Electromagnetism

Electromagnetism is the physics of the electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles.

 

The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets.

 

While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted developed an experiment which provided evidence that surprised him. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.

 

At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.

 

His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.

 

Ørsted was not the first person to discover that electricity and magnetism are related. He was preceded in this discovery by 18 years by Gian Domenico Romagnosi, an Italian legal scholar. An account of Romagnosi's discovery was published in 1802 in an Italian newspaper, but it was overlooked by the scientific community.

 

A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, the basis of operation for electrical generators, induction motors, and transformers). Similarly, a changing electric field generates a magnetic field. Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity—the electromagnetic field.

 

This unification, which was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside, is one of the triumphs of 19th century physics. It had far-reaching consequences, one of which was the understanding of the nature of light. As it turns out, what is thought of as "light" is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an electromagnetic wave. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.

 

The theoretical implications of electromagnetism led to the development of special relativity by Albert Einstein in 1905.

 

The electromagnetic force

The force that the electromagnetic field exerts on electrically charged particles, called the electromagnetic force, is one of the four fundamental forces. The other fundamental forces are the strong nuclear force (which holds atomic nuclei together), the weak nuclear force (which causes certain forms of radioactive decay), and the gravitational force. All other forces are ultimately derived from these fundamental forces.

 

As it turns out, the electromagnetic force is the one responsible for practically all the phenomena encountered in daily life, with the exception of gravity. Roughly speaking, all the forces involved in interactions between atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. It also includes all forms of chemical phenomena, which arise from interactions between electron orbitals.

 

According to quantum electrodynamics, electromagnetic force is the mathematical by-product of interaction of real charged particles with virtual photons. In 3-dimensional space such interaction (with spin-1 virtual particles) results in inverse square law.

 

Units

Electromagnetic units are part of a system of electrical units based primarily upon the magnetic properties of electric currents, the fundamental cgs unit being the abampere. The units are:

• abampere (current)
  
• abcoulomb (charge)
  
• abfarad (capacitance)
  
• abhenry (inductance)
  
• abohm (resistance)
  
• abvolt (electric potential)
  
• abwatt (power)
  

In the electromagnetic cgs system, electrical current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.

SI electromagnetism units
Symbol	Name of Quantity	Derived Units	Unit	Base Units
I	Magnitude of current	ampere (SI base unit)	A	A = W/V = C/s
q	Electric charge, Quantity of electricity	coulomb	C	A·s
V	Potential difference or Electromotive force	volt	V	J/C = kg·m2·s−3·A−1
R, Z, X	Resistance, Impedance, Reactance	ohm	Ω	V/A = kg·m2·s−3·A−2
ρ	Resistivity	ohm metre	Ω·m	kg·m3·s−3·A−2
P	Power, Electrical	watt	W	V·A = kg·m2·s−3
C	Capacitance	farad	F	C/V = kg−1·m−2·A2·s4
	Elastance	reciprocal farad	F−1	V/C = kg·m2·A−2·s−4
ε	Permittivity	farad per metre	F/m	kg−1·m−3·A2·s4
χe	Electric susceptibility	(dimensionless)	-	-
G, Y, B	Conductance, Admittance, Susceptance	siemens	S	Ω−1 = kg−1·m−2·s3·A2
σ	Conductivity	siemens per metre	S/m	kg−1·m−3·s3·A2
B	Magnetic flux density, Magnetic induction	tesla	T	Wb/m2 = kg·s−2·A−1 = N·A−1·m−1
Φm	Magnetic flux	weber	Wb	V·s = kg·m2·s−2·A−1
H	Magnetic field strength,Magnetic field intensity	ampere per metre	A/m	A·m−1
	Reluctance	ampere-turn per weber	A/Wb	kg−1·m−2·s2·A2
L	Inductance	henry	H	Wb/A = V·s/A = kg·m2·s−2·A−2
μ	Permeability	henry per metre	H/m	kg·m·s−2·A−2
χm	Magnetic susceptibility	(dimensionless)
Π and Π *	Electric and Magnetic hertzian vector potentials	n/a	n/a