A magnet or an electric current will produce a magnetic field, which can be visualized as a series of circular field lines around a wire segment.
All magnets have two poles, one called the north pole and one called the south pole. Like poles repel and unlike polls attract (in analogy to positive and negative charges in electrostatics). North and south poles always exist in pairs (there are no magnetic monopoles in nature), so if one were to split a permanent magnet in half, two smaller magnets would be created, each with a north pole and south pole.
Since magnetic forces act at a distance, we define a magnetic field (B-field) to represent magnetic forces. The direction of magnetic field lines is defined to be the direction in which the North end of a compass needle points, or going from North pole to South pole of a magnet.
The properties of magnetic field lines can be summarized by these rules:
1. The direction of the magnetic field is tangent to the field line at any point in space
2. The strength of the field is proportional to the closeness of the lines.
3. Magnetic field lines can never cross
4. Magnetic field lines are continuous, forming closed loops without beginning or end. They go from the north pole to the south pole.
The last property is related to the fact that the north and south poles cannot be separated. It is a distinct difference from electric field lines, which begin and end on the positive and negative
MCAT Official Prep (AAMC)
• The magnetic field direction is the same direction a compass needle points, which is tangent to the magnetic field line at any given point.
• The strength of the B-field is inversely proportional to the distance between field lines. It is exactly proportional to the number of lines per unit area perpendicular to the lines.
• A magnetic field line can never cross another field line. The magnetic field is unique at every point in space.
• Magnetic field lines are continuous and unbroken, forming closed loops. Magnetic field lines are defined to begin on the north pole of a magnet and terminate on the south pole.
B-field: A synonym for the magnetic field.
Magnetic field lines: A graphical representation of the magnitude and the direction of a magnetic field.