2a Intro to Electronics Theory

8/2/2019 2a Intro to Electronics Theory

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Introduction to Electronics Theory

English for EEIT


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Introduction to Electronics

Definitions

Conventional current Voltage & Current

Resistance

Ohms Law


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Electricity is

the electric charge.

the electromagnetic field energy sent out by batteries andgenerators.

the flowing motion of electric charge.

the amount of imbalance between quantities of electrons andprotons.

the classes of phenomena involving electric charges.

the flowing motion of electrical energy (electric power, Watts ofelectricity)

the electric potential or e-field (Volts of electricity)

the glowing nitrogen/oxygen plasma (sparks of electricity)

a field of science (Basic Electricity, Advanced Electricity)

(There are many definitions of the word electricity)


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Conventional current


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Current electricity

Current electricity is generated and used as asource of energy in homes and industry.

Current electricity is a flow of charged particles,usually through a circuit

In all dry conductors, the flow is of electrons andtherefore of negative charge.

The electrons flow from negative to positive. Theyare attracted to the positive terminal and repelled

from the negative terminal.


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However

It was once thought that current flowed

from positive to negative.Consequently: Though the electrons flowfrom negative to positive, conventionalcurrent flows from positive to negative

(although conventional current doesn'treally exist


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Benjamin Franklin

This was put forward by

Benjamin Franklin (1706 -90) and it became sowidely accepted that it isstill used today and calledconventional current, eventhough it was found to bewrong in 1898 when SirJoseph J Thomsondiscovered the electron*.


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Voltage & Current


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What is electric current?

Electric current or power that results from the

movement of electrons in a conductor from anegatively charged point to a positivelycharged point.

For the purpose of this class we will useelectricity to mean electric current.


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Representing and measuring current

Current is represented by the symbol, I and ismeasured in amperes (amps or A) with an ammeter.

An ammeter measures the rate at which electrons* areflowing through a circuit* at a given point.In a circuit, an ammeter is shown as follows:


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Current

Current is not used up, whatflows into a component mustflow out.

Current is measured with an

ammeter, connected in

series. To connect in series you

must break the circuit and putthe ammeter across the gap,

as shown in the diagram.


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Current

The need to break the

circuit to connect in seriesmeans that ammeters aredifficult to use on solderedcircuits. Most testing in

electronics is done withvoltmeters which can beeasily connected withoutdisturbing circuits.


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Formula for Current

One amp is equal to the flow of one

coulomb of charge per second. Thistranslates to the following formula:current = charge/time (I=Q/t)

Or in unit terms:Amps=Coulombs/Seconds


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Physics Formulas:

Charge, Current & Time

The questions on the following pages test

your ability to use the formula:current=charge/time

You should also learn the formula in theinternationally-agreed characters:

I=Q/t


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Question 1

A current of 2 A flows for 30 seconds through a lamp.How much charge has moved?

60 coulombs of charge has moved.

The formula is: I=Q/t (Amps=Coulombs/Seconds)or Q=t x I (Coulombs=Amps*Seconds)

Thus: Q = 2 30

Q = 60 coulombs


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Question 2

A current of 12 A flows for 20 minutes into anMicrowave oven.

How much charge has the Microwave used?

The Microwave has used 14400 coulombs of charge.

The formula is: I=Q/t (Amps=Coulombs/Seconds)

or Q=t*I (Coulombs=Amps*Seconds)Thus: Q =12 1200

Q = 14400 coulombs


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Question 3

If 18400 C of charge flows through an air

conditioning unit every hour, what current does itdraw?

The Air Conditioning Unit draws 5.1 amps


Hence: I=18400/(60x60)

I=18400/3600

I=5.1 amps


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Question 4

A current of 3 A flows into a television set.

How long would it take 1500 C of charge to flow

through it?It takes 500 seconds for 1500 C of charge to flow

through the set.


Hence: 3=1500/t3 t = 1500

t=1500/3

t = 500 seconds


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TEST

Current is represented by which symbol?

I O U

Current is represented by the symbol, I.
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TEST

The electrons in a current flow from where towhere?

FROM POSITIVE TO NEGATIVE

FROM NEGATIVE TO POSITIVE

FROM HERE TO ETERNITY

The electrons in a current flow from negative topositive.


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TEST

Conventional current flows from where to where?

FROM RUSSIA WITH LOVEFROM POSITIVE TO NEGATIVE

FROM NEGATIVE TO POSITIVE

Conventional current flows from positive tonegative

(although conventional current doesn't really exist).


