Electric Circuit Variables and Elements

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.

MCBLOYOLA

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

MCBLOYOLA

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

MCBLOYOLA

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.

MCBLOYOLA

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)

MCBLOYOLA

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

MCBLOYOLA

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

MCBLOYOLA

ELECTRIC CURRENT

(a) (b)

(c) (d)

ramp

exponential sinusoidal

MCBLOYOLA

direct current

ELECTRIC CURRENT AND CHARGE

Mathematically,

I for constant current, i for a time-varying current

I =Q

t A

Q = It C

MCBLOYOLA

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

MCBLOYOLA

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.

MCBLOYOLA

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

MCBLOYOLA

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

MCBLOYOLA

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

MCBLOYOLA

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.

MCBLOYOLA

CIRCUIT ELEMENTS

Non-linear element Linear element: satisfies both the homogeneity and additivity properties

MCBLOYOLA

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

MCBLOYOLA

CIRCUIT ELEMENTS

Resistor: a good example of a linear element

MCBLOYOLA

CIRCUIT ELEMENTS

Diode: a good example of a non-linear element

MCBLOYOLA

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

MCBLOYOLA

CIRCUIT ELEMENTS

SPST (Single-pole, single-throw)

MCBLOYOLA

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

MCBLOYOLA

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.

MCBLOYOLA

Please refer to course syl labus.

REFERENCES

MCBLOYOLA