1.3 Electromagnetics — Universal constants and SI units

Image by Gerd Altmann from Pixabay

Units in Measurements: All physical quantities need numbers and units like meters for length, kilograms for mass, seconds for time, and amperes for current.

Basic Units: Mechanics uses length, mass, and time as fundamental units; electromagnetics adds current. The SI system (International System of Units) is built on these fundamental units.

Derived Units: Other units, like coulombs for charge or Teslas for magnetic flux density, are derived from combinations of these fundamental units.

SI Definitions for Units: The meter is defined by the speed of light, the kilogram by a specific alloy bar, and the second by the cesium atom’s radiation. The ampere is defined by a specific current in ideal conductors.

Universal Constants in Electromagnetics:

Speed of light in free space c = 3 ×10⁸ meters per second.

Permittivity of free space (ε₀) = 8.854 × 10^−12 farads per meter. Permeability of free space (μ₀) = 4π×10^−7 Henries per meter.

Relationships between Field Quantities in Free Space:

Electric flux density (D) is related to electric field intensity (E) by

​D = ε₀E

Magnetic flux density (B) is related to magnetic field intensity (H) by

B = μ₀H ​

Remember:

SI units are crucial for measuring physical quantities. SI definitions set standards for units like meter, kilogram, second, and ampere. Constants like speed of light, permittivity, and permeability in free space are essential in electromagnetics.