When electrical energy moves, both an *E* field and an *H* field are required. The power density at a point in space is given by the vector cross product of the *E*- and *H* field intensities. This vector *P* in Equation 1.10 is called *Poynting's vector*. This vector exists at

all points in space and points in the direction of energy flow. For a pair of conductors carrying power from a battery to a load, the *E* field crosses between the conductors and the *H* field circles the current carrying conductors (Fig. 1.3).

Note that the *E* and *H* fields have directions perpendicular to the current path and perpendicular to the direction of power flow. The *E* field has units of volts per meter and the *H* field has units of amperes per meter, so the product has units of watts per meter squared.

N.B.

The energy supplied to a load is carried in the space between the connecting conductors and not in the conductors. This concept applies at all frequencies including dc. The *P* vector is greatest at points near the conductors.

It is important to view every pair of conductors as a transmission line. Fields carry the energy in the space between conductors at all frequencies. Fields that leave the circuit represent radiated energy. In this case, a component of Poynting's vector points in the direction of radiation. When there is heat loss, then a component of Poynting's vector points ...

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