physics//electromagnetism//magnetic field

A magnetic field is the influence that moving charges spread around them, which pushes sideways on other moving charges, and it is what motors, generators, transformers, loudspeakers, hard drives and MRI machines are built on. Every current makes one, a permanent magnet makes one (from the aligned spins of its electrons), and the Earth makes one large enough to steer a compass.

A magnetic field is the influence that moving charges spread around them, which pushes sideways on other moving charges, and it is what motors, generators, transformers, loudspeakers, hard drives and MRI machines are built on. Every current makes one, a permanent magnet makes one (from the aligned spins of its electrons), and the Earth makes one large enough to steer a compass.

It is measured in tesla. The scale is wide and worth having in the head: the Earth's field is about 25 to 65 microtesla, a fridge magnet a few millitesla at its surface, an MRI scanner 1.5 to 3 tesla, and the field the brain's own currents leave outside the skull tens to hundreds of femtotesla, a billionth of the Earth's. The push on a moving charge, the Lorentz force, is at right angles to both its motion and the field, so a magnetic field bends a charge's path without speeding it up.

The field around a straight wire is weak and circles it. Wind the wire into a coil and the turns add their fields in the middle, which is the idea of the inductor and of every electromagnet.

What a circuit feels is the magnetic flux: how much field passes through the loop, field times area. A steady flux does nothing to the loop; a changing one drives a current in it, which is electromagnetic induction.

Measuring weak fields is a specialty of its own, from the magnetometer in a phone to the optically pumped magnetometers that read brain activity.