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TEST

Current is measured in what?

AMPS

OHMSVOLTS

Current is measured in amperes (amps or A).

Ohms is resistance Volts is potential difference across a circuit


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TEST

One amp is equal to the flow of charge at:

1 OHM1 COULOMB PER SECOND

1 VOLT

One amp is equal to the flow of charge at onecoulomb per second.


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Voltage

In order for a current to flow, something

has to make it flow. The energy available to drive a flow of

current is called potential difference orelectromotive force (EMF). It is provided

by an energy source such as a battery orpower supply and is measured in volts bya voltmeter:


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Voltage

One volt is the energy required to drive a current

of one amp* through a circuit with a resistance ofone ohm.

Voltage is represented by the symbol, V.


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Voltage is the Cause, Current is

the Effect

Voltage attempts to make a current flow, andcurrent will flow if the circuit is complete.

Voltage & Current

The switch is closed making a complete circuit socurrent can flow.


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Voltage but No Current

The switch is open so the circuit is brokenand current cannot flow.


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Voltage, V

Voltage is a measure of theenergy carried by thecharge.

Voltage is supplied by thebattery (or power supply).

Voltage is used up incomponents

Voltage is measured with avoltmeter, connected in

parallel across a component.


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Voltage at a point and 0V (zero

volts)

Voltage is a differencebetween two points,

Usually refer to voltage at apoint as the differencebetween that point and a

reference point of 0V (zerovolts).

Zero volts is normally thenegative terminal of thebattery or power supply. Y


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Zero volts for circuits with a dual

supply

On complex circuit diagramsusing a dual supply the earthsymbol is often used toindicate a connection to 0V.

The diagram shows a 9V

dual supply, the positiveterminal is +9V, the negativeterminal is -9V and the middleterminal is 0V.


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Voltage and Current for

components in Series

Voltages add up for componentsconnected in series.

Currents are the same through allcomponents connected in series.

In this circuit the 4V across the

resistor and the 2V across the LEDadd up to the battery voltage: 2V +4V = 6V.

The current through all parts

(battery, resistor and LED) is 20mA.


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Voltage and Current for

components in Parallel

Voltages are the same across all

components connected in parallel. Currents add up for components

connected in parallel. In this circuit the battery, resistor and

lamp all have 6V across them. The 30mA current through the resistor

and the 60mA current through thelamp add up to the 90mA current

through the battery.


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Voltage & Current Summary

Voltage is the energy that drives the flow of current.

Voltage is measured in volts with a voltmeter:

Voltage is represented by the symbol, V.

One volt is the energy required to drive a current ofone amp through a circuit with a resistance of oneohm.

An increase in voltage means an increase in energyand therefore an increase in current.

A decrease in voltage means a decrease in energyand therefore a decrease in current.


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Voltage & Current Summary

Voltages add up for components connectedin series.

Currents are the same through allcomponents connected in series.

Voltages are the same across all

components connected in parallel.

Currents add up for components connectedin parallel.


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Current Electricity Summary

Current electricity is a flow of charged particles.

In all dry conductors the flow is of electrons.

Electrons flow from negative to positive.

Conventional current flows from positive to negative

(although conventional current doesn't really exist).

Current is represented by the symbol, I.

Current is measured in amperes (amps or A) with anammeter.

One amp is equal to the flow of one coulomb* of chargeper second. This translates to:current=charge/time (I=Q/t)

Or in unit terms: Amps=Coulombs/Seconds


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Resistance


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Conductors

All conductors* offer resistance to the flow of current*.This resistance is determined by the conductor's

atoms*.Silver and copper atoms offer negligible resistance to

an electric current because a significant proportion oftheir electrons* are free to move from atom to atom.

Silver and copper, having negligible resistance, arecommonly in used as conductors.


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Insulators

Glass and synthetic polymer atoms offer veryconsiderable resistance to an electric current

because a significant proportion of their electronsare not free to move from atom to atom.

Glass and synthetic polymers, having veryconsiderable resistance, are commonly used as

insulators*.A conductor's length, temperature, and cross-sectional

area also affects its resistance.


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Open Circuit

With an open circuit there is no flow of electrons


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Insulator

An insulator acts in a similar manner to an open circuit, noelectrons flow


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Conductor

With a conductor the circuit is completed and current flows


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Semi-Conductor

With semi-conductors,some energy flows and

some is used up creatingheat.A semiconductor is amaterial with an electrical

conductivity that isintermediate between thatof an insulator and aconductor.


