The electric field (E field) around a conductor naturally extends to nearby conductors and even far into space. To limit the E field, especially in practical applications, changes in geometry can be employed. The figure illustrates a grounded conductor, known as a shield, surrounding the central conductor. The E field lines leave the center conductor and terminate on the inside surface of the shield, effectively confining the E field to the space between the two conductors. This shielding geometry prevents an E field or charge from existing on the outside of the shield. If an external field were present, it would terminate on the outer walls of the shield, inducing a charge distribution on the external surface.
The shield, along with its central conductor, constitutes a shielded cable. When a voltage is applied between the shield and the center conductor, the E field is contained within the cable. While grounding the shield is common in most applications, it is not a strict requirement for effective shielding.
Shielding extends beyond cables; conducting boxes can also serve as shields for circuits. When a box surrounds a circuit, the E field activity is confined to the box, preventing external E fields from entering. Although achieving a perfectly sealed box is impractical due to ventilation and lead requirements, shielding remains a crucial tool in electronic design. Various techniques address the imperfections of shields or boxes, especially in the presence of rapidly changing signals, a topic explored in later chapters.
In the context of typical shielded cables, the shield often comprises a conducting braid made of small tinned wires to ensure flexibility and electrical connection at the ends. Despite the shielding, some internal E field may leak out through the cable, terminating on induced charges on external conductors. This leakage is described as mutual capacitance, and when the E field varies, induced charges change, implying current flow. This current flow can act as a source of a signal in an external circuit, and the signal in the cable is considered coupled to an external conductor through mutual capacitance, also known as leakage capacitance. In most cases, leakage capacitances are on the order of a few picofarads per foot of cable.