Basic Concepts of DC Circuits
Introduction
An electric circuit is an interconnection of
electrical elements.
Systems of Units
Charge
The most basic quantity in an electric
circuit is the electric charge.
Charge is an electrical property of the
atomic particles of which matter consists,
measured in coulombs (C).
The charge
e
on an electron is

1.602 x
10

19
C.
Charge (cont.)
Current
A unique feature of electric charge or
electricity is that it is mobile; it can be
transferred where it can be converted to
another form of energy.
When a conducting wire is connected to a
battery, the charges are compelled to
move; positive charges in one direction
and negative charges in the opposite
direction.
Current (cont.)
This motion of charges is what creates an
electrical current.
Electric current is the time rate of change of
charge, measured in amperes (A).
1 ampere = 1 coulomb / second
It is conventional to take the current flow as the
movement of positive charges, although current
is actually due to negatively charged electrons.
Current (cont.)
A direct current (dc) is a current that
remains constant with time.
An alternating current (ac) is a current
that varies sinusoidally with time.
The Relationship
Mathematically, the
relationship between
current
i,
charge
q
,
and time
t
is
The charge
transferred between
time
t
0
and
t
is found
by integrating both
sides;
Voltage
To move an electron in a particular
direction requires some work or energy
transfer. This work is performed by an
external electromotive force (emf),
typically a battery.
This emf is also known as potential
difference or voltage.
Voltage (cont.)
The voltage between
two points
a
and
b
is
the energy (or work)
needed to move a
unit charge from
a
to
b
.
where
w
is energy in
joules (J) and
q
is
charge (C). The
voltage is measured
in volts (V).
1 volt = 1 joule /
coulomb
= 1
newton*meter/ coulomb
Voltage Polarity
The plus (+) and
minus (

) signs are
used to define
reference direction or
voltage polarity.
v
ab
=

v
ba
The polarity can be
interpreted in two
ways:
Power
Although current and voltage are the two
basic variables, they are not sufficient by
themselves.
For practical purposes, we need to know
how much power a device can handle and
how much energy is consumed over a
period of time.
Power (cont.)
To relate power and
energy to voltage and
current, we recall that:
power is the time rate of
expending or absorbing
energy, measured in
watts (W).
We write this relationship
as
p
is power in watts (W),
w
is energy in joules (J)
and
t
is time in seconds
(s).
Power (cont.)
The power
p
is a time

varying quantity
and is called instantaneous power.
p
> 0, power is absorbed
p
< 0, power is supplied
power absorbed =

power supplied
Power (cont.)
The law of conservation of energy must
be obeyed in any circuit. For with this
reason, the sum of power in a circuit must
be zero.
Energy
Energy is the capacity to do work,
measured in joules (J).
The energy absorbed or supplied by an
element from time
t
0
to
t
is
Circuit Elements
An element is a basic building block of a
circuit.
There are two types of elements: active
and passive.
Circuit Elements (cont.)
Active elements are
capable of generating
energy
Generators
Batteries
Amplifiers
Passive elements
cannot create energy
Resistors
Capacitors
Inductors
Circuit Elements (cont.)
The most important active elements are
voltage and current sources because they
deliver power to the circuit.
There are two types of sources:
independent and dependent.
Circuit Elements (cont.)
Independent sources
provide a specified
voltage or current
that is completely
independent of other
circuit variables.
Dependent sources
have their source
quantity controlled by
another voltage or
current.
Circuit Elements (cont.)
There are 4 possible types of dependent
sources: voltage controlled voltage source,
current controlled voltage source, voltage
controlled current source, and current
controlled current source.
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