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Resistance

Resistance is represented by the symbol R andis measured in ohms or

The ohm is named after physicist Georg Ohm(1787 - 1854).


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TEST

Resistance does what?

ENABLES THE FLOW OF CURRENTRESISTS THE FLOW OF CURRENT

Resistance resists the flow of current.


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Test

Resistance is...

HIGH IN A CONDUCTOR, LOW IN AN INSULATOR

LOW IN A CONDUCTOR, HIGH IN AN INSULATOR

Resistance is low in a conductor and high in an

insulator.


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Test

Resistance is measured in what?

ROMMELSHINDENBURGS

OHMS

Resistance is measured in ohms.


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Test

Resistance is represented by which

symbol?V C R

Resistance is represented by the symbol,

R.


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Resistance Summary

Resistance resists the flow of current.

Resistance is determined by a conductor'satoms, length and area.

Resistance is low in a conductor, medium in asemi-conductor and high in an insulator.


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Ohm's Law

In Voltage & Current we explained therelationship between voltage and current.

In Resistance we explained therelationship between resistance andcurrent.

Here we explain the relationship betweenall three.


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Ohm's Law

Georg Simon Ohm (1787-1854) formulatedthe relationships among voltage, current, andresistance as follows:

The current in a circuit is directlyproportional to the applied voltage and

inversely proportional to the resistance ofthe circuit.


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Ohm's Law

Ohm's Law most simply translates to theequation,

V = IR, or:

Voltage = Current Resistance

where V is the applied voltage in volts, I is

current in amperes, and R is resistance inohms().

This is all, however, at a constant temperature


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Ohm's Law

In reality Ohm's Law is

stated like this:

Current is directly

proportional tovoltage for a metalconductor at aconstant temperature.


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Water Model: Ohms = Ohms

Resistance is the Same for both

(Note: The Higher the Resistance the Smaller the Orifice)


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Water Model: Amps = Amps

Current is the Same for both

(Note: The Higher the Resistance the Smaller the Orifice)


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Power = Volts x Amps (Work)


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Example 1

We can demonstrate the relationship between voltage*,current* and resistance* in the circuit below which

connects a cell*, an ammeter* and a resistor

The cell provides a voltage of 1 volt.The resistor has a value of 1 ohm.The ammeter reads a current of 1 amp.If we replace the 1 ohm resistor with a 2 ohm resistor, what

will the ammeter read?


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The ammeter reads 0.5 amps.

A 2 ohm resistor is twice as resistant as a 1 ohmresistor, so it is twice as hard for the current to flow.

The resistor allows half of the 1 amp current to pass.By Ohm's Law: V(volt) = I(amp) x R(ohm)

Solving for I: I = V/R

Now replacing the symbols with the values given:

I = 1(volt)/2(ohm)

I = 0.5

I = 0.5 amps


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Example 2

The circuit below connects a cell, an ammeter and a resistor.

The cell provides a voltage of 2 volts.

The resistor has a value of 2 ohms.

The ammeter reads a current of 1 amp.

If we replace the 2 ohm resistor with a 1 ohm resistor,what will the ammeter read?


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The ammeter reads 2 amps.

A 1 ohm resistor is half as resistant as a 2 ohm resistor,so it is twice as easy for the current to flow. The

resistor allows twice the 1 amp current to pass.By Ohm's Law: V = IR (Volts = Amps x Ohms)

Solving for I: I = V/R


I = 2/1I = 2

I = 2 amps


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Example 3

The circuit below connects a cell, an ammeter and aresistor.

The cell provides a voltage* of 4 volts.

The resistor has a value of 2 ohms*.

The ammeter reads a current* of 2 amps.

If we replace the 2-ohm resistor with an 4-ohm resistor, what

will the ammeter read?


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The ammeter reads 1 amp.

A 4-ohm resistor* is twice as resistant as a 2-ohmresistor, so it is twice as hard for the current* to flow.

The resistor allows half of the 2 amp current to pass.By Ohm's Law: V = IR (Volts = Amps x Ohms)Solving for I: I= V/R


I = 4(volt)/4(ohm)I =1

I = 1 amp


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Summary

The relationship between voltage, resistanceand current is expressed in Ohm's Law which

most simply translates to:V = IR

or

Voltage = Current

Resistance(Volts = Amps x Ohms)

Which is true only at a constant temperature.


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Practical

What is the value of the resister?

4 V/ 02 A=200 R

2a Intro to Electronics Theory

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