Voltage Explained Simply: Meaning, Formula, and Examples

 Voltage

We must now begin to refer to a circuit element, which can be best defined in general terms to start with. Such electrical devices as fuses, light bulbs, resistors, batteries, capacitors, generators, and spark coils can be represented by combinations of simple circuit elements. We begin by showing a very general circuit element as a shapeless object possessing two terminals at which connections to other elements may be made (Fig. 2.8). There are two paths by which current may enter or leave the element.

 In subsequent discussion,s we will define particular circuit elements by describing the electrical characteristics that may be observed at their terminals. In Fig. 2.8, a dc current is sent into terminal A, through the general element, and back out of terminal B. Let us also assume that pushing a charge through the element requires an expenditure of energy. We then say that an electrical voltage (or a potential difference) exists between the two terminals, or that there is a voltage “across” the element. Thus, the voltage across a terminal pair is a measure of the work required to move charge through the element. The unit of voltage is the volt,4, and 1 volt is the same as 1 J/C. Voltage is represented by V or v. A voltage can exist between a pair of electrical terminals,s whether a current is flowing or not. An automobile battery, for example, has a voltage of 12 V across its terminals even if nothing whatsoever is connected to the terminals. According to the principle of conservation of energy, the energy that is expended in forcing charge through the element must appear somewhere else. When we later meet specific circuit elements, we will note whether that energy is stored in some form that is readily available as electric energy or whether it changes irreversibly into heat, light, or some othenon-electricalal form of energy. We must now establish a convention by which we can distinguish between energy supplied to an element and energy that is supplied by the element itself. We do this by our choice of sign for the voltage of terminal A with respect to terminal B. If a positive current is entering terminal A of the element and an external source must expend energy to establish this current, then terminal A is positive with respect to terminal B. (Alternatively, we may say that terminal B is negative with respect to terminal A.) The sense of the voltage is indicated by a plus-minus pair of algebraic signs. In Fig. 2.9a, for example, the placement of the + sign at terminal A indicates that terminal A is v volts positive with respect to terminal B. If we later find that v happens to have a numerical value of −5 V, then we may say either that A is −5 V positive with respect to B or that B is 5 V positive with respect to A. Other cases are shown in Fig. 2.9b, c, and d. Just as we noted in our definition of current, it is essential to realize that the plus-minus pair of algebraic signs does not indicate the “actual” polarity of the voltage but is simply part of a convention that enables us to talk unambiguously about “the voltage across the terminal pair.” The definition of any voltage must include a plus-minus sign pair! Using a quantity v1(t) without specifying the location of the plus-minus sign pair is using an undefined term. Figure 2.10a and b do not serve as definitions of v1(t); Fig. 2.10c does.