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Electric Circuit Variables and Elements (c) credits to the owner
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1 T S Y 2 0 1 4 - 2 0 1 5
EE101 EE CIRCUITS 1
MCBLOYOLA
W E E K 1
Electric Circuit Variables and Elements
MCBLOYOLA
LEARNING OUTCOMES
Discuss the electric circuit variables such as the quantities electric charge, current, voltage, power and energy including their definition, characteristics and properties, symbol, unit, and formula.
Discuss the different types of electric circuit elements such as, linear and non-linear, passive and active, independent and independent sources and switches.
Differentiate an open and close circuit.
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What is an electric circuit?
Electric circuit or electric network
is an interconnection of electrical elements linked together in a closed path so that an electric current may flow continuously.
Can you give real example of an electric circuit? MCBLOYOLA
A simple electric circuit
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ELECTRIC Charge
Charge is an electrical (and intrinsic) property of the atomic particles of which matter consists, measured in coulombs (C).
The charge of an electron is negative and equal in magnitude to 1.602 10-19 C which is called as electronic charge.
= -1.602 10-19 C
-1 C of charge = 6.24 x 1018 electrons
Q for constant charge, q for a charge that varies with time
ELECTRIC CHARGE
A unique feature of electric charge is the fact that it is mobile;
it can be transferred from one place to another
it can be converted to another form of energy
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ELECTRIC CURRENT
When a conducting wire (consisting of several atoms) is connected to a battery (a source of electromagnetic force), the charges are compelled to move.
This motion of charges create electric current.
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ELECTRIC CURRENT
Electric current is the time rate of change of charge, measured in amperes (A). (Alexander & Sadiku, 2011)
Electric current is the time rate of flow of electric charge past a given point (Dorf & Svoboda, 2011)
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ELECTRIC CURRENT
Keep in mind that electric current is always through an element.
Conventional current flow: (a) positive current flow, (b) negative current flow
Conventional: take the current flow as the movement of positive charges, that is, opposite to the flow of negative charges
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DIRECT CURRENT ALTERNATING CURRENT
A direct current (dc) is a current that remains constant with time.
An alternating current (ac) is a current that varies sinusoidally with time (time-varying current)
ELECTRIC CURRENT
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ELECTRIC CURRENT
(a) (b)
(c) (d)
ramp
exponential sinusoidal
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direct current
ELECTRIC CURRENT AND CHARGE
Mathematically,
I for constant current, i for a time-varying current
I =Q
t A
Q = It C
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VOLTAGE
To move electron in a conductor in a particular direction requires some work or energy transfer
This work is performed by an external electromotive force (emf)
Also known as voltage or potential difference
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VOLTAGE
Voltage (or potential difference) is the energy required to move a unit charge through an element, measured in volts (V). (Alexander & Sadiku, 2011)
Voltage across an element is the work (energy) required to move a unit positive charge from the negative () terminal to the positive (+) terminal. (Dorf & Svoboda, 2010)
Defining the voltage between two points. MCBLOYOLA
VOLTAGE
Keep in mind that voltage is always across the element or between two points.
vab > 0 means the potential of a is higher than potential of b.
vab < 0 means the potential of a is lower than potential of b.
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VOLTAGE
Mathematically,
Where:
w, W = energy in joules (J)
q, Q = charge in coulombs (C)
v, V = voltage in volts (V); usually written with subscripts (i.e , V= voltage across points a and b)
dq
dwv
V for constant voltage (dc voltage), v for a time-varying voltage (ac voltage)
V =W
Q V
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POWER AND ENERGY
Power is the time rate of expending or absorbing energy, measured in watts (W).
Mathematical expression:
vidt
dq
dq
dw
dt
dwp
Passive sign convention: when current enters through the positive terminal of an element, p = +vi MCBLOYOLA
POWER AND ENERGY
The law of conservation of energy
Energy is the capacity to do work, measured in joules (J).
Mathematical expression:
0p
t
t
t
tvidtpdtw
0 0
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POWER AND ENERGY
Units for w, W
joules, J
kilowatthours, kWh
t
t
t
tvidtpdtw
0 0
P for power which is constant, p for a time-varying power
= P = VI
=
P =
W
t
W = Pt = VIt
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CIRCUIT ELEMENTS
Active Elements: capable of generating/supplying energy
Passive Elements: absorb energy
Independent sources
Dependent sources A dependent source is an active element in
which the source quantity is controlled by another voltage or current.
They have four different types: VCVS, CCVS, VCCS, CCCS. Keep in mind the signs
of dependent sources.
An active element that provides a specified voltage or current that is completely
independent of other circuit variables.
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CIRCUIT ELEMENTS
Non-linear element Linear element: satisfies both the homogeneity and additivity properties
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LINEARITY
Homogeneity:
Requires that if the input is multiplied by a constant, then the output is multiplied by the same constant
Additivity
Requires that the response to a sum of inputs is the sum of the responses to each input applied separately
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CIRCUIT ELEMENTS
Resistor: a good example of a linear element
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CIRCUIT ELEMENTS
Diode: a good example of a non-linear element
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CIRCUIT ELEMENTS
SWITCHES
Used in circuits to connect and disconnect elements and circuits
Have two distinct states
Open
Closed
Ideally, a switch acts
A short circuit when it is closed
An open circuit when it is open
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CIRCUIT ELEMENTS
SPST (Single-pole, single-throw)
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Short circuit and open circuit as ideal meters
An ideal ammeter measures the current flowing through its terminals and has zero voltage across it; acts like short circuits
An ideal voltmeter measures the voltage across its terminals and has terminal current equal to zero; acts like open circuits
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Short circuit and open circuit as special cases of resistance
9/22/2013
29
An element with resistance approaching zero or R = 0 (G = ) is a short circuit.
An element with resistance approaching infinity or R = (G = 0) is an open circuit.
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Please refer to course syl labus.
REFERENCES
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