A magnetic field is an invisible force field created by the movement of electric charges. These fields represent one half of the electromagnetic force that governs how charged particles interact. While unseen, a magnetic field’s presence is detected by the force it exerts on other moving charges or on specific materials, such as iron. A stationary electric charge only produces an electric field, but once that charge begins to move, a magnetic field appears around it. Understanding how this motion generates a field requires examining three distinct mechanisms: the macroscopic flow of current, the microscopic properties of atoms, and the dynamic interplay of energy in empty space.
Magnetic Fields from Electric Current
The most straightforward way to generate a magnetic field is through the steady flow of electric charge, known as current, through a conductor like a wire. This relationship was first documented in 1820 by Hans Christian Ørsted, who observed that a compass needle would deflect when placed near a wire carrying current. Ørsted’s discovery established that electricity and magnetism were two facets of the same force. The strength of the magnetic field generated is directly proportional to the magnitude of the current flowing through the conductor. The field lines created by a straight wire carrying current form concentric circles around the wire, with the direction determined by the charge flow.
This principle is used in devices called electromagnets. By coiling a conducting wire into a tight helix, or solenoid, the individual magnetic fields produced by each loop combine and reinforce one another. This concentration creates a strong, uniform magnetic field inside the coil, which can be turned on or off by controlling the electric current. Electromagnets rely entirely on this macroscopic flow of charges and are distinct from permanent magnets because their influence only exists while the current is flowing.
Magnetic Fields from Atomic Structure
The magnetism found in permanent magnets, such as those made of iron, arises from activity occurring at the atomic level, independent of any external electric current. Electrons contribute to a material’s magnetic properties in two primary ways. The first is the electron’s orbital motion around the nucleus, which acts like a tiny current loop, producing a small magnetic moment. The second, and more significantly, is an intrinsic property called spin, which behaves as if the particle were a tiny spinning magnet.
In most materials, the magnetic moments generated by these electrons cancel each other out because the electrons are paired and their motions are oriented in opposite directions. However, certain ferromagnetic materials like iron, cobalt, and nickel, contain atoms with unpaired electrons. In these materials, groups of neighboring atoms spontaneously align their magnetic moments into small, uniformly magnetized regions called domains. When the domains are randomly oriented, the material shows no net magnetism. When an external magnetic field is applied, these domains align themselves in the same direction, collectively generating the strong, persistent magnetic field that characterizes a permanent magnet.
Magnetic Fields from Changing Electric Fields
A third mechanism for generating a magnetic field involves the dynamic relationship between electricity and magnetism in empty space. This concept is formalized in Maxwell’s equations, a set of four formulas that describe the behavior of electric and magnetic fields. James Clerk Maxwell proposed that a changing electric field generates a magnetic field.
This idea completed a symmetrical relationship with Michael Faraday’s earlier discovery, which showed that a changing magnetic field generates an electric field. The combination of these two principles means that an electric field and a magnetic field can continuously create each other in an ongoing cycle. This self-sustaining interplay allows energy to propagate through a vacuum without the need for charges or conductors. This process describes the nature of electromagnetic waves, which include light, radio waves, and X-rays. In these waves, the electric and magnetic fields oscillate perpendicular to each other and perpendicular to the direction of travel. The magnetic field is generated precisely when the electric field changes its strength or direction, making this the mechanism responsible for all forms of electromagnetic radiation.