2. 2
Dr. Firas Obeidat – Philadelphia University
Introduction
electrical systems are found everywhere, such as homes,
schools, hospitals, factories, transportation … etc.
Any electrical system can be represented by electrical circuit
and mathematical equations to study and analyze its physical
behavior.
An electric circuit is an interconnection of electrical elements.
In electrical circuit analysis, we often find ourselves seeking
specific currents, voltages, or powers.
Pay close attention to the role of “+” and “−” signs when
labeling voltages, and the significance of the arrow in defining
current; they often make the difference between wrong and
right answers.
3. 3
Dr. Firas Obeidat – Philadelphia University
Units and Scales
In order to state the value of some measurable quantity, we
must give both a number and a unit, such as “3 meters.”
The most frequently used system of units is International
System of Units (SI).
4. 4
Dr. Firas Obeidat – Philadelphia University
Units and Scales
Comparison of units of the various systems of unit.
5. 5
Dr. Firas Obeidat – Philadelphia University
Units and Scales
The SI uses the decimal system to relate larger and smaller
units to the basic unit, and employs prefixes to signify the
various powers of 10.
7. 7
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Charge
the most basic quantity in an electric circuit is the electric
charge.
all matters are made of fundamental building blocks known as
atoms and that each atom consists of electrons, protons, and
neutrons.
charge e on an electron is negative and equal in magnitude to
(-1.602 ×10-19 C). while a proton carries a positive charge of
the same magnitude as the electron (+1.602 ×10-19 C).
In 1 C of charge, there are (1/1.602 ×10-19=6.24×1018) electrons.
Charge is an electrical property of the atomic
particles of which matter consists, measured
in coulombs (C).
8. 8
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Charge
9. 9
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Current
Electric current is the time rate of change of
charge, measured in amperes (A). Or
the net amount of charge that passes
through the wire per unit time.
𝒊 =
𝒅𝒒
𝒅𝒕
1 Ampere = 1 Coulomb per second (C/s).
If the current does not change with time, but
remains constant, we call it a direct current (DC).
A direct current (DC) is a current that remains
constant with time.
A common form of time-varying current is the
sinusoidal current or alternating current (AC).
Alternating current (AC) is a current that varies
sinusoidally with time.
DC
current
AC
current
i = current in amperes
q = charge in coulombs
t = time in sec
10. 10
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Current
The direction of current flow is conventionally taken as the
direction of positive charge movement.
In fig.a he direction of the arrow and the value 3 A indicate
either that a net positive charge of 3 C/s is moving to the
right or that a net negative charge of −3 C/s is moving to the
left each second. In fig.b there are again two possibilities:
either −3 A is flowing to the left or +3 A is flowing to the
right.
11. 11
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Current
12. 12
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Current
Safety Considerations
It is important to realize that even small levels of current through
the human body can cause serious, dangerous side effects.
Most individuals can withstand currents up
to perhaps 10 mA for very short periods of time without serious
side effects, any current over 10 mA should be considered
dangerous.
currents of 50 mA can cause severe shock, and currents of over
100 mA can be fatal.
In most cases the skin resistance of the body when dry is
sufficiently high to limit the current through the body to relatively
safe levels for voltage levels typically found in the home.
when the skin is wet due to perspiration, bathing, etc., or
when the skin barrier is broken due to an injury, the skin
resistance drops dramatically, and current levels could rise to
dangerous levels for the same voltage shock.
13. 13
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Voltage
Voltage (or potential difference) is the energy
required to move a unit charge through an
element, measured in volts (V).
𝟏 𝑽𝒐𝒍𝒕 = 𝟏
𝒋𝒐𝒖𝒍𝒆
𝒄𝒐𝒖𝒍𝒐𝒎𝒃
=𝟏
𝒏𝒆𝒘𝒕𝒐𝒏−𝒎𝒆𝒕𝒆𝒓
𝒄𝒐𝒖𝒍𝒐𝒎𝒃
let us suppose that a dc current is sent into
terminal ‘a’, through the general element, and
back out of terminal ‘b’. Let us also assume that
pushing charge through the element requires an
expenditure of energy. We then say that an
electrical voltage (or a potential difference) exists
between the two terminals, or that there is a
voltage “across” the element.
v=
𝒅𝒘
𝒅𝒒
v = voltage in volt
w = energy in joule
t = time in sec
14. 14
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Voltage
The plus (+) and minus (-) signs are used to
define reference direction or voltage
polarity.
The vab means that Point a is at a potential
of vab volts higher than point b.
vab=-vba
For example, in fig.a and fig.b are two
representations of the same voltage. In fig.a,
point a is +9V above point b; in Fig.b, point
b is -9V above point a.
A constant voltage is called a DC voltage,
whereas a sinusoidally time-varying voltage
is called an AC voltage. A dc voltage is
commonly produced by a battery; ac
voltage is produced by an electric generator.
15. 15
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Power
Power is the time rate of expending or
absorbing energy, measured in
watts (W).
𝒑 =
𝒅𝒘
𝒅𝒕
=
𝒅𝒘
𝒅𝒒
×
𝒅𝒒
𝒅𝒕
=vi
If the power has a (+) sign, power is
being delivered to or absorbed by the
element.
If the power has a (-) sign, power is being
supplied by the element.
When the current enters through the
positive polarity of the voltage as in fig.a.
In this case, p=+vi implies that the
element is absorbing power. However, if
p=-vi as in fig.b the element is releasing
or supplying power.
P=power in watts
w= energy in joule
q = charge in coulombs
i = current in amperes
v = voltage in volt
t = time in sec
16. 16
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Power
Two cases of an element with
an absorbing power of 12 W:
(a)p=4×3=12 W,
(b) p=4×3=12W.
Two cases of an element with
an absorbing power of 12 W:
(a)p=-4×3=-12 W,
(b) p=-4×3=-12W.
Passive sign convention is satisfied when the current enters
through the positive terminal of an element and p=+vi. If the
current enters through the negative terminal, p=-vi.
17. 17
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Power
18. 18
Dr. Firas Obeidat – Philadelphia University
Basic Electrical Quantities - Power
