22.5.1 The Concept of a Field

Faraday was particularly impressed with the pattern that iron filings make when sprinkled around a magnet, as seen in FIGURE 22.21. The pattern’s regularity and the curved lines suggested to Faraday that the space itself around the magnet is filled with some kind of magnetic influence. Perhaps the magnet in some way alters the space around it. In this view, a piece of iron near the magnet responds not directly to the magnet but, instead, to the alteration of space caused by the magnet. This space alteration, whatever it is, is the mechanism by which the long-range force is exerted.

FIGURE 22.21 Iron filings sprinkled around the ends of a magnet suggest that the influence of the magnet extends into the space around it.

The figure illustrates the magnetic field lines formed due to iron filings sprinkled around two bar magnets facing each other.
FIGURE 22.21 Full Alternative Text

FIGURE 22.22 illustrates Faraday’s idea. The Newtonian view was that A and B interact directly. In Faraday’s view, A first alters or modifies the space around it, and particle B then comes along and interacts with this altered space. The alteration of space becomes the agent by which A and B interact. Furthermore, this alteration could easily be imagined to take a finite time to propagate outward from A, perhaps in a wave-like fashion. If A changes, B responds only when the new alteration of space reaches it. The interaction between B and this alteration of space is a local interaction, rather like a contact force.

FIGURE 22.22 Newton’s and Faraday’s ideas about long-range forces.

The figure consists of three parts that illustrate Newton’s and Faraday’s ideas about long-ranged forces using two charged particles.
FIGURE 22.22 Full Alternative Text

Faraday’s idea came to be called a field. The term “field,” which comes from mathematics, describes a function that assigns a vector to every point in space. When used in physics, a field conveys the idea that the physical entity exists at every point in space. That is, indeed, what Faraday was suggesting about how long-range forces operate. The charge makes an alteration everywhere in space. Other charges then respond to the alteration at their position. The alteration of the space around a mass is called the gravitational field. Similarly, the space around a charge is altered to create the electric field.

NOTE   The concept of a field is in sharp contrast to the concept of a particle. A particle exists at one point in space. The purpose of Newton’s laws of motion is to determine how the particle moves from point to point along a trajectory. A field exists simultaneously at all points in space.

Faraday’s idea was not taken seriously at first; it seemed too vague and nonmathematical to scientists steeped in the Newtonian tradition of particles and forces. But the significance of the concept of field grew as electromagnetic theory developed during the first half of the 19th century. What seemed at first a pictorial “gimmick” came to be seen as more and more essential for understanding electric and magnetic forces.