
What's
Electricity?


Charge Is Fundamental

Electric charge is a fundamental property of some
particles of matter, for example, electrons and
quarks. Charge can have a polarity of either
negative or positive.
An electron is an elementary particle having one negative elementary charge
(symbole).
A proton, composed of three electrically charged quarks,
has one net positive charge (+e).
The mathematical symbol for charge is q,
from the 18th century phrase "quantity of electricity". The
standard unit of electric charge is the coulomb, abbreviated C
(capital C because Coulomb was a person).
Around 1910, Robert Millikan became the first person to measure the
elementary
charge, a fundamental physical constant. His value was extremely
close to today's definition :
e ≡ 1.602 176 634 x 10 ^{19} C
Inversely, 1 C ≅ 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 just 30 microseconds!

The Atom

Surrounding every electric charge is a space (an electric
field) acting to move the charge toward opposite polarities
and away from like polarities.
Electrons, being lightweight and energetic, are electrically attracted
toward the more massive and
stationary protons.
Meanwhile, protons clump together with other protons, and also with
neutrons (particles composed of three quarks totaling zero
charge). These protons and neutrons are held together by the strong nuclear force,
which is much stronger than the electric force.
The positive clump of particles (called a nucleus) and
all its attracted electrons is called an atom.
The attracted 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. A femtometer 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 1923, Louis de Broglie, a French physics graduate student, presented
his 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
De Broglie wavelengths are usually provided in nanometer (nm) or Angstrom
(Å) subunits :
1 nm = 10^{ 9 } m
1 Å = 10^{ 10 } m
NOTE : Equation [1] also converts a light's wavelength to its photon momentum. Although
photons
have no mass, they have momentum thanks to quantum mechanics.
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.
Because it's so spread out in comparison, an electron can't even exist closer than a
couple of electron wavelengths from 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, themselves, can bond together by sharing electrons, forming
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
where many electrons
are
delocalized. That is, they aren't dedicated to any
particular atom or chemical bond. Metallic structures have a lattice of positive ions (atoms
lacking one electron) sitting in a "sea" of mobile electrons.
The mobile electrons can flow en masse through the lattice much like
water flows through a sieve. The rate of flow of this electric
charge is called electric
current, symbol I,
from the French phrase "intensité de courant".
So, current (I) is the amount
of charge (q)
passing a certain point per unit of time (t)
:


[2]

The unit of electric current is
the ampere or amp, abbreviated A (capital A because
AndréMarie Ampère
was a person).
Materials that support electric current, such as copper,
tin, nickel, silver, and gold, are
called conductors.
Materials that don't support electric current, such as wood, rubber,
ceramics, plastic, and glass are called insulators.

Voltage — Separated Charge

Potential energy arises whenever opposite electric
charges are separated, just as it does when a stone is lifted off the
earth. The energy rise comes from the energy spent in separating the
charge or lifting the stone.
The unit of energy (symbol E) is the
joule (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.
Electrical voltage (symbol V)
is defined as the potential energy of separated charge, per unit of charge (q) :


[3]

The unit of measurement for voltage is the volt (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 this name, however, emf is not a force but rather a potential energy.

Power

The transfer of energy (i.e., work) can be accomplished quickly or slowly but
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 give you
the same amount of gravitational potential.
Power (symbol P) is thus defined as
energy transfer per time. In other words, It's the quickness of energy
transfer :


[4]

The unit of measurement for power is the watt, abbreviated W
(capital W because Watt was a person). 1 W is the power needed to
transfer 1 joule of energy
in 1 second.
Equation [4] can also be written :
E = P × t
In fact, your electric service is ordinarily billed in units of
kilowatthours (P × t), not joules.
One kilowatthour (kWh) is
1,000 watts of power for 1 hour (3,600 seconds), making the total
transfer of energy equal to 3.6 megajoules :
1 kW h = 1,000 W x 3,600 s = 3,600,000 J = 3.6 M J of energy
An alkaline AA battery delivers 9 kilojoules (k J) of energy in its
useful life. An average bolt of lighting delivers 1,000,000 k J in
just 30 microseconds!

Power = Volts × Amps

Take a look at the following algebraic identity, where the two q' s cancel each other out :


[5]

Since the above three ratios are the definitions of
P, V,
and I (see equations
[4],
[3], and
[2] ),
we see that the power P of
an electric current I flowing across a
voltage V is :


[6]

One amp of current across a one volt energy difference equals 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. So, 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) :


[7]

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 [7] 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, times
the resistance :


[8]

Divide both sides of equation [8] by R to
find that the current through a resistor equals the applied voltage divided by
the resistance :


[9]

A trick to help remember the three permutations of Ohm's Law (equations [7], [8] and [9])
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 [6] (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 :


[10]

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 :


[11]

Resistor Construction
Like garden hoses, conductors 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 coil up a long, thin piece of wire.
Wirewound resistors can be precise and also handle large currents.

Other resistors are constructed from a material, such as carbon, that falls
in between a conductor and
an insulator. 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.



