Electrical Engineering Definitions

7/27/2019 Electrical Engineering Definitions

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Electrical Engineering Definitions

SI units

Electric Charge, Electrostatic Force, Electric

and Magnetic Fields

Electric Voltage, Current, and Power

DC and AC sources

Conductors, Resistance, Capacitance

Ohms Law


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Currently, there are two main sets of measurement standards in use

The U.S Customary System

The International System (SI for short)

The primary system of measure for the world is the SI system often

referred to as the metric system

The scientific and engineering community use the SI system, so it isimportant to learn the system


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Some commonly used prefixes for the international

system of units

Prefix Symbol Power of 10 Prefix Symbol Power of 10

exa E deci d

peta P centi c

tera T milli mgiga G micro

mega M nano n

kilo k pico phecto h femto f

deka da atto a

1810

1510

12

109

10

610

3

102

10

110

110

210

3

10

610

910

12

10

1510

1810

PEN


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The seven basic SI units

Measured Quantity Unit Symbol

Length Meter m

Mass Kilogram kgTime Second s

Electric Current Ampere A

Thermodynamic Temperature Kelvin KAmount of a Substance Mole mol

Luminous Intensity Candela cd


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Example finding net charge:

What is the magnitude of charge, in coulombs, of 756.23 x 1017

electrons?

18

number of electrons

6.24 10 electrons/Cnetq

x=

17

18

756.23x10 electrons

6.24 10 electrons/C

netq

x

=

12.12netq C=


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Electrical Charge

To characterize electrical charge behavior we must start at the atom

which consist of

1. Proton(s): positive charges

2. Neutron(s): neutral charges

3. Electron(s): negative charges

Charge properties

1. Like charges repel one another

2. Unlike charges attract one another3. The force of repulsion or attraction obeys the inverse square law

EXAMPLE 4.1
http://example4.1.jnt/http://example4.1.jnt/


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A atom that has the same number of positive particles (protons)

and negative particles (electrons) has a net neutral charge.

-

-

+N

N+

-

N+

N

-

-

+N

N+

-

N+

N


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If an atom loses an electron, it has a net positive charge

-

+N

N+

-

N+

N

--

+N

N+

-

N+

N

-


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Once an electron is removed from an atom, which requires

some form of work, we are left with a positively charged atom

and a free, negatively charged, electron

The free electron may

1. Be captured by a nearby positive atom

2. Remain free and migrate to the surface of the material

which occurs because like charges repel

Note: option 2 assumes the energy applied to dislodge the

electron from the atom was not sufficient to completely ionize

the electron from the material


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The repulsion and attraction phenomena of charge can be

best explained by looking at the equation for the force

present between two charges, or Coulombs law

Electrostatic Force

Electrostatic Force is the force exerted by one body ofcharge on another. This force can be calculated for

simple point charges by using the following equation.

F - is the force between the charges, q1 and q2

r - is the distance of separationk - is a proportionality constant = 8.99 x 109 Nm2/C2

- is a unit vector pointing from q1 to q2

EXAMPLE 4.2

122

21

rF r

qqk=

12r


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The Electrostatic Force depends on the presence of at least

two charges a fixed distance apart.

We can generalize the force exerted by a single point charge

by defining a field, known as the Electric Field..

Electric Field

Note the units are in Newton/Coulomb or force/unit charge

0q

FE =

This is in terms of the force a test charge would experience whenplaced in the electric field produced by other charges.


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Graphically representing Electric Field lines

++

1. Field lines originate on a positive

charge and terminate on a

negative charge. If there is no

negative charge to terminate on,

the lines will continue out to infinity

2. The number of field lines isproportional to the magnitude of

the net charge

3. The density of the field lines at agiven point is proportional to the

magnitude of the field at that point


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Electric Current

How is moving charge is described, or how do we quantify the amount ofcharge we can move through a wire connected to a battery.

Electric Current

Electric Current, symbolized by I, is the quantitative measure of the

flow rate of electric charge carriers. It is measured by determining the

number of coulombs of charge that pass a specific point in the period

of time. The unit of electric current is the Ampere.

dq CoulombsI Ampere

dt Second = = =


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Example of current

A piece of wire is connected across the terminals of a battery for a

period of 3 seconds. During this time, it is determined that 18

coulombs of charge move through the wire. What is the current in

the wire while it is connected to the battery terminals?

18 63

CI ASec

= =

dq

I dt=


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Magnetic Field

All changing Electrical fields have an

accompanying Magnetic Field. The magnetic

field is the result of charge movement or

Current . If a current exist you have an

associated magnetic field.

The magnetic field produced by a current is

related the magnitude of the current and thegeometry and other physical characteristics

of the conductor the current is in.

BATTERY-WIRE COIL-NAIL


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Magnetic Flux due to a Current

(a)(a)

(b)(b)

(a)(a)

(b)(b)

2

7

0

(Weber)

cross-sectional area of the coil

permeability of core material, in freespace

4 x 10 /

number of turns in the coil

length of the coil

current in the coil

cm

c

N AI

l

A

H m

N

l

I

=

=

=

= =

=

=

=


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Voltage (potential): is defined as electrical potential energyper unit charge.

Voltage can be thought of as a measure of stored electrical

energy that has the ability to do work, such as moving other

charges via an electric field.

This stored energy is equal in magnitude to the work done to

move a charge from point A to B where A and B are of

different potentials.

