Circuit Elements

Electrical Circuit Analysis > Electric Circuit

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An electrical circuit is an interconnection of electrical circuit elements. These circuit elements can be categorized into two types, namely active circuit elements and passive circuit elements.

Some Definitions/explanations of electrical terms

Charge

(unit: coulomb, C; letter symbol: q or Q )
The electric charge is the most basic quantity in electrical engineering, and arises from the atomic particles of which matter is made.

(unit: volt, V; letter symbol: v or V )
The potential difference, also known as voltage, is the work done (or energy required) to move a unit positive charge from one point to another (across a circuit element). Thus the change in work done $dw$ when a charge $dq$ moves through a potential difference of v

$$dw = v.dq$$

Current

(unit: ampere, A; letter symbol: i or I )
The electric current is the rate of charge flow in a circuit.

$$i = {dq \over dt} \,\quad q = âˆ«i dt$$

Energy

(unit: joule, J; letter symbol: w or W )
The Energy is the capacity to do work. Thus in electrical quantities this may be expressed as

$$âˆ« dw = âˆ« v.dq = âˆ« v.i.dt$$

Power

(unit: watt, W; letter symbol: p or P ) The electric power is the rate of change of energy.

$$p = (v)(i) = ({dw \over dq})({dq \over dt})$$

$$p = {dw \over dt}$$

Common usage of letter symbols

It is common practice to use the simple letters (such as $v$, $i$, $p$, $w$) to represent quantities which are varying with time, and capital letters (such as $V$, $I$, $P$, $W$) to represent quantities which are constants. But this need not always be done and is a useful practice rather than a rule. With representation of elements, obviously this practice does not exist as they are not time variables.

The letter $j$ is normally used for the imaginary operator $âˆš-1$ as $i$ is almost invariably used to denote current.

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