Electric charge is a fundamental property of some
elementary particles, like electrons and
quarks, and can be either negative
or positive in polarity. Charge, as well as its motion, are
linked to an electromagnetic force
field wherein opposite charges attract and like charges
The electron has one negative charge and the proton, which is made up of charged quarks, has one
net positive charge.
Electrons, which are tiny, fast moving and abundant, settle into
orbitals around heavy, proton laden nuclei. A positive nucleus and its
surrounding negative charge is called an atom.
Atoms, themselves, bond electromagnetically with other atoms to form
fields. Tennis balls bounce,
buildings stand, and aspirin thins the blood, all thanks to
symbol for charge is Q and its unit of
measurement is the coulomb, abbreviated C
(capital C because Coulomb was a person).
One coulomb equals the combined charge of about 6¼ billion billion
The amount of charge that passes through an
alkaline 'AA' battery over its useful life is about 5,000 C (5 kC).
The amount of charge that passes through an average bolt of lightning
is just 15 C.
In the periodic table, the metals have
electrons that are delocalized (not associated with a
single atom or chemical bond). The metallic structure consists of
positive ions (atoms with a surplus proton in the
nucleus) amid a "sea" of
Free electrons can flow en masse through a metal much like
does through a pipe. Flowing charge is called electric current
and materials that support current are called electrical
The standard unit of current is
the ampere or amp which is abbreviated A (capital A because Ampère was a person).
The symbol for current is I from the French
phrase "Intensité de courant".
One ampere is the flow of one coulomb of charge per second.
I = Q/t
Well-known metals include copper, tin, nickel, silver and gold.
Materials that don't allow electric charge to flow are called insulators. Insulators include wood, rubber,
ceramics, plastic and glass.
The force that causes electric charge to attract and repel is called an
electromotive force or emf because, like all forces, it
has the potential to change an object's motion. It does this by transferring
energy to it.
standard unit of energy is the joule, abbreviated J (Joule was a
person). The symbol for energy is E.
A 'AA' battery can deliver 9 kJ over
its useful lifetime. A bolt of lightning can deliver 1,000,000 kJ in
just 30 microseconds! Energy is clearly more significant than
the number of charge carriers (electrons) involved. [see Section
Potential energy results from separating electric charge much as it
does from lifting an apple.
is a measure of the energy between two points per charge.
It's symbol is V and its standard unit is the
volt , abbreviated V (Volta was a person).
So, volts is the number of joules per coulomb.
V = E/Q
Power ( symbol P) is the
of energy transfer (i.e., joules per second).
P = E/t
E/t = E/Q x Q/t.
In other words,
P = VI.
So, power equals voltage times current.
The unit of measurement for power is the watt, abbreviated W (Watt was
a person). One watt is the power consumed by a 1A current flowing
between two points separated by 1V.
Resistance is an opposition to the flow of current, analogous to friction.
Even a good conductor has some resistance and so a long, thin wire
has more resistance than a short, fat one. The same goes for a water
A resistor is a component engineered to possess a certain
amount of resistance. One way to make a resistor is to coil up a
long, thin piece of wire. Wirewound resistors can be
precise and handle large currents.
Another way to make a resistor is to use materials that fall in
between a conductor and an insulator. Carbon, for example, has
delocalized electrons to conduct charge.
resistors are made from tiny carbon particles bound with clay. Carbon-film
resistors use helical tracks of carbon film.
Electrical energy is dissipated (i.e., given up) in a resistor, converted mostly into
thermal energy (heat). Experiments show that the energy loss (voltage drop) across a particular resistor divided by the current through
it is a constant. This constant ( symbol R)
is the resistance of that resistor.
R = V/I
This relationship is called Ohm's Law. The standard unit of resistance is the ohm, abbreviated Ω
(capital Omega because Ohm was a person).
One ohm is the
resistance between two points separated by 1V, conducting a current of
With the help of Ohm's Law, we can calculate the power dissipated by a resistor
from either the voltage across it or the current through it:
P = VI = V x (V/R) = V2/R
P = VI = (IR) x I = I2 R