1.20 Glossary

B field (Section 1.13): The B field is the field of magnetic induction. The unit of B field intensity at a point is the tesla. The B field is a vector field, as it has intensity and direction at every point in space. The B field does not change intensity across a boundary when the permeability changes value. When the B field is represented by lines, these lines always form closed loops. The B and H fields are related by permeability B = μ₀μ_R · H, where μ₀ is the permeability of free space or 4π10⁻⁷ and μ_R is the relative permeability.

Capacitance (Section 1.7): The ability of a physical space to store electric field energy. In circuit applications it is the ability of a conductor geometry to store electric field energy (capacitor). If the electric field energy is not contained by conductors, it must be moving at the speed of light. Examples of fields in motion are transmission lines or antennas. Space has a capacitance per unit volume.

Capacitor (Section 1.7): A circuit component designed to store electric field energy.

Charge (Section 1.4): A quantity of electrons (negative charge). The absence of electrons (positive charge). The unit of charge is the coulomb. A coulomb of charge flowing past a point in one second is an ampere.

Charged (Section 1.4): A mass that has an excess or a lack of electrons on its surface.

Current (Section 1.6): The flow of electrons or ions. Electrons can flow in a conductor. Ions can travel in space or in a chemical solution.

D field (Section 1.10): The electric field representation that originates and stops on charges and is independent of the dielectric constant. The D field is continuous between charges located on the surface of separate conductors. The D field and the electric field are related by the dielectric constant D = ε ₀ ε _RE, where ε ₀ is the permittivity of free space and ε _R is the relative permeability. See the definition of permittivity.

Displacement current (Section 1.10): When an electric field changes intensity, it is equivalent to a displacement current in space. It is not electron flow. As an example, this current flows in the space between the plates of a capacitor when the voltage across the capacitor changes. A displacement current has an associated magnetic field.

Efield (Section 1.5): The force field that exists around all electric charges. When the charges are located on physical masses, the forces appear to act on the masses. Charges that create an electric field may be located in space, on the surface of conductors, or trapped inside an insulator (dielectric). An E field can exist without the presence of fixed charges when it is moving at the speed of light. As an example, the E field that leaves an antenna or flows down a transmission line. The E field intensity at a point in space is measured in units of volts per meter. The E field has intensity and direction at every point in space. It is a vector field.

Fall time (Section 1.14): The length of time it takes for a signal to go from 80% to 20 % of its initial value. Fall time applies to step changes in voltage, current, or field intensity.

Faraday's law (Section 1.16): A voltage is induced in any loop by the changing magnetic flux crossing that loop.

Field energy (Section 1.13): For a region where the E field is constant, the energy stored is 1/2 V/ ε ₀, where V is the volume of space. In a region where the H field is constant, the energy stored is to 1/V/μ₀, where V is the volume of space. See Poynting's vector.

Fourier spectrum (Section 1.14): The harmonic content that makes up any repetitive wave form. A single event has a spectrum that exists at all frequencies. There is no energy at any one frequency. For a single event, it is practical to discuss the voltage in a band of frequencies.

Ground (Section 1.5): A conducting surface that is large compared to the components or conductors located nearby. A conducting surface designated as the reference or zero of potential.

Hfield (Section 1.13): The magnetic field around a element current. The force of a magnetic field is against a second magnetic field. When two conductors carry current, the forces are felt on the conductors. If the H field is not confined by conductors, it moves at the speed of light. Examples are transmission lines and antennas. The H field has units of amperes per meter. It is a vector field as it has intensity and direction at every point in space. The H field is discontinuous at transitions in permeability. Around a current carrying conductor, the value of H is given by Ampere's law as I/2πr, where r is the distance from the conductor.

Inductance (Section 1.13): The ability of a physical space to store magnetic field energy. Inductance has units of henries. The inductance of a conductor geometry is the ratio of total magnetic flux generated per unit current. In circuit applications, it is the ability of a conductor geometry to store magnetic field energy. If this field energy is not sustained by conductors, it must be moving at the speed of light. Examples are transmission lines and antennas. Space has an inductance per unit volume.

Inductor (Section 1.13): A conductor geometry designed to store magnetic field energy for the flow of current.

Lenz's law (Section 1.13): The flux generated by an induced current is always opposite in direction to the flux generating the current.

Magnetic field energy (in an inductor) (Section 1.13): Field energy can be introduced into an inductor by accelerating charges. It takes work to move a charge into an existing current path that creates the magnetic field. The total work is 1/2LI².

Mutual capacitance (Section 1.8): The ratio of charge induced on a second conductor by a voltage on a first conductor when all other conductors are at zero potential.

Mutual inductance (Section 1.17): The magnetic flux coupled to a second conductor loop from a current flowing in a first conductor loop. The resulting loop voltage can be of either polarity.

Permeability of free space: The ratio between the B and H fields in free space. If B is in tesla and H is in amperes per meter then B = μ₀H, where μ₀ = 4π × 10⁻⁷.

Permittivity of free space (Section 1.10): The ratio between the D field and E field in free space. D/E = ε ₀ = 8.854 × 10⁻¹² F/m.

Poynting's vector (Section 1.18): The vector cross product of E and H at a point in space. It represents the power crossing per unit of area and the vector points in the direction of power flow at this point in space.

Rise time (Section 1.14): The length of time a signal takes to go from 20% to 80% of its final value. Applies usually to step functions of voltage.

Skin effect (Section 1.8): The flow of current on the surface of conductors at high frequencies. The magnetic field from a changing current keeps that very current from entering the conductor.

Self-capacitance (Section 1.7): The ratio of charge on a conductor to voltage on that same conductor with all other conductors at zero volts. The capacitance C of a capacitor.

Self-inductance (Section 1.13): The ratio of magnetic flux generated by a circuit to current in that same circuit. Usually given the symbol L. The voltage across an inductor is L dI/dt. This is usually the definition of inductance as flux cannot be measured directly.

Tesla (Section 1.13): The unit of magnetic field intensity for the B field.

Transmission line (Section 1.1): Any two parallel conductors, one of which can be a conducting plane.

Voltage (Section 1.5): (Properly a voltage is a potential difference) The work required to move a unit electrical charge between two points. In most circuit applications, these end points are on conductors and the work is done in the space between the conductors. Work is the product of force and distance when the force is in the direction of motion. The force that does work on the unit electrical charge can only result from the two electric fields. A voltage difference can exist between any two points in space and between points on conductors.

Notes

¹ A more thorough but elementary treatment of these fields is given in the author's book “Grounding and Shielding—Circuits and Interference” 5th Edition, John Wiley, publisher.

² Published skin depth equations usually relate to plane electromagnetic sine waves impinging on an infinite conducting plane. The depth that is calculated by this method is generally used as an approximation for other conductor geometries. As an example, it provides a rough measure of the current penetration in a round conductor, a trace or in a conducting plane.