1.dc circuits

8/10/2019 1.DC Circuits

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Unit 101

Electrical and Electronic Principles

BTEC HND

Sri Lanka Telecom Training CentreWelisara


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Course Contents

Chapter 1 DC circuits

DC circuit theory

Voltage

Resistance

Circuit components

Resistors

Diodes

Circuit layout

Ohms law, power and energy

Application of Kirchhoff's voltage & current laws

DC networksNetworks with one DC power source

Diode resistor circuit

Measurement in DC circuits

Multimeter

Measurements


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Chapter 2 DC circuits with capacitors

Capacitors

Types

Construction

Function

Charging and discharging of a capacitorMeasurement of voltage

Current & time

Graphical representations

Time constants

DC network that includes a capacitorDC power source with capacitors

DC power source


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Chapter 3 MagnetismMagnetic field

Electromagnetic induction

Chapter 4 AC Theory


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Units

Quantity Unit

Length metre, m

Mass kilogram, kg

Time second, s

Electric current ampere, A

Temperature kelvin, K

Intensity candela, cd

Amount of substance mole, mol


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Units

Prefix Name multiplier

T tera 10G giga 109M mega 10k kilo 103m mili 10;3 micro 10;

n nano 10;9p pico 10;


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Charge

Units- coulomb (C)

One coulomb is one ampere second

1 = 6.24 108Definition-The quantity of electricity which flows pass a given

point in an electric circuit when a current of one ampere ismaintained for one second

=


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Exercise

I. A charge of 35 mC is transferred between two points in a

circuit in a time of 20 ms. Calculate the value of current

flowing

II. If a current of 12 0 A flows for a time of 15 s,

determine the amount of charge transferredIII. 80 coulombs of charge was transferred by a current of 0.5

A. Calculate the time for which the current flowed


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Electric Potential (Voltage) & EMF

Units - Volt (V)

One volt is defined as the difference in potential between two

points in a conductor which, when carrying a current of one

ampere, dissipates a power of one watt

DefinitionA change in electric potential between two points inan electric circuit is called a potential difference


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AC & DC

The term DC means Direct Current .

It is also used to describe unidirectional voltages

A DC voltage refers to a unidirectional voltage that may only be

An alternating or AC supply in which the current flows

alternately in opposite directions


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Kirchhoffs Current Law

At any junction in an electric circuit the total current flowing

towards that junction is equal to the total current flowing away

from the junction,

= 0 + = 3 + + 5

+ 3 5 = 0


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Exercise

Find the unknown currents


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Kirchhoffs Voltage Law

The algebraic sum of the voltage equals zero for any closed

electrical path in an electrical circuit

= 0

A closed electrical path is called a loop

+ + = 0

+ = 0

+ + = 0


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Exercise

Find unknown voltages


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Resistors in Series

= + + 3 + + = + + 3 + +

= = + + 3 + +


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Resistors in Parallel

= + + 3 + +

= +

+

+ +

= =

:

:

::


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Example


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Exercise

(a)

(b)

(d)(c)


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Ohms law

The potential difference developed between the two ends of a

resistor is directly proportional to the value of current flowing

through.

The constant of proportionality is called the resistance = =

=


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Exercise

A current of 5.5 mA flows through a 33 kresistor.

Calculate the voltage thus developed across it

If a voltage of 24 V exists across a 15 resistor then what

must be the current flowing through it?


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Internal Resistance of a DC source

Ideal Voltage Source

The generated voltage is equal to its terminal voltage when

supplying current

Practical Voltage Source

Terminal Voltage is less than generated voltage when supplyingcurrent

=


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Exercise

1. Following figure shows a DC source connected to an external

circuit

I. Find the terminal voltage

of the DC source

II. Find the terminal voltage

when external circuit isopen circuited

2. A battery of emf 6 V has an internal resistance of 0.15

Calculate its terminal voltage when delivering a current of

I. 0.5 A

II. 2 A

III. 10 A


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Exercise

3. A battery of emf 12 V supplies a circuit with a current

of 5 A. If, under these conditions, the terminal voltage is

11.5 V, determine

I. The battery internal resistance

II. The resistance of the external circuit


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Energy

Units-Joule

Definition - The property of a system that enables it to do

work

The most common form into which energy is transformed

is heat. Thus one of the effects of an electric current is to

produce heat

=

=

=


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Exercise

1. A current of 200 mA flows through a resistance of 750

for a time of 5 minutes. Calculate

I. The voltage developed

II. The energy dissipated


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Power

UnitsWatts (W)

Definition - The rate at which energy is dissipated

=

= =

=


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Exercise

1. A resistor of 680 , when connected in a circuit,

dissipates a power of 85 mW. Calculate

I. The voltage developed across it

II. The current flowing through it

2. A current of 1.4 A when flowing through a circuit for 15

minutes dissipates 200 KJ of energy. Calculate

I. The voltage

II. Power dissipated

III. The resistance of the circuit


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Resistance


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Conductors

Zero voltage drop in ideal conductors

Short circuit

Two points are connected by conductor

Open circuit

No connection between two points in a circuit


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Charging a Capacitor (DC)

When a voltage is applied across a capacitor, the positive

voltage attracts electrons from the positive plate, leaving it

positively charged

The negative voltage repels electrons to the negative plate,

making it negatively chargedWhen a capacitor charges a current (I) will flow for a period of

time (t) as the electrical charge (Q) moves around the circuit

=


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Charging a Capacitor (DC)

If this supply voltage is then removed, the charge is held in

place by the attraction between the opposite charges on the

plates

1.dc circuits

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