Basic Electric Circuits Session 1

Basic Electric Circuits – © 2020 Mouli Sankaran Email: mouli.sankaran@yahoo.com 2
Session 1: Focus
 Introduction to Electric Circuits
 System of Units
 What is an Electric Circuit?
 Electric Current
 Electric Potential
 Flashlight Circuit
 Power
 Home Work Problems

An Introduction to
Electric Circuits

Electric Circuits on a
Printed Circuit Board (PCB)
4

Introduction to Electric Circuits
 Circuit analysis has long been a traditional introduction
to the art of problem solving from an engineering
perspective.
 It’s extremely unlikely for any engineer to encounter a
system that does not in some way include electrical
circuitry.
 As circuits become smaller and require less power,
power sources have become smaller and cheaper.
◦ Embedded circuits are seemingly everywhere
 So, Engineers of all disciplines need to understand
Electric Circuits

Systems with Electric Circuits
Hubble Space Telescope: NASA
Ground
Control
Oil Rig

Evolution of Circuit Technology
 Circuit technology has changed drastically over the
years
 In the early 1960s the space on a circuit board
occupied by the base of a single vacuum tube
◦ Today that same space could be occupied by an Intel
Pentium integrated circuit chip containing 50 million
transistors
Vacuum Tube
Intel’s Pentium Chips

System of Units

System of Units
 The system of units we employ is the International
System of Units, the Système International des Unités
◦ Normally referred as SI standard system
 SI is composed of the basic units
◦ meter (m), kilogram (kg), second (s), ampere (A), kelvin (K),
mole (mol) and candela (cd)

Standard SI Prefixes & Engineering Units
 These standard prefixes are employed throughout our study of
electric circuits
 It is fairly common to see numbers expressed in what are
frequently termed as “engineering units.”
 In engineering notation, a quantity is represented by a number
between 1 and 999 and an appropriate metric unit using a power
divisible by 3.
◦ 0.048 W  48 mW
◦ 4.8 x 10-1 A  480 mA
◦ 12000 g  12 kg

What is an Electric Circuit?

What is an Electric Circuit?
 An electric circuit is an interconnection of electrical
components.
 The most elementary quantity in an analysis of
electric circuits is the electric charge.
 Our interest in electric charge is centered around its
motion
◦ Since charge in motion results in an energy transfer
 Especially of those situations in which the motion of
charge is confined to a definite closed path
 An electric circuit is essentially a pipeline that
facilitates transfer of charge from one point to
another.

Electric Current

Electric Current
 The time rate of change of charge constitutes an
electric current
 i (t) and q(t) represent current and charge, respectively
◦ Lowercase letters represent time dependency, and capital
letters are reserved for constant quantities
 The basic unit of current is the ampere (A), and 1 A is
1 coulomb per second.
i(t) is the instantaneous current
q(t0) is the charge that has flown from: – ∞ to t0

Current Flow

Current Flow
 We know that current flow in metallic conductors
results from electron motion
 The conventional current flow, which is universally
adopted, represents the movement of positive charges
 The figure below indicates that at any point in time, 2 C
of charge pass from left to right each second through
the wire
◦ Which means that electrons are flowing from right to left
 It is mandatory to specify not only the magnitude (2 A)
of current but also its direction ( )
Positive current flow

Current Flow … contd.
 A single electron has a charge of −1.602 × 10−19 C
 Thus, 6.24 x 1018 electrons have -1 C of charge
◦ 1/ 1.602 × 10−19 = 6.24 x 1018
 Which means that, in the below circuit
◦ Electrons flow from right to left
◦ 2 Ampere is a flow of 2 Coulomb of charges per second
◦ 2 * 6.24 x 1018 electrons flow in one second
Positive current flow

Example: Positive Current Flow
 What is the direction of movement of electrons in the
below figure?
 How many electrons flow in a second?
◦ Net positive charge of 3C move left to right in a second
◦ That means, electrons are flowing from right to left and
◦ Total of 3 * 6.24 x 1018 electrons flow in a second
3
1 A is 1 coulomb per second
One electron has -1.602 × 10−19 Coulomb of charge
1/ 1.602 × 10−19 = 6.24 x 1018 electrons have -1 C of charge

Representation of Current
 Current of 3A indicates 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
 Two methods of representation for the exact same current (3A)
are given below
 It is convenient to think of current as the motion of positive
charge, even though it is known that current flow in metallic
conductors results from electron motion.

Example: Negative Current Flow
 What is the direction of movement of electrons in the
below figure?
 How many electrons flow in a second?
◦ Electrons flow from left to right
◦ 3 * 6.24 x 1018 electrons flow in a second
Negative current flow

Quiz 1
 In the below circuit, if 7 A of current has been flowing
for 5 seconds:
◦ How many electrons would have flown in total and in which
direction?
◦ Electrons would have flown from right to left
◦ 5 * 7 * 6.24 x 1018 electrons in 5 seconds
7

Quiz 2
 In the figure below electrons are moving from left to
right to create a current of 1 mA.
 Determine I1 and I2
 I1 = -1 mA
 I2 = +1 mA

AC and DC
 In our daily lives, two types of currents are often
encountered
◦ Alternating Current (AC)
◦ Direct Current (DC)
 Common current found in every household is AC which
is used to run the refrigerator, TV, etc.
 Batteries, which are used in automobiles and flashlights,
are examples of DC sources
AC DC Exponential Damped Sinusoidal

Typical Current Magnitudes

Electric Potential

Voltage or Potential Difference
 Voltage across a terminal pair is a measure of the
work required to move charge through the element
 The unit of voltage is the volt
 1 volt is the same as 1 joule per coulomb.
 Voltage is represented by V or v
A general two-terminal
circuit element
Note: Work or energy, w(t) or W, is measured in joules (J);
1 joule is 1 newton meter (N m)

 Assume, a DC current is sent into terminal A, through
the general element, and back out of terminal B
 Also assume that pushing charges 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
A general two-terminal
circuit element

 The variable (v) represents the voltage between points
A and B and it is assumed that point A is at a higher
potential than point B
 If v = 5V then the difference in potential of points A
and B is 5 V and point A is at the higher potential
Terminal A is 5 V positive
with respect to terminal B

 If a unit positive charge is moved from point A
through the circuit to point B
◦ It will give up energy to the circuit and have 5 Joules less
energy when it reaches point B
 It means that the voltage source is delivering energy to
the circuit elements connected to the points A and B
◦ Or the circuit elements are said to absorb energy from the
voltage source
Terminal A is 5 V positive
with respect to terminal B

 v = -5V means that the potential between points A and
B is 5 V and point B is at the higher potential
 The same can also be represented as v = 5 V, with point
B at a higher potential with respect to A
◦ Note the changes to the signs on the terminals A and B
 Hence, when we define either current or voltage, it is
absolutely necessary that we specify both magnitude
and direction.
Terminal A is -5 V
with respect to
terminal B

Home Work Problems

S1_HW_Problem_1
 Find the unknown voltage and current
 V1:
 I :

S1_HW_Problem_2
 Find the unknown voltage and current
 V1:
 I :

S1_HW_Problem_3
 Compute the power absorbed by each part in the
figure
 (a):
 (b):
 (c):

S1_HW_Problem_4
 Compute the power being absorbed by the circuit
element (a):
 Compute the power being generated by (b):
 Determine the power being delivered at t = 5 ms
(c):

Session 1: Summary
 Introduction to Electric Circuits
 System of Units
 What is an Electric Circuit?
 Electric Current
 Electric Potential
 Flashlight Circuit
 Power
 Home Work Problems

References
Ref 2Ref 1

Basic Electric Circuits Session 1

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