Applied electronics is a term that requires a bit of definition before delving into the theoretical side of electronic theory. Generally, the term refers to the physical phenomena involved in electronic conduction or current flow. It also refers to various ways in which these phenomena and the properties of resistance, inductance, and capacitance interact to govern the characteristics, ratings, and limitations of electronic devices. The term is useful in describing the application of electronics theory to the various branches of electrical and electronic engineering. Electron theory, on the other hand, is limited to constructing a viable working model representing our understanding of how electronic conduction occurs based upon careful observation and experimentation.
Electronic conduction (current) is the result of the flow of electrons in a conductive material, such as a copper wire. Metals such as copper, lead, tin and aluminum are the most familiar conductors of electricity. There are two components required to produce electron flow: a source of electromotive energy creating a difference of electronic potential between two points, and a conductive path. A simple example of this is the lighting bolt. Incredibly powerful discharges of electricity strike the earth some 100 times each second, or 8 million times a day. (http://www.nssl.noaa.gov/primer/lightning/ltg_clima tology.html)
A lightning bolt is the result of a huge buildup of electromotive energy, or force between earth and clouds with strongly circulating winds and high concentrations of water vapor. The circulating water vapor in the form of rain, snow, or hail pellets accumulates a significant electro-static charge within the clouds vaporous structure. At some point, the difference of potential between the cloud and the earth becomes large enough to exceed the dielectric breakdown point of the atmosphere. The result is an exchange of energy between the earth and the clouds, which we see as a flash of lightning. An average bolt of lightning carries a negative electric current of 40 kiloamperes (kA) (although some bolts can be up to 120 kA), and transfers a charge of five coulombs and 500 MJ, or enough energy to power a 100 watt lightbulb for just under two months. (http://en.wikipedia.org/wiki/Lightning)
Electronic devices, with the exception of some high-powered radar devices, operate at far less voltage and current levels. Common electronic circuits with low current flow demand use low voltage 1.5 to 9 Volt DC batteries as the primary source of electromotive force (EMF). Household devices that require greater amounts of current flow plug into a 120 Volt AC line. These devices contain an internal electronic power supply circuit that rectifies and regulates the AC voltage into the appropriate DC voltage required. Larger batteries, typically in the range of 12 to 28 Volts are common in applications where higher voltages and currents are required.
Electronic current flow results from the application of an electromotive force, or difference of potential, to a conductive material. The application has to be "closed" in that both the negative and the positive sources of potential difference must be applied. Electrons in the conductor will flow from negative to positive because of the displacement of negatively charged electrons in the outer rings of the electron shell of the material. These electrons flow away from the negatively charged source and towards the positively charged source.
Theoretically, electron flow is a natural phenomenon of the equalization of the electronic difference of potential between the negative and positive charges presented by the battery or power supply. Whatever the true state of electrons, either negatively charged particles, waves, or packets of energy, or all the above, is irrelevant to basic electron theory. Electron flow is observable and measurable by the use of the Ammeter. A Voltmeter is a device used to measure the electromotive force creating the difference of potential between two points.
Although metals are good conductors of electricity, copper is the most common conductor used in the construction of electronic circuits. It is plentiful, easily formed into strings of wire, lightweight, inexpensive and easily connected to other wires by soldering. Solder is a combination of lead and tin, which melts at a relatively low temperature and bonds the three metals together. Solder also conducts electricity, making it invaluable in the construction of circuits and electronics components. Gold and silver are also good conductors, but are too expensive to be practical, although there are some applications where silver and gold are useful in the manufacture of corrosion-resistant connections.
In solid-state devices, silicon serves as a substrate conductor carefully infused, or doped, with metallic elements such as arsenic or germanium. Combinations of silicon and arsenic produce a layer with the characteristics of having an extra electron. Conversely, a combination of silicon and germanium produces a layer exhibiting the properties of the lack of an electron. Multi-layered devices can have either negatively or a positively doped control layers, or "bases". Electronic engineers and technicians refer to these as "NPN" or PNP" -type devices. Diodes and transistors are the most common examples of these devices.
The interaction of electron flow and varying resistance at the boundary layer results in the property of amplification through the control of current flow. Transistors and Integrated Circuits, which are unified assemblies of hundreds and sometimes thousands of micro-electronic devices, commonly referred to as "Active" devices. "Passive" devices are the building blocks of electronic circuits, coming in the form of resistors, capacitors and inductors. Vacuum tubes, now considered obsolete, are voltage-controlled devices, using a variation in the difference of potential between two metallic plates to achieve amplification by controlling the level of current flow in the output side of the circuit.
A basic electronic circuit consists of various active and passive components that perform different functions. A resistor opposes the flow of electricity, creates a "voltage drop", and dissipates energy in the form of heat. A capacitor stores energy electro-statically between two plates separated by a non-conducting material, or insulator. An inductor, or coil, stores energy in the form of an electro-magnetic field. These devices in many different combinations and forms are used in the construction of the millions of electronic circuits that give us all the modern conveniences we enjoy today.