Electric Circuits I - Philadelphia University · 2017. 7. 23. · 2 Dr. Firas Obeidat – Philadelphia University Introduction electrical systems are found everywhere, such as homes,

Electric Circuits I Circuit Variables

1

Dr. Firas Obeidat

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.


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).


Units and Scales

Comparison of units of the various systems of unit.


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.


Units and Scales

Powers of Ten


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).


Basic Electrical Quantities - Charge


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



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.


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.


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


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.


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



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.

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Electric Circuits I - Philadelphia University · 2017. 7. 23. · 2 Dr. Firas Obeidat – Philadelphia University Introduction electrical systems are found everywhere, such as homes,