A magnetic field is a vector field describing magnetic influence throughout space and time. It is conventionally represented by the magnetic flux density symbol and measured in teslas. Magnetic fields exert forces on moving electric charges and on currents and interact with magnetic dipoles and magnetic materials. A stationary isolated electric charge does not experience a magnetic force from its own static electric field, whereas a moving charge can experience a force dependent on its velocity and the magnetic field. Magnetic fields can be generated by electric currents permanent magnets changing electric fields and more generally by moving charges. They are therefore closely integrated with electric fields within electromagnetism.
The magnetic force on a moving charge depends on charge magnitude velocity magnetic field strength and the angle between velocity and magnetic field. The force is perpendicular to both the velocity and magnetic field when the velocity has a component perpendicular to the field. Consequently a magnetic field can change the direction of a charged particles motion without necessarily changing its speed. This property produces circular or helical trajectories for charged particles moving through uniform magnetic fields. The resulting behaviour is used in particle detectors mass spectrometers cyclotrons and many other technologies. The magnetic force on a current carrying conductor follows the corresponding current carrying wire relationship.
Magnetic field lines are a graphical method for representing field structure. Outside an ordinary bar magnet they are conventionally drawn from the north pole toward the south pole and inside the magnet they continue to form closed loops. Unlike electric field lines magnetic field lines do not begin or end at isolated magnetic charges in classical electromagnetism. This reflects the absence of experimentally established isolated magnetic monopoles in conventional electromagnetism. Magnetic field lines are conceptual representations rather than physical filaments. Their density can be used to indicate relative field strength but quantitative calculations require the vector field itself.
Electric currents are important sources of Magnetic Fields. A straight current carrying conductor produces a circular magnetic field around the conductor with strength depending on current and distance. Coils can concentrate magnetic fields and solenoids can produce relatively uniform fields within their interiors. Electromagnets use electrical current and magnetic cores to create controllable magnetic fields. The direction around a current carrying conductor can be determined using the appropriate right hand rule. These relationships form the basis of motors relays transformers inductors magnetic sensors and numerous electrical machines.
Magnetic Fields also arise from permanent magnetism. In materials such as ferromagnetic substances microscopic magnetic moments can interact and align within domains. When sufficient alignment occurs the material can exhibit a persistent macroscopic magnetic field. Permanent magnet behaviour depends on atomic structure domain configuration material composition temperature and magnetic history. Heating or applying sufficiently strong opposing fields can alter magnetic ordering. Magnetic materials are therefore not simply passive sources of fixed fields but physical systems whose magnetic states can depend on their environment and history.
Earth has a large scale Magnetic Field generated primarily by processes in its electrically conducting outer core. The field is approximately dipolar at large scales but changes over time and is affected by interactions with the solar wind and surrounding space environment. A magnetic compass aligns approximately with the horizontal component of Earths field and has historically provided a practical method of navigation. Earths magnetic field also influences charged particles in near Earth space and contributes to the structure of the magnetosphere. Magnetic observations therefore have applications in navigation geology space physics and studies of Earths interior.
Magnetic Fields can store energy and are essential in inductors and electromagnetic systems. When current builds in an inductor the associated magnetic field stores energy that can later be returned to the circuit. Changing magnetic fields also induce electric fields according to electromagnetic induction. This coupling means that electric and magnetic fields cannot always be treated as independent phenomena. A changing magnetic field can produce an electric field and a changing electric field can contribute to magnetic field generation. These relationships are captured quantitatively by Maxwell equations and form the basis of electromagnetic wave propagation.
Common misconceptions include believing that magnetic fields act on all charges equally or that magnetic force is always attractive. Magnetic forces on moving charges depend on velocity and direction, and interactions between magnetic dipoles can involve attraction or repulsion depending on orientation. Another misconception is that magnetic field lines are physical objects. They are visualisation tools. Magnetic fields are nevertheless physically measurable through their effects on charged particles currents magnetic materials and electromagnetic systems. Their role extends from atomic and condensed matter physics to electrical engineering planetary science medical imaging and modern information technology.
