Primary
Magnetic Field ○◂｜Definition｜1st｜20260105003619-00-⌔
Magnetic field
In magnetism and electromagnetism, magnetic field is a physical property of space that quantifies the magnetic influence at a given location. Magnetic fields deflect moving electric charges (including electric currents), apply torques on magnets to twist them in the direction of the magnetic field, and attract or repel magnets and magnetic material such as iron. In addition, a time-varying magnetic field induces electrical currents. Magnetic fields are created by magnetic materials and by moving electric charges (including electrical current). The latter is important in creating electromagnets: devices that precisely control magnetic fields by changing the current through the electromagnet.
Magnetic fields are used throughout modern science and technology. In electrical engineering and electromechanics it is important in the design and use of electric motors, generators, transformers, electromagnets, and inductors among many other devices. In material science, magnetic forces give information about the charge carriers in a material through the Hall effect in addition to other uses.
In geology and geophysics, Earth’s magnetic field gives information about earth’s interior while local magnetic field measurements are used in mineral exploration and other measurements. Too, Earth’s magnetic field creates a magnetosphere which shields the Earth’s ozone layer and the rest of the planet from the solar wind. In physics the relationship between the magnetic and electric fields forms the field of electrodynamics which is important to understand a wide range of phenomena including light (also known as electromagnetic radiation) and the properties of antenna and transmission lines.
Since both strength and direction of a magnetic field may vary with location, it is described mathematically by assigning a vector to each point of space, making it a vector field.1 There are two different, but closely related, vector fields which are called “magnetic field”. These are written as B and H.2 While the best names for these fields is the subject of long running debate, the underlying physics is uncontested.3
Printed 2026-06-28.
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Link to original Footnotes
More precisely, magnetic field is a pseudovector field due to its properties under inversion. ↩
The letters B and H were originally chosen by Maxwell in his Treatise on Electricity and Magnetism (Vol. II, pp. 236–237). For many quantities, he simply started choosing letters from the beginning of the alphabet. See Ralph Baierlein (2000). “Answer to Question #73. S is for entropy, Q is for charge”. American Journal of Physics. 68 (8): 691. Bibcode:2000AmJPh..68..691B. doi:10.1119/1.19524. ↩
John J. Roche (2000). “B and H, the intensity vectors of magnetism: A new approach to resolving a century-old controversy”. American Journal of Physics. 68 (5): 438. Bibcode:2000AmJPh..68..438R. doi:10.1119/1.19459. ↩
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