Electronics is the understanding, development, and application of devices that use the flow of electrons to store and manipulate data pertaining to images, sound, and other forms of information. On the theoretical level, electronics draws on knowledge developed in many other scientific disciplines, such as chemistry, condensed-matter physics, mathematics, quantum theory, and thermodynamics. As an applied science, electronics grew out of electrical engineering, which is concerned with every aspect of electricity: its generation, control, distribution, and applications.
Electrical and electronic circuits
Electrical circuits are designed for many purposes. They consist of components—typically capacitors, inductors, potentiometers, resistors, and switches—linked together by conductors, such as metal wires. Each component has specific electrical characteristics—resistance, capacitance, and inductance—that contribute to the characteristics of the circuit and govern its electrical behavior.
Electronic circuits are electrical circuits that include electronic components, such as vacuum tubes and semiconductor devices. The electrical characteristics of such components are more complex than those of electrical components. The resistance of a diode, for example, depends on the direction of current flow, being extremely high in one direction but negligible in the other.
Transducers and applications
In general, transducers are devices that convert energy from one form into another; in the context of electronics, one form of energy is electrical. As such, transducers form the interfaces between electronic circuits and their environments. The electrical energy can be the input or the output of the transducer. A microphone, for example, is a transducer that converts sound energy into electrical energy in the form of a signal that can be manipulated by an electronic circuit. A loudspeaker, on the other hand, is a transducer that converts the electrical output of an amplifier circuit into sound energy. Other types of transducers produce electrical signals in response to stimuli such as heat, light, or pressure or convert electrical signals into light or motion, for example.
The expansion of electronics at the cost of mechanical devices has been intimately linked to the development of transducers for specific tasks. Just as the British inventor John Logie Baird’s mechanical image-scanning system for television was rapidly superseded by an electronic scanning system developed by EMI, so the development of compact analog and digital displays enabled electronic devices to dominate the wristwatch market.
Vacuum-tube diodes as rectifiers
A simple vacuum tube consists of two electrodes sealed within an evacuated glass tube. One electrode—the cathode—is constructed from an alkali metal, such as cesium. When such metals become hot, some of their electrons "boil off"—they leave the surface of the metal. This is known as the thermionic effect, and vacuum tubes are sometimes called thermionic valves. If the other electrode—the anode—is at a positive potential relative to the cathode, thermionic electrons are drawn from the cathode to the anode and an electrical current flows through the device. There are no thermionic electrons available around the anode. Consequently, if the potential difference is reversed, no current flows through the device.
This type of vacuum tube is called a diode because it has only two electrodes. The ability of such devices to conduct electricity in one direction but not the other makes them useful for rectifying alternating current. That is, they produce a direct current from an alternating current. This action is similar to that of a nonreturn valve in a hydraulic system, which is why vacuum-tube diodes are called valves in some countries.
Because of the unique direction of electron flow through a diode, the cathode is sometimes called the emitter and the anode the collector. When the potential difference between the electrodes is favorable for current flow, the diode is in forward bias. If the cathode is positive relative to the anode, the diode is in reverse bias.
