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Definition of electronics: Electronics is the branch of science that deals with the study of flow and control of electrons (electricity) and the study of their behavior and effects in vacuums, gases, and semiconductors, and with devices using such electrons. This control of electrons is accomplished by devices that resist, carry, select, steer, switch, store, manipulate, and exploit the electron.

Some of the basic electrical units and definitions are mentioned below:

Passive: Capable of operating without an external power source. Typical passive components are resistors, capacitors, inductors and diodes (although the latter are a special case).

Active: Requiring a source of power to operate.    Includes transistors (all types), integrated circuits (all types), TRIACs, SCRs, LEDs, etc.

DC:
Direct Current. The electrons flow in one direction only.  Current flow is from negative to positive, although it is often more convenient to think of it as from positive to negative.  This is sometimes referred to as "conventional" current as opposed to electron flow.


AC: Alternating Current. The electrons flow in both directions in a cyclic manner - first one way, then the other.  The rate of change of direction determines the frequency, measured in Hertz (cycles per second).

Frequency: Unit is Hertz, Symbol is Hz, old symbol was cps (cycles per second). A complete cycle is completed when the AC signal has gone from zero volts to one extreme, back through zero volts to the opposite extreme, and returned to zero.  The accepted audio range is from 20Hz to 20,000Hz.  The number of times the signal completes a complete cycle in one second is the frequency.

Voltage: Unit is Volts, Symbol is V or U, old symbol was E . Voltage is the "pressure" of electricity, or "electromotive force" (hence the old term E).  A 9V battery has a voltage of 9V DC, and may be positive or negative depending on the terminal that is used as the reference.  The mains has a voltage of 220, 240 or 110V depending where you live - this is AC, and alternates between positive and negative values.  Voltage is also commonly measured in millivolts (mV), and 1,000 mV is 1V.  Microvolts (uV) and nanovolts (nV) are also used.

Current: Unit is Amperes (Amps), Symbol is I . Current is the flow of electricity (electrons).  No current flows between the terminals of a battery or other voltage supply unless a load is connected.  The magnitude of the current is determined by the available voltage, and the resistance (or impedance) of the load and the power source.  Current can be AC or DC, positive or negative, depending upon the reference.  For electronics, current may also be measured in mA (milliamps) - 1,000 mA is 1A.  Nanoamps (nA) are also used in some cases.

Resistance: Unit is Ohms, Symbol is R or Ω . Resistance is a measure of how easily (or with what difficulty) electrons will flow through the device.  Copper wire has a very low resistance, so a small voltage will allow a large current to flow.  Likewise, the plastic insulation has a very high resistance, and prevents current from flowing from one wire to those adjacent.  Resistors have a defined resistance, so the current can be calculated for any voltage.  Resistance in passive devices is always positive (i.e. > 0)

 
Learn More on Basics of Electronics:

Rules of Electrical Circuits: * A voltage of 1V across a resistance of 1 Ohm will cause a current flow of 1 Amp, and the resistor will dissipate 1 Watt (all as heat).

What is an electronic circuit? A circuit is a structure that directs and controls electric currents, presumably to perform some useful function. The very name "circuit" implies that the structure is closed, something like a loop.

Current: Charge is mobile and can flow freely in certain materials, called conductors. Metals and a few other elements and compounds are conductors. Materials that charge cannot flow through are called insulators. Air, glass, most plastics, and rubber are insulators, for example. And then there are some materials called semiconductors, that seemed to be good conductors sometimes but much less so other times. Silicon and germanium are two such materials. The flow of charge is called electrical current. Current is measured in amperes (a), amps for short (named after another French scientist who worked mostly with magnetic effects).

Wiring Symbols: There are many different representations for basic wiring symbols, and these are the most common.  The conventions I use for wires crossing and joining are marked with a star (*) - the others are a small sample of those in common use, but are fairly representative.  Many can be worked out from their position in the circuit diagram (schematic).

Voltage: Voltage is something is a type of "pressure" that drives electrical charges through a circuit.
Bodies with
opposite charges attract, they exert a force on each other pulling them together. The magnitude of the force is proportional to the product of the charge on each mass.

What is charge?  Charge may be defined as the quantity of unbalanced electricity in a body (either positive or negative) and construed as an excess or deficiency of electrons. Charge comes in two forms, positive (+) , and  negative charge ( - ) .

Batteries: Charges can be separated by several means to produce a voltage. A battery uses a chemical reaction to produce energy and separate opposite sign charges onto its two terminals. As the charge is drawn off by an external circuit, doing work and finally returning to the opposite terminal, more chemicals in the battery react to restore the charge difference and the voltage. The particular type of chemical reaction used determines the voltage of the battery, but for most commercial batteries the voltage is about 1.5 V per chemical section or cell.

Resistors: A Resistor is an electrical device that resists the flow of electrical current. It is a passive device used to control, or impede the flow of, electric current in an electric circuit by providing resistance, thereby developing a drop in voltage across the device. The value of a resistor is measured in ohms and represented by the Greek letter capital omega. Resistors usually have a brown cylindrical body with a wire lead on each end, and colored bands that indicate the value of the resistor.

Ohm’s Law: Ohm's law describes the relationship between voltage, V , which is trying to force charge to flow, resistance, R , which is resisting that flow, and the actual resulting current I .

Power: Power is the Electric energy produced per unit time.

Capacitors: In simple words, we can say that a capacitor is a device used to store and release electricity, usually as the result of a chemical action. Also referred to as a storage cell, a secondary cell, a condenser or an accumulator. A Leyden Jar was an early example of a capacitor.

Inductors: An inductor is an electrical device (typically a conducting coil) that introduces inductance into a circuit. An inductor is a passive electrical component designed to provide inductance in a circuit. It is basically a coil of wire wrapped around an iron core. simplest form an inductor is made up of a coil of wire. The inductance measured in henrys, is proportional to the number of turns of wire, the wire loop diameter and the material or core the wire is wound around.

Semiconductor devices: A conductor made with semiconducting material. Semiconductors are made up of a substance with electrical properties intermediate between a good conductor and a good insulator. A semiconductor device conducts electricity poorly at room temperature, but has increasing conductivity at higher temperatures. Metalloids are usually good semiconductors.

Silicon: Silicon, atomic number 14 on the periodic table, is a semiconducting material from which integrated circuits (computer chips of all types--processors, memory chips, etc.; CCDs; transistors; etc.) are created.

Silicon is one of the most common elements. Silicon is also the semiconductor material out of which almost all modern transistors are made.

Diodes: A Diode is an electronic device that allows current to flow in one direction only. It is a semiconductor that consists of a p-n junction. They are used most commonly to convert AC to DC, because they pass the positive part of the wave, and block the negative part of the AC signal, or, if they are reversed, they pass only the negative part and not the positive part.

Electronic Component name abbreviations: Here is a list of Electronic Component name abbreviations widely used in the electronics industry.

What is Energy Star?

Home Electrical Wiring Types and Rules

Surface Mount Device (SMD) or Surface Mount Electronic Components

Circuit Symbol / Circuit Schematic Symbols of Electronic Components

All about Semiconductor

Ball Grid Array (BGA) Package

SMD Surface Mount Electronic Components for SMT

How to Solder – Hand Soldering Tutorial

Printed Circuit Board: Design, Diagram and Assembly

Active and Passive Electronic Components

Electronic Components, Parts and Their Function

How Electronic / Electrical Circuit Works

How to Generate Electricity – How is Electricity Generated
 

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