KAHIBARO
Table of Contents
Electric charge as the source of electromagnetic interaction
Electromagnetism is the fundamental interaction that acts between particles with electric charge. It explains why opposite charges attract, why like charges repel, why atoms hold electrons around nuclei, why light exists, and why electric and magnetic phenomena are connected.
In this interaction, the key property is electric charge. Particles such as the electron and proton carry charge, while some particles such as the neutron are electrically neutral overall. The electromagnetic interaction acts on charged particles and does not act directly on particles with zero electric charge.
A useful basic fact is that there are two kinds of electric charge, positive and negative. The proton has positive charge $+e$, and the electron has negative charge $-e$, where $e$ is the elementary charge:
$$
e \approx 1.602 \times 10^{-19}\ \text{C}
$$
The unit of charge is the coulomb, written $\text{C}$.
Opposite charges attract, and like charges repel.
The proton carries charge $+e$ and the electron carries charge $-e$.
Range and strength
Electromagnetism is a long range interaction. This means its effects can extend over very large distances. Unlike the strong interaction, it does not quickly disappear outside the nucleus. This long range behavior is possible because its mediator, the photon, has zero rest mass.
At the level of everyday matter, electromagnetism is much stronger than gravitation. For example, the electric repulsion between two electrons is enormously larger than their gravitational attraction. This is why electromagnetic effects dominate the structure of atoms, molecules, solids, liquids, and gases.
However, large objects are often electrically neutral overall, so their positive and negative charges nearly cancel. In that case, gravitational effects can become more noticeable on large scales.
Electromagnetism in particle physics
In particle physics, electromagnetism is one of the four fundamental interactions. It affects all charged elementary particles, including charged leptons and quarks. Since quarks are charged, electromagnetism contributes to hadron behavior as well, although the strong interaction is usually more important inside hadrons and nuclei.
Electromagnetism also controls many observable processes involving charged particles, such as scattering, radiation, and energy loss in matter. A charged particle moving through a detector often interacts electromagnetically with atoms in the material, producing ionization and excitation.
Electric and magnetic aspects
The name electromagnetism reflects the fact that electric and magnetic effects are two aspects of one unified interaction. A charge at rest produces an electric effect. A moving charge can also produce a magnetic effect. Similarly, changing electric and magnetic fields are linked to each other.
This unity becomes especially important in relativity. Observers in different states of motion may describe the same phenomenon with different mixtures of electric and magnetic fields, but the underlying interaction is the same.
Photon as the exchange particle
In modern particle physics, the electromagnetic interaction is described as being mediated by the photon. The photon is the exchange particle, or gauge boson, of electromagnetism.
The photon has these important properties:
| Property | Photon |
|---|---|
| Electric charge | $0$ |
| Rest mass | $0$ |
| Spin | $1$ |
| Interaction role | Mediates electromagnetism |
Because the photon has zero rest mass, the electromagnetic interaction has infinite range in principle.
In quantum language, charged particles interact by exchanging photons. Sometimes these photons are real, as in visible light or gamma rays. In many interaction diagrams, the exchanged photon is virtual, meaning it is an internal carrier of the interaction rather than a directly observed free particle.
Electromagnetism is mediated by the photon.
Because the photon is massless, the electromagnetic interaction is long range.
Coupling to charged particles
Any particle with electric charge couples to the electromagnetic field. The strength of this coupling depends on the particle's charge. For example, an electron and a proton have charges of equal magnitude but opposite sign, so they interact strongly with electromagnetic fields, but in opposite ways.
Quarks also couple electromagnetically because they carry fractional electric charge. For example:
| Quark | Charge |
|---|---|
| Up quark $u$ | $+\frac{2}{3}e$ |
| Down quark $d$ | $-\frac{1}{3}e$ |
| Strange quark $s$ | $-\frac{1}{3}e$ |
| Charm quark $c$ | $+\frac{2}{3}e$ |
| Bottom quark $b$ | $-\frac{1}{3}e$ |
| Top quark $t$ | $+\frac{2}{3}e$ |
This is why protons, which contain quarks $uud$, have total charge $+e$, while neutrons, which contain quarks $udd$, have total charge $0$.
Classical and quantum descriptions
Electromagnetism can be described at two levels. In classical physics, it appears as electric and magnetic fields acting continuously in space and time. In modern particle physics, it is described by quantum electrodynamics, usually called QED.
QED is the quantum theory of electromagnetism. It describes how charged particles and photons interact. It is one of the most successful theories in physics and gives extremely accurate predictions.
For beginners, it is enough to remember that the classical picture uses fields, while the quantum picture uses photon exchange. Both describe the same fundamental interaction in different ways.
A simple interaction picture
A basic electromagnetic interaction between two charged particles can be represented schematically by photon exchange.
In this sketch, $\gamma$ represents a photon. The diagram is not a literal path in space, but a symbolic way to represent an interaction process.
Conserved electric charge
A central rule in electromagnetic processes is conservation of electric charge. The total electric charge before an interaction must equal the total electric charge after the interaction.
For example, if an electron emits a photon, the electron remains negatively charged and the photon carries no charge, so total charge is unchanged.
Electric charge is conserved in every electromagnetic process.
If the total charge before a process is $Q_{\text{before}}$, then
$$
Q_{\text{before}} = Q_{\text{after}}
$$
Importance of electromagnetism
Electromagnetism is responsible for an enormous range of physical phenomena. It binds electrons to nuclei, creates chemical bonds between atoms, produces light, radio waves, X rays, and gamma rays, and governs most interactions of charged particles with matter.
In nuclear and particle physics, electromagnetism is especially important because it provides many of the signals we detect experimentally. Charged particles leave tracks, deposit energy, and emit radiation through electromagnetic interactions, allowing us to observe processes that happen at extremely small scales.
Relation to the broader picture
Among the four fundamental interactions, electromagnetism occupies a special place because it is both long range and strong enough to dominate much of ordinary matter. In modern theory, it is also closely connected to the weak interaction through the electroweak framework, although that larger unification belongs to a separate discussion.
For now, the key idea is simple: electromagnetism is the interaction of charged particles, and its quantum messenger is the photon.