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The evolution of electromagnetic theory spans from the early observations of magnetism to the complex principles of quantum electrodynamics (QED). Key figures like James Clerk Maxwell and discoveries such as Earth's dynamo-generated magnetic field have shaped our understanding. Technological advancements and record-breaking magnetic fields further illustrate the impact of this scientific journey.

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## History of Electromagnetism

### Ancient Awareness of Magnetism

Ancient civilizations were aware of magnetism

### Scientific Study of Magnetism

Petrus Peregrinus de Maricourt

In 1269, Petrus Peregrinus de Maricourt described the magnetic field on a spherical magnet's surface

William Gilbert

In 1600, William Gilbert proposed that Earth behaves like a giant magnet

Hans Christian Ørsted and André-Marie Ampère

In the 18th and 19th centuries, Hans Christian Ørsted and André-Marie Ampère made pivotal discoveries about the interaction between electricity and magnetism

### Maxwell's Unification

In the 19th century, James Clerk Maxwell unified the fields of electricity, magnetism, and optics into a coherent theoretical framework

## Quantum Electrodynamics

### Definition and Advancements

Quantum electrodynamics (QED) is a quantum theory of electromagnetic interactions that has surpassed previous semi-classical approaches

### Contributions to the Standard Model

QED is a cornerstone of the Standard Model of particle physics, which unifies the fundamental forces of the universe

## Earth's Magnetic Field and Dynamo Theory

### Generation of Earth's Magnetic Field

Earth's magnetic field is generated by the dynamo effect in the planet's liquid outer core

### Characteristics of Earth's Magnetic Field

Earth's magnetic field is not static and exhibits fluctuations in intensity and periodic reversals

### Dynamo Theory

Dynamo theory provides a framework for understanding the complex processes that generate Earth's magnetic field

## Technological Applications of Magnetic Fields

### Alternating-Current Motor

The development of the alternating-current (AC) motor by Nikola Tesla revolutionized the use of electricity

### Hall Effect

The Hall effect, discovered by Edwin Hall, is used to measure magnetic field strength and determine the nature of charge carriers in conductive materials

### Magnetic Circuits

Magnetic circuits use the concept of magnetic vector potential to calculate magnetic flux and are crucial in the design of electromagnets and the study of magnetic materials' properties