

So, one amp of current across one volt of voltage generates one watt
of power.

Resistance

In the ordinary world, there are no perfect conductors of electric current. Even in
metals,
electrons collide with ions, losing energy in
the form of heat.
This frictionlike impedance to electric current is called resistance.
In electronics, resistance can be used to limit currents and to
establish
potential differences. Components called resistors
are engineered to provide precise amounts of resistance.
Ohm's Law
Experiments show that the ratio of the voltage V across a
certain
resistor to the current I flowing through it is a constant.
This constant
ratio is defined to be the resistor's resistance (symbol R) :


[8]

The unit of resistance is the ohm, abbreviated
Ω (capital Omega because Ohm was a person).
A 1 Ω resistor across 1 V of voltage will pass 1 A of current.
Equation [8] is called Ohm's Law. Multiply both sides of
Ohm's Law by I to find that
the voltage across a resistor equals the current through the resistor
multiplied by
the resistance :


[9]

Divide both sides of equation [9] by R to
find that the current through a resistor equals the applied voltage divided by
the resistance :


[10]

A trick to help remember the three above permutations of Ohm's Law (equations [8], [9] and [10])
is to substitute Vulture,
Rabbit, and Indian
for V, R,
and I.
The
Vulture sees the Indian beside the Rabbit. The Rabbit sees the Vulture
over the Indian, and the Indian sees the Vulture over the Rabbit.
Power Dissipation
A resistor must be able to dissipate all the heat generated by the
internal electronion collisions; otherwise, it'll overheat and burn
out. Therefore, each resistor has a power rating.
In equation [7] ( P = V × I ) ,
we can use Ohm's Law to replace V with
IR and thus find out how much power a
certain resistance R must shed
when a current I flows through it :


[11]

Or we can use Ohm's Law to replace I with V/R to find
out how much power the resistance R
must shed when a
voltage V is applied across it :


[12]

Resistor Construction
Like garden hoses, conductive wires that are long and thin offer more
resistance to current than do ones that are short and fat.

One way to manufacture a resistor is to simply coil up a long, thin piece of wire.
Such wirewound (a.k.a.
sandblock or cement)
resistors can be precise and can handle large currents.

Other resistors are constructed from a material, such as carbon,
which falls between a conductor and
an insulator. As such, carbon has relatively few
delocalized electrons.
These vintage, carbon composition resistors are composed of tiny carbon particles bound with clay.

Many modern resistors are made from lasercut, helical tracks of carbon
or metal film.