When we talk about Voltage, or potential difference, we are

really describing the voltage difference between two points

W JouleVolt

q Coulomb

= =

Definition of Electric Voltage

V lt E l


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Voltage Example

Any measure of potential must include a reference point


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Electrical Power

Electrical power is a measure of the electrical work, or energy used, per

unit time, the following relationship is known as Watts Law

Instantaneous Power

The instantaneous power, of an electrical device is defined as

the work that is done per unit of time. In terms of the voltage, V,

and current, I ,

EXAMPLE 4.3

WattSecond

JouleIVP ===


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DC Signal

Has a constant voltage and current, neglecting the changes

occurring during power on and power off.

Common sources

Batteries

DC power supply

constant

constant

V

I

=

=t

C S


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AC Signal

Alternating Current

Alternating Current is a fluctuating current that is associated with a

changing potential difference (AC Voltage). The most common

alternating current pattern is associated with a sinusoidal change involtage.

Common Sources

Household power

Signal generator

Peak, peak-to-peak

RMS

EXAMPLE 4.4

( ) sin(2 )v t A ft =

t


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Resistance

Physically resistance is a measure of a materialsopposition to charge flow or current.

Resistance is measured in units called Ohms

The higher the resistance of a material, the more potential

difference is required to maintain a current.

The resistance of a material is temperature dependant.


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Figure 4.6. This water pipe illustrates the concept of resistance. The smaller center section of the pipe

has a larger resistance to water flow.

Resistive MaterialResistive Material

Figure 4.7 The electrical equivalent of the water-based example given in figure 4.6. A resistive material connected to

two conduct ing copper wires.


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Type Characteristics

Carbon Least expensive, wide available range of values

and tolerances, typically used for low power and

low frequency applications

Metal Film Used in higher voltage applications and where

high precision is called for. These devices exhibit

internal capacitance, due to the metal film

deposits, which can cause changes in the device

impedance at higher frequencies.

Wirewound Used for medium to high voltage applications

requiring high power handling. However due to

their geometry they exhibit high inductive

properties, making them suitable only for lower

frequency applications.

Numeric 1 Numeric 2 Numeric 3 Multiplier ToleranceColor


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Numeric 1 Numeric 2 Numeric 3 Multiplier Tolerance

Black 0 0 0 1

Brown 1 1 1 10 1%

Red 2 2 2 100 2%

Orange 3 3 3 1KYellow 4 4 4 10K

Green 5 5 5 100K 0.5%

Blue 6 6 6 1M 0.25%

Violet 7 7 7 10M 0.10

Gray 8 8 8 0.05%White 9 9 9

Gold 0.1 5%

Silver 0.01 10%

Color

Four band Resistor

The first two bands indicate numericvalues.

The third band is the multiplier.

The fourth band is the tolerance.

Example:

[Red2][Green5][Yellow10k][Silver10%]

Resistor value = 25 x 10k = 250 kOhm 10%

Five band Resistor (High precision)

The first three bands are numerical

values.

The fourth band is the multiplier.

The fifth band is the tolerance.

Example:

[Blue6][Gray8][Red2][Blue1M][Brown1%]

Resistor value = 682 x 1M = 682 MOhm 1%


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Capacitance

Capacitance

Capacitance, simply stated is the amount of charge that a

capacitor is capable of holding per unit of voltage applied.

Where Q is the net charge, V is the voltage, and C is the

capacitance. Units are Farads (F).

V

QC =

Capacitoris a device capable of storing energy in an electrical field

Energy Stored by a capacitor:

Units of Joules

21

2U CV=


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Example of a parallel plate capacitor

+ -

Plate

Separation

distance, d

+ -

Plate

Separation

distance, d

Figure 4.8. A conceptual example of a basic

capacitor. A capacitor consists of two metal

plates separated by an insulating d ielectric

material.

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

PlateSeparation

Distance, d

Electric Field Lines

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

PlateSeparation

Distance, d

Electric Field Lines

Figure 4.9. An illust ration of the electric field

lines existing between the plates of a capacitor.

The energy that is stored in a capacitor is stored

in this electric field.

d

xAC

)F/m1085.8( 12=


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Type CharacteristicsPaper Cheap, low to high voltage, low frequency, low

capacitance/volume, non-precision general

purpose applications

Mica Very stable, high precision, good for tunedcircuit applications, high capacitance/volume,

low leakage current

Tantalum or Aluminum Polarized, largest capacitance/volume, high

loss, commonly used for power supply filtering.

Ceramic High voltage, available in both low loss and high

loss, Capacitor tolerance can run from +100%

to -20%

Codes see: http://wiki.xtronics.com/index.php/Capacitor_Codes
http://wiki.xtronics.com/index.php/Capacitor_Codeshttp://wiki.xtronics.com/index.php/Capacitor_Codes


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There exist a nice relationship that ties together the quantities

Voltage, Current, and Resistance known as Ohms Law

Ohms Law

Ohms Law gives a relationship between a materialsresistance, R, the voltage across it, V, and the current flowing

through it, I.

IRV=

Ohms law gives us an equation we can use to find voltage, current, orresistance if we know two of the quantities. MEMORIZE IT

EXAMPLE 4.5

CHECK 4.5


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What You should know

1. Basic SI units

2. How charges behave (repel, attract)

3. Definition of current

4. Definition of voltage

5. Definition of resistance6. Ohms law

7. Watts law

8. Difference between a DC and AC signal9. How to find peak, peak-to-peak, and RMS voltage

values

Electrical Engineering Definitions

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