ELECTRICAL CHARGE, CURRENT, AND POTENTIAL

Static electricity involves bodies that are positively or negatively charged. When two bodies are rubbed together, the electrons may be removed from one (giving a positively charged body) and transferred to 

The time rate of change of electrical charge through a conductor is called current (I) and the unit of current is the ampere (A). One ampere corresponds to the flow of one coulomb of charge per second. A stroke of lightening involves the flow of a very high current for a very short time (-1,000 A for about 10 sec). The potential difference responsible for current flow in a conductor is measured in volts (V). A positive flow of current corresponds to flow from a plus to a minus terminal. 

A semiconductor is a material having electrical conductivity between a conductor and a nonconductor. Silicon, which is in the fourth column of the periodic table (valence = 4), is normally a nonconductor. It may be converted to a semiconductor by diffusing a small amount ( 1 part in 10 ) of boron (valence = 3) or phosphorous (valence 5) throughout its structure. This is called doping. When boron is the dopant, it is called a positive type (p-type) semiconductor but when phosphorous is the dopant it is called an n-type semiconductor. When a potential difference (voltage) is applied across an n-type semiconductor, the unattached electrons where phosphorous atoms are located move toward the positive terminal. When a boron atom is in a p-type semiconductor, there is an unfilled bond site called a hole. Holes tend to act as positively charged particles and move toward the negative terminal when a potential difference is applied. 

A liquid that conducts current is called an electrolyte while one that does not is called a dielectric. When a crystal such as copper sulfate (CUSO4) dissolves in water, it yields Cu++ and SO ions. When a potential difference is applied to immersed electrodes, positive ions migrate to the 

The flow of current in a conductor is analogous to the flow of fluid in a pipe where the current is equivalent to the rate of fluid flow, and the potential difference is equivalent to pressure drop along the pipe. Just as work is done when lifting a weight in mechanics, work dW) is done when voltage (V) causes a displacement of a charge (dq). 

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