Welter Class Notes
Introduction to Electricity
Electricity is a form of energy that results from the accumulation or motion of
electrons, either statically as an accumulation of charge or dynamically as a
current.
Background: Electrical Structure of Matter
An atom of matter consists of positively
charged protons, negatively charged
electrons and neutral neutrons. Atoms are
electrically neutral because the numbers of
protons equal the number of electrons.
Electric charges are carried by the protons
and electrons.
Protons (and neutrons) are located in the
nucleus. They are tightly bound and unable to
leave the atom except through nuclear processes. Electrons are on the exterior of
an atom. They are exposed to other atoms and to forces that are capable of
ejecting them or stripping them from the atom.
Consider, first, the electrical effect in solids. The atoms of a solid are held
tightly in place; their nuclei are not free to move about within the solid. Since
these nuclei contain all of the protons, the amount of positive charge in a solid
remains constant and fixed in position. However, it is possible for the negative
charges within a solid to move, for electrons have the ability to move from atom to
atom.
As such, all electric charges in solids are due to an excess or deficit (shortage) of
electrons.
Objects that are charged negatively have an excess of electrons. Objects charged
positively have a deficit of electrons. Both types always have their normal number
of protons.
Welter Class Notes
Electric Charge
Fundamental Law of Electric Charge:
1. Opposite electric charges attract each other.
2. Similar electric charges repel each other.
3. Charged objects attract some neutral objects.
An object becomes charged due to an excess or deficiency of electrons. Knowing
the exact number of excess or deficient electrons in an object allows one to
determine the object's charge.
The charge of one electron, called the elementary charge, is 1.60x10-19 coulombs
(C).
One coulomb (C) of charge represents an excess or deficit of 6.24 x 1018 electrons.
The quantity of charge (Q) on an object is equal to the number of elementary
charges on the object (N) multiplied by the elementary charge (e).
Q = Ne
Example 1: Use this to determine the charge of the following objects.
Object
# of Excess or Deficient Electrons
Quantity and Kind of Charge (Q) on
Object in Coulombs (C)
A
1.0 x 103 excess electrons
-1.6 x 10-16
B
1.0 x 106 deficient electrons
+1.6 x 10-13
C
2.0 x 109 excess electrons
-3.2 x 10-10
Solution:
Given: N = 1.0 x 103 excess electrons
e = 1.60 x 10-19 C
Need: Q
Q = Ne
Q = (1.0 x 103) x (1.60 x 10-19 C)
Q = -1.6 x 10-16 electrons
Hint: Solve for the charge and then make the kind of charge – ve if there is an excess of electrons and +
ve if there is a deficiency of electrons.
Welter Class Notes
Example 2: Approximately 5.5 x 10-1 C of electric charge move through the filament
each second when an ordinary household light bulb is turned on. How many electrons
are there?
Solution:
Given: Q = 5.5 x 10-1 C
e = 1.60 x 10-19 C
Need: N
Q = Ne
N = Q/e
N = 5.5 x 10-1 C / 1.60 x 10-19 C
N = 3.4 x 1018 electrons
Electric Current
When electric charges move from one place to another, this is considered an
electric current – a flow of electrons. All electrical devices operate due to the
flow of electrons through their components.
Current is the rate of flow of electric
charges.
Where I is the current, Q is the
quantity of charge, and t is the time.
The unit for current is the ampere (called amps for short).
1 ampere = 1 coulomb / 1 second
where one coulomb equals 6,240,000,000,000,000,000 electrons. That’s a lot of
electrons moving past a given point in a second!
Welter Class Notes
Example 3: How much current is flowing if 8 coulombs of charge flow past a point in 6
seconds?
Given:
I
I
I
I
=
=
=
=
Q=8C
t = 6 seconds
Need: I
Q/t
8C /6s
1.33 A
1A
Example 4: If the fuse can withstand 10 Amps and the object it is connected to can
provide a continuous 20 coulombs, how long should it take to flow through the fuse if
you want the maximum amount of current to pass through the fuse and you do not
want the fuse to blow?
Given:
t = Q/I
t = 20C/10A
t=2s
I = 10 A
Q = 20 coulombs
Need: t