A magnetic field is a region where magnetic forces are exerted. It surrounds every magnet and every moving electric charge. The field is invisible, but it can be mapped with iron filings or a compass. Field lines run from the north pole of a magnet to the south pole outside the magnet, and from south to north inside. The denser the field lines, the stronger the field. The unit of magnetic field strength is the tesla. Earth's magnetic field is about 50 microtesla. An MRI machine uses a field of 1.5 to 3 tesla.

Magnetic fields are produced by moving charges. A current-carrying wire creates a circular magnetic field around it. A coil of wire creates a stronger field, and a coil with an iron core creates a stronger one still. That is how electromagnets work. Magnetic fields also exert forces on moving charges. That is how electric motors work. A current-carrying wire in a magnetic field experiences a force perpendicular to both the current and the field. The force is described by the Lorentz force law. Magnetic fields also affect compasses, which align with Earth's field, and they protect the planet from charged particles from the sun.

Magnetic field facts

• Definition. Region of magnetic force.
• Source. Moving electric charge.
• Unit. Tesla (T).
• Earth's field. About 50 microtesla.
• MRI. 1.5 to 3 tesla.

A magnetic field is not the same as an electric field. An electric field acts on charges regardless of motion. A magnetic field acts only on moving charges.