What is 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 has one negative elementary charge (e).
A proton, made up of three charged quarks, has one positive
elementary charge (+e).
The mathematical symbol for charge is q,
from the 18th century phrase "quantity of electricity". The
unit of charge is the coulomb, abbreviated C
(capital C because Coulomb was a person).
Around 1910, Robert Millikan became the first person to measure the electron's
charge, a fundamental physical constant. His value was extremely
close to today's value:
e = 1.602 x 10^{19}C
(Inversely, 1C = about 6¼ billion billion electrons.)
An alkaline AA battery supplies about 5,000C during its
useful life. An average bolt of lightning delivers just 15C.

The Atom

An electric charge is surrounded by a
force field
that attracts oppositely charged particles and repels similar ones.
Electrons, which are lightweight and energetic, are attracted into
the space around protons, which are heavier.
Meanwhile, protons clump together with other protons, and also with
neutrons (three quarks with a combined charge of zero),
forming a nucleus that's
held together by the strong nuclear force, not the
electric force.
A nucleus and its associated electrons is called an atom.
The diameter of a nucleus is between 1.6 and 15 femtometers,
abbreviated fm.
1 fm (often called a fermi) = 1 quadrillionth (10^{15}) of a meter
An atom is a nucleus surrounded by a "cloud" of
electrons
The wavelength of an electron is longer than the nuclear diameter.
Hence, an electron can't exist closer to a nucleus than a couple of
electron wavelengths—the electric force isn't powerful enough to squash an electron into a
proton to make a neutron.
Repelling one another, the electrons crowd around
the atomic nucleus as close as they can get, forming what's called a
cloud.
Hydrogen, the lightest atom, has one proton and one electron but large
atoms can have over
a hundred of each. Each atomic size is one element
in the periodic table of the chemical elements.
Chemical elements can bond together, sharing
electrons to form compounds, molecular elements, and
fancy materials.
Tennis balls bounce, buildings stand, and aspirin thins the blood,
all thanks to electric charge.

CURRENT — CHARGE IN MOTION

In the table of the elements, the metals have many
electrons that are delocalized. That is, they're not
associated with a single atom or chemical bond.
The metallic structure is 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 flowing charge is called an
electric
current. Materials that support current (like metals) are
called conductors.
The unit of 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 of current (I) is the passage of 1C
of charge (q)
in 1 second of time (t)
:


[1]

Wellknown 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.

VOLTAGE — SEPARATED CHARGE

The electric force is sometimes called the
electromotive force or emf because, like all forces, it
has the potential to change the motion of an object by transferring energy to it.
The unit of energy (symbol E) is the
joule, abbreviated J (capital
J because Joule was a person).
Potential energy rises when opposite electric charge is separated, just as it does when a
rock is lifted from the earth. The gain in potential energy comes from the
energy spent separating the
charge or lifting the rock.
Voltage (symbol V) is simply
defined as the energy stored by separated charge, per unit of charge (q):


[2]

The unit of measurement for voltage is the volt,
abbreviated V (capital V because Volta was a person). One volt
equals one joule of energy per coulomb.

POWER

Energy exchange (that is, work) can be done quickly or slowly but
doing it quickly takes more power.
For example, more power is
needed
to run up a hill than to walk up, even though both ways
add equally to your gravitational potential energy.
So power (symbol P) is defined as
the energy transferred per second. In other words, it's the
rate of energy exchange:


[3]

The unit of measurement for power is the watt, abbreviated W
(capital W because Watt was a person). 1W is the power needed to
transfer 1 joule of energy
in 1 second.
Equation 3 says that E = Pt.
Your energy bill, in fact, is based on
kilowatt hours (power x time).
1 kilowatt hour (kWh) is
1000 watts of power for 1 hour, which translates to 3.6 megajoules of
energy:
1 kWh = 1000 W x 3600 s = 3,600,000 J = 3.6 MJ of energy
An alkaline AA battery delivers 9 kilojoules (kJ) of energy over its
useful life while an average bolt of lighting delivers 1,000,000 kJ in just 30 microseconds!

POWER = VOLTS x AMPS

Now take a look at the next algebraic identity, where the two q's on the right
side of the equation
cancel out:


[4]

Since we've already defined the three terms in this equation as
P, V,
and I
(see Eq. 3, 2, and 1), we know that the power of a given voltage and
current is:


[5]

1 watt is the power of a 1 amp current flowing between two
points having a potential difference of 1 volt.

RESISTANCE

In the ordinary world, there are no perfect electrical conductors. Even in
metals,
electrons collide with positive ions, losing energy in
the form of heat.
This type of impedance to electric current is called resistance.
Resistance is used productively in electronic circuits to control currents and
establish voltage levels. Components called resistors
are engineered to provide this resistive impedance.
Experiments show that the ratio of the voltage across a given
resistance to the current through it is a constant. If V is tripled, for example,
I will also triple.
Resistance (symbol R) is defined as this constant ratio:


[6]

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.

OHM'S LAW

Equation 6 is called Ohm's Law. Multiply both sides of
Ohm's Law by I to see that
the voltage across a resistance equals the current times the resistance:


[7]

Divide both sides of Eq. 7 by R to see
that the current through a resistance equals the applied voltage divided by
the resistance:


[8]

In equation 5, we can replace V with
IR to find the power that a resistor
must dissipate as heat when a current flows through it:


[9]

Or we can replace I with V/R to
find the power a resistor must dissipate when a
voltage is placed across it:


[10]

A resistor that can't dissipate enough power will overheat and
eventually burn out.
RESISTOR CONSTRUCTION

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

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,
like carbon. 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.



