Electromagnetism Resource

The magnetic field is a vector field that exists in all regions of space.

Humans lack any direct means of detecting magnetic fields, but magnetic fields can be detected by observing their effects on other magnets or magnetically permeable substances.

An example is a compass. A compass contains a bar magnet that aligns itself with Earth's magnetic field.

Compasses also work with any other magnetic field.

Another effective means of seeing a magnetic field is with iron filings. Iron filings are a fine iron powder that can be sprinkled around magnets.

The image below shows iron filings sprinkled on a sheet of paper placed over a bar magnet.

The iron filings align themselves along the magnetic field in lines. These lines are the shape of the magnetic field.

Iron filings do not give the direction of the magnetic field. In the illustration above, it is not possible to tell which pole is which by the iron filings alone. A compass must be used to determine the direction. The "north" end of the compass needle points in the direction of the magnetic field.

The closer the iron filings are to each other, the stronger the magnetic field.

Every magnet has two poles, a north pole, and a south pole. The magnetic poles are where the field lines appear to "leave" and "enter" the magnet.

Magnetic poles are also where the magnetic field lines are the most concentrated as they leave (or enter) a magnet.

In a bar magnet, the poles are at the ends. The poles is also the area where the magnetic flux density is the greatest, as the iron filings are the most concentrated there.

The picture below shows the magnetic field of a horseshoe magnet. Note how strong the magnetic field is at the poles, as they are closer together than with a bar magnet.

Magnetic Fields and Magnetic Flux

The magnetic field is a vector quantity. The magnitude of the magnetic field has the symbol \( B \). The unit of magnetic field is the tesla or T.

The magnetic field may also be defined as the magnetic flux density; or magnetic flux per unit area.

The magnetic flux represents the total magnetic field \( B \) over an area.

The unit of magnetic flux has three interpretations: the weber (Wb), the tesla metre squared (T.m²), or the volt second (Vs). Note that 1 T = 1 Wb.m^-2.

The magnetic field may also be represented as magnetic flux, with the symbol \( \Phi \) (Greek letter capital 'phi').

Magnetic fields are vector fields like electric fields, but with an important difference: there are no net "sources" or "sinks" of magnetic fields.

In other words, magnetic field lines do not "start" at a north pole, and "terminate" at a south pole. Magnetic field lines are always closed loops, with no start and no end.

The picture below shows the simplest magnetic field: a magnetic dipole.

Outside the magnetic object, the magnetic field lines point towards the south pole, and come away from the north pole.

Note how all the field lines form closed curves. The south pole is on the left of the dipole, the north pole is on the right. The closer the lines are to each other, the stronger the field.

Behaviour of Magnets and Magnetic Fields

For a permanent magnet, every atom is a magnetic dipole. That means that if you cut a magnet in half, you will not get a separate north and south pole.

You will get two magnets, each with their own north and south poles.

Magnetic field direction is commonly detected using another magnet, e.g. a compass. The illustration below shows how this is done using compasses. The compass points in the direction of the magnetic field. As can be seen below, the magnetic field lines go from the north pole to the south pole. In other words, south sucks".

Compasses were originally designed to detect magnetic north. The compass needle itself is a small magnet. The part of a compass needle that points north is a magnetic north pole. The needle gives the direction of the magnetic force, the magnetic field going in the direction the compass needle is pointing. Note that this means that the compass needle always points towards a south pole. The magnetic pole in Earth's northern hemisphere is actually a south pole!

Magnetic Attraction and Repulsion

Magnets are capable of interacting with each other. The images below show two situations: attraction between unlike poles, and repulsion between like poles.

The image below shows magnetic attraction. The iron filings in the middle go pretty much straight from one magnet to another. These magnets may also produce a noticeable force, and may actually pull themselves together so they are touching.

The image below shows magnetic repulsion. The iron filings in the middle are quite sparse, and the field lines look "squeezed outwards" with a "dead spot" in the centre between the magnetic poles. These magnets may also produce a noticeable force, and may actually push themselves apart.