What is a Magnetic Field?
A magnetic field is the area around a magnet, magnetic object, or an electric charge in which magnetic force is exerted.
The invisible area around a magnetic object that can pull another magnetic object toward it or push another magnetic object away from it is called a magnetic field.
Fig.no.1: Magnetic Field lines directed from north pole to south pole of a permanent magnet.
What makes a magnetic field?
Magnetic fields are created by moving electric charges. When electrons, which have a negative charge, move around in certain ways, a magnetic field can be created. These fields can be created inside the atoms of magnetic objects or within wires (electromagnetism).
How do we measure a magnetic field?
We measure a magnetic field by its strength and by the direction it points.
Every magnetic field is a bit different. Some magnetic fields are large, some are strong, some are small, and some are weak. For example, the Earth's magnetic field is large but weak.
Physical proximity (how close or far away) really counts in magnetism. The closer you stand to a magnet, the stronger the magnetic field will be. The farther away you are from a magnet, the weaker the magnetic field becomes. (A magnetic field never ends - it simply gets weaker and weaker the farther away you go, in principle, even to infinity!)
Magnetic field is represented by a capital letter $B$, and is measured in units of Tesla (International System of Measurement or SI).
There are also many other units and terms used in the field of electromagnetism, including Weber, Maxwell, Gauss, and even 109 gamma!
How can we see the forces in a magnetic field?
Sometimes we draw field lines to show the direction of the forces at different locations within a magnetic field. Field lines exit the magnet at its north pole, travel around in the air, and re-enter the magnet through its south pole. Field lines don't start in one place and stop in another; magnets travel in "closed paths," which means they will continue to travel the same path again and again.
Remember that the magnetic field is present everywhere around the magnet, not just along the field lines that we draw, but even between the field lines. The lines simply help us visualize the direction the field is flowing at various locations around the magnet and even within the magnet.
Magnetic fields may be represented mathematically by quantities called vectors that have direction as well as magnitude. Two different vectors are in use to represent a magnetic field: one called magnetic flux density, or magnetic induction, is symbolized by B; the other, called the magnetic field strength, or magnetic field intensity, is symbolized by H.