By Syed Hussain Ather – Updated Mar 24, 2022
A magnetic field is the invisible region around a magnet where magnetic forces act. For a typical dipole, field lines emerge from the north pole, loop through space, and return to the south pole – the same pattern that shapes Earth’s geomagnetic field.
Earth’s field shields the planet from solar wind, protecting the ozone layer and life on the surface. The field also prevents the loss of atmospheric gases, playing a crucial role in maintaining our environment.
Place a magnet beneath a sheet of paper, sprinkle iron filings on top, and observe the pattern they form. The resulting arrangement reveals the direction and concentration of the magnetic field lines around the magnet.
Magnetic field intensity is measured in Tesla (T). The denser the field lines between the poles, the stronger the field.
Whenever electric charges move, they produce a magnetic field. In a straight wire carrying current I, the field circles the wire in concentric loops, a relationship described by Ampère’s law:
\(B = \dfrac{\mu_0 I}{2\pi r}\)
where \(\mu_0\) (4π×10⁻⁷ H/m) is the permeability of free space and r is the distance from the wire.
The right‑hand rule helps determine the direction of magnetic fields around currents: point the thumb in the direction of conventional current, and the curled fingers indicate the field’s direction.
Magnetism depends on the arrangement of electrons within atoms. The main categories are:
When a charged particle q moves with velocity v in an electric field E and a magnetic field B, its total force is given by the Lorentz equation:
\(F = qE + q\mathbf{v}\times\mathbf{B}\)
The magnetic component, \(q\mathbf{v}\times\mathbf{B}\), depends on the cross‑product of velocity and magnetic field, and it is zero when v is parallel to B.
The cross‑product produces a vector perpendicular to both input vectors. Using the right‑hand rule, point your index finger along one vector, middle finger along the other, and your thumb points in the direction of the resulting cross‑product.
One of the most familiar uses of magnetic fields is in Magnetic Resonance Imaging (MRI). Machines generate fields of 0.2–0.3 T, aligning hydrogen nuclei in the body. When the field is switched off, the nuclei return to their original orientation, emitting signals that are used to construct detailed internal images.
From the protective shield around Earth to the powerful diagnostics in hospitals, magnetic fields are fundamental to both our natural world and modern technology. Understanding how they form, interact, and can be harnessed empowers scientists and engineers to innovate across disciplines.
