Electric
**charge** is a fundamental quantity
like mass, distance, or time.

Charge
is observable and measurable by the force it exerts on other charges.

There
are two types of charges: positive and
negative.

Like
charges repel one another: positive
repels positive, negative repels negative.

Opposite
charges attract one another: positive
attracts negative, negative attracts positive.

The
variable *q* or *Q* is used to represent an amount of charge (may be positive or
negative).

The
SI unit of charge is the **Coulomb**,
abbreviated C.

Since
all matter contains protons and electrons, there is charge present (often in
great quantities) in every object.
Typically, however, the number of protons in an object essentially equals
the number of electrons – adding the amounts of charge gives a total of zero –
the object is said to have no net charge and to be **neutral**. An object is
charged or has a net charge when there are unequal numbers of protons and
electrons. This occurs almost always as
a result of electrons being transferred to or from an object.

Charge
is **conserved**! The net amount of electric charge produced in
any process is zero.

__Means
of becoming charged:__

**Conduction** is the transfer of charge
from one object to another, usually as a result of contact.

**Induction** involves the rearrangement
of charge within an object due to the presence of an external charge (or
electric field). There is no contact.

Both
insulators and conductors can possess charge.

Key
difference: charge can travel freely
through **conductors**. Charge cannot travel freely through **insulators**, but rather tends to be
“locked” in place.

Explanation: It is now known that electrons are carriers
of charge. In conductors (metals) the
electrons are not tightly bound to nuclei and easily “roam” from one atom to
another. In insulators (e.g. rubber,
plastic, wood, etc.) electrons are held more tightly in orbits around nuclei
and do not easily move from one atom to another.

The
force one charge, *q*_{1},
exerts on another, *q*_{2}, has
a magnitude given by:

*F* =__ k q_{1} q_{2 }__

*r*^{2}

where *r* is the distance between *q*_{1} and *q*_{2} and *k* is a
constant.

The
direction of this force is either toward or away from the other charge –
depending on whether it is attraction or repulsion.

*k* = 9.0 ´ 10^{9} N m^{2}/C^{2}

The
smallest possible amount of charge is that on an electron or proton. This amount is called the fundamental or
elementary charge, *e*.

*e* = 1.602 ´ 10^{-19} C

An
electron has charge: *q* = -*e*
= -1.602 ´ 10^{-19} C

A
proton has charge: *q* = +*e* = 1.602 ´ 10^{-19} C

Furthermore, any amount of charge greater than the elementary charge is an exact integer multiple of the elementary charge! Weird, eh?

*q* = *ne*, where *n* is an integer

For
this reason, charge is said to be “quantized”.
It comes in quantities of 1.602 ´ 10^{-19} C.

*V* = * ^{W}/_{q}*
or

*V* = Electric potential (A.K.A.
Voltage, Potential Difference, Electromotive Force or EMF)

*W* = Work done to move charge
between two points

*E* = Potential energy due to
position (separation) of charge

*q* = Amount of charge

SI
unit for electric potential: the Volt

1
Volt = ^{1 Joule}/_{1 Coulomb} or
V = ^{J}/_{C} (The number of volts indicates the
number of joules work or energy per coulomb.)

*I*
= ^{Q}/_{t}

*I* = electric current – rate
at which charge flows in a certain pathway

*Q* = amount of charge flowing
past a certain point

*t* = time

SI
unit for electric current: the Ampere

1
Ampere = ^{1 Coulomb}/_{1 second} or
A = ^{C}/_{s} (The number of amperes indicates the
number of coulombs of charge flowing per second.)

*P*
= *VI*

The
power for an electrical device is equal to the voltage times the current. This value will indicate (in

*R*
= * ^{V}/_{I}* more commonly written as:

*R* = electric resistance (this
is the “resistance” to flow of charge through a device)

An
object or device with greater resistance will require a greater voltage to
produce a certain amount of current.

SI
unit for resistance: the Ohm

1
Ohm = ^{1 Volt}/_{1 Ampere}
or W = ^{V}/_{A} (The
number of ohms indicates how many volts are required to produce 1 ampere of
current.)

For
certain materials and devices the resistance will be constant over a wide range
of voltages and currents; such a device or material is said to be “ohmic”. Ohmic materials include metals like copper, silver,
etc. Common carbon based resistors are
also ohmic. Nonohmic devices have a resistance that changes depending
on voltage and current. A light bulb
filament is nonohmic because its resistance increases
as its temperature increases. Semiconductors, such as diodes and transistors,
and electric motors are also nonohmic.