The motion of electric charges constitutes an electric current, denoted by the letters i or I. As a matter of vocabulary, we say that a current flows along a path, from A to B, or through an element, as shown in Figure 7.1.

Note that a complete description of current requires both a value and a reference direction, as shown in Figure 7.1.

By definition, current is the time rate of change of charge, or

(7.1)

We consider the network elements to be electrically neutral. That is, no net charge can accumulate in the element. Charges may not accumulate or be depleted at any point. Any charge entering the element must be accompanied by an equal charge leaving the element.

7.1.2 Voltage

Charges in a conductor may move in a random manner. To move the charges in a conductor in a particular direction requires some work or energy transfer.

We define the voltage v_{AB} between two points A and B in an electric circuit as the energy (work) needed to move a unit of charge from A to B.

Mathematically,

(7.2)

As a matter of vocabulary, we say that a voltage exists across an element, or between two points ...

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