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Fundamental Quantities

In the study of electric circuits, we deal with the fundamental phenomenon of the movement of electrically charged particles or simply charged particles. The fundamental quantities that are used to describe how rapidly charged particles move in a circuit and in what way they do so in the circuit are current and voltage.

Current  is sometimes referred to as the ``through'' quantity and voltage  as the ``across'' quantity. In the physical context, current is the flow of electric charge through a component or apparatus, whereas voltage is the potential difference between two points in a circuit. Current flows from high potential to low potential. In particular we define the current, I, flowing in a component or apparatus as the amount of charge passing through that component or apparatus per unit of time. Denoting charge by q, we may write current I as
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Two other important quantities that are frequently used in describing physical systems are power and energy. If a small quantity of electric charge tex2html_wrap_inline3093 is displaced from a point A to a point B, then the change in its potential energy  (or work done) is equal to tex2html_wrap_inline3095, where V denotes the voltage between A and B. In order to measure how rapidly work is done, we consider the amount of work done per unit of time. This quantity is called power, and is usually denoted by P .
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The unit of current is the ampere (A), that of voltage is the volt (V), that of energy is the joule (J), and that of power is the watt (W). Prefixes are often used to emphasize the significant figures when the magnitudes are too large or too small. Common prefixes and their corresponding multipliers are shown in Table 1.1. For example, 1ns denotes 1tex2html_wrap_inline310110tex2html_wrap_inline3103s, and 6kV denotes 6tex2html_wrap_inline310110tex2html_wrap_inline3107V. 

 table34
Table 1.1: Prefixes of units 


next up previous
Next: Direction and Polarity Up: BASIC CIRCUIT CONCEPTS Previous: BASIC CIRCUIT CONCEPTS

Michael Tse
Tue Mar 10 13:15:28 HKT 1998