Magnetic Field In and Around a Bar Magnet
As discussed previously, a magnetic field is a change in energy
within a volume of space. The magnetic field surrounding a bar
magnet can be seen in the magnetograph below. A magnetograph can
be created by placing a piece of paper over a magnet and sprinkling
the paper with iron filings. The particles align themselves with
the lines of magnetic force produced by the magnet. The magnetic
lines of force show where the magnetic field exits the material
at one pole and reenters the material at another pole along the
length of the magnet. It should be noted that the magnetic lines
of force exist in three dimensions but are only seen in two dimensions
in the image.
It can be seen in the magnetograph that there are poles all along
the length of the magnet but that the poles are concentrated at
the ends of the magnet. The area where the exit poles are concentrated
is called the magnet's north pole and the area where the entrance
poles are concentrated is called the magnet's south pole.
Magnetic Fields in and around Horseshoe
and Ring Magnets
come in a variety of shapes and one of the more common is the
horseshoe (U) magnet. The horseshoe magnet has north and south
poles just like a bar magnet but the magnet is curved so the poles
lie in the same plane. The magnetic
lines of force flow from pole to pole just like in the bar magnet.
However, since the poles are located closer together and a more
direct path exists for the lines of flux to travel between the
poles, the magnetic field is concentrated between the poles.
a bar magnet was placed across the end of a horseshoe magnet or
if a magnet was formed in the shape of a ring, the lines of magnetic
force would not even need to enter the air. The value of such
a magnet where the magnetic field is completely contained with
the material probably has limited use. However, it is important
to understand that the magnetic field can flow in loop within
a material. (See section on circular magnetism for more information).
General Properties of Magnetic Lines
lines of force have a number of important properties, which include:
- They seek the path of least resistance between opposite