Direct Current Density

Current is known as the flow of charge. By inheritance, the direction of this flow is with the movement of positive charge. The amount of charge δQ flowing through (that is, perpendicular) to a surface, in time δt is given by the formula

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Passive networks and reciprocity theorem

The ABCD parameters for a 2-port network are:

V1 = AV2 −BI2                   (28)

I1 = CV2 −DI2                    (29)

Figure shown below shows a two-port network whose terminals RS are short-circuited with an input voltage V across terminals PQ. Then V2 in the above equations is 0 and the equations become:

V =−BI2                    (31)

and    I1 =−DI2                  (32)

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Magneto motive force and magnetic field strength

The Magneto-motive force (mmf) is the cause due to the presence of the a magnetic flux in a magnetic circuit,

mmf,Fm=NI amperes

where N is used to represent the number of conductors (or turns) and I represents the current in amperes. We sometimes express the unit of mmf as ‘ampere-turns’. As we know, since ‘turns’ have no dimensions, the SI unit of mmf is the ampere. Magnetic field strength (or magnetizing force),

H= NI/l ampere per metre

where l stands for the mean length of the flux path in metres.

Thus mmf =NI= Hl amperes.

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