Paul Peter Urone; Roger Hinrichs

Einstein is said to have been fascinated by a compass as a child, perhaps musing on how the needle felt a force without direct physical contact. His ability to think deeply and clearly about action at a distance, particularly for gravitational, electric, and magnetic forces, later enabled him to create his revolutionary theory of relativity. Since magnetic forces act at a distance, we define a magnetic field to represent magnetic forces. The pictorial representation of magnetic field lines is very useful in visualizing the strength and direction of the magnetic field. As shown in Figure 22.15, the direction of magnetic field lines is defined to be the direction in which the north end of a compass needle points. The magnetic field is traditionally called the B-field.

Three diagrams illustrating magnetic field lines. Figure a shows a bar magnet with a number of compasses arranged along the magnet on either side. The needles of the compasses at the north pole of the magnet point away from the pole. The needles of the compasses at the south pole of the magnet point toward the pole. The needles of compasses in between the two poles point parallel to the magnet, toward the south pole. Figure b shows lines running from the north pole out and around to the south pole. Figure c shows lines as closed loops running from the north pole outside the magnet and around the south pole, and then up through the magnet to the north pole.

Figure 22.15 Magnetic field lines are defined to have the direction that a small compass points when placed at a location. (a) If small compasses are used to map the magnetic field around a bar magnet, they will point in the directions shown: away from the north pole of the magnet, toward the south pole of the magnet. (Recall that the Earth’s north magnetic pole is really a south pole in terms of definitions of poles on a bar magnet.) (b) Connecting the arrows gives continuous magnetic field lines. The strength of the field is proportional to the closeness (or density) of the lines. (c) If the interior of the magnet could be probed, the field lines would be found to form continuous closed loops.

Small compasses used to test a magnetic field will not disturb it. (This is analogous to the way we tested electric fields with a small test charge. In both cases, the fields represent only the object creating them and not the probe testing them.) Figure 22.16 shows how the magnetic field appears for a current loop and a long straight wire, as could be explored with small compasses. A small compass placed in these fields will align itself parallel to the field line at its location, with its north pole pointing in the direction of B. Note the symbols used for field into and out of the paper.

Figure a: magnetic field of a circular current loop with a current moving counter-clockwise. The field lines are also roughly circular, running up through the center of the current loop, and back down outside the loop. Figure b: a straight wire with a current running straight up. The magnetic field lines circle the wire in a counter-clockwise direction. Figure c: a right hand with the thumb pointing up, parallel to a wire with the current running upward. The figures of the hand curl around the wire in the counter-clockwise direction to show the direction of the magnetic field when current is up. The symbol to represent magnetic field lines running out of the surface and toward the viewer—B out—is a circle with a sold circle inside. The symbol to represent magnetic field lines running into the surface and away from the viewer—B in—is represented with a circle with an x inside it. When the current is running straight up, B out is to the left and B in is to the right.

Figure 22.16 Small compasses could be used to map the fields shown here. (a) The magnetic field of a circular current loop is similar to that of a bar magnet. (b) A long and straight wire creates a field with magnetic field lines forming circular loops. (c) When the wire is in the plane of the paper, the field is perpendicular to the paper. Note that the symbols used for the field pointing inward (like the tail of an arrow) and the field pointing outward (like the tip of an arrow).

Making Connections: Concept of a Field

A field is a way of mapping forces surrounding any object that can act on another object at a distance without apparent physical connection. The field represents the object generating it. Gravitational fields map gravitational forces, electric fields map electrical forces, and magnetic fields map magnetic forces.

Extensive exploration of magnetic fields has revealed a number of hard-and-fast rules. We use magnetic field lines to represent the field (the lines are a pictorial tool, not a physical entity in and of themselves). The properties of magnetic field lines can be summarized by these rules:

The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.
The strength of the field is proportional to the closeness of the lines. It is exactly proportional to the number of lines per unit area perpendicular to the lines (called the areal density).
Magnetic field lines can never cross, meaning that the field is unique at any point in space.
Magnetic field lines are continuous, forming closed loops without beginning or end. They go from the north pole to the south pole.

The last property is related to the fact that the north and south poles cannot be separated. It is a distinct difference from electric field lines, which begin and end on the positive and negative charges. If magnetic monopoles existed, then magnetic field lines would begin and end on them.

22.3 Magnetic Fields and Magnetic Field Lines