4.5 An Introduction to Electromagnetic Models

How Does It Work?

A signal propagating along a pair of wires is a simple example of electromagnetic energy transfer, yet clearly, a great deal takes place. Charges in the originally neutral wire drift slightly out of place giving rise to a charge imbalance and currents. Charge imbalance gives rise to an electric field. Currents give rise to a magnetic field.

Or is it the electric and magnetic fields exerting forces on the charges and currents and guiding the energy as they propagate along the line at the speed of light?

Or is the flow of energy driving the train with the fields and current merely adjuncts along for the ride?

All this happens simultaneously. Which causes which?

Tired of hearing economists claim on the one hand things may get better, but on the other hand they may get worse, Harry Truman is said to have asked for a one-handed economist. He’d probably have felt the same way about physicists. Consider a few of the more promising models.

Harry Truman (1884-1972)

Charges and Currents: On the one hand, perhaps charges and currents are primary, their presence and motions giving rise to fields, and the fields in turn guiding the energy flow. One charge influences its neighbor which influences its neighbor in turn, and the influence propagates on down the wire. Charges and currents are primary, and electromagnetism arises from their actions.

Fields: On the other hand, perhaps fields are primary. Changing electric fields give rise to changing magnetic fields and vice versa. Fields exert forces on charges and currents, and also convey or guide energy.

Energy: On yet another hand, energy may be interpreted as a fundamental quantity as well. The electrochemical energy of the battery transforms to electromagnetic energy. Electromagnetic energy flows from place to place, fields are an aspect or characteristic of energy, and changes in potential and kinetic energy do the work associated with inducing the motion of charges and currents. This perspective implies that energy is somehow a real entity instead of being a property of other entities. Mathematically, this energy-first approach is described by “Hamiltonian mechanics.”

Next time: 4.5.1 Potentials and Actions At a Distance

Full Table of Contents [click here]

Chapter 4 Electromagnetism Comes of Age

• 4.1 Heaviside, Poynting, & Energy Flow

• 4.2 Early Models of Electromagnetism

  • 4.2.1 Ancient Views

  • 4.2.2 Fluid Theories of Electricity

• 4.3 Actions at a Distance: Fluids or Fields?

  • 4.3.1 Skin Depth and Litz Wire

  • 4.3.2 Inductive Telegraphy at a Distance

  • 4.3.3 Cross Coupling

• 4.4 Signals Along a Transmission Line

  • 4.4.1 How Do Transmission Lines Work?

  • 4.4.2 The Catt Question

• 4.5 An Introduction to Electromagnetic Models

• 4.6 Hertz & Radiation Fields

• 4.7 How Does Radiation Work?

• 4.8 Summary & Conclusions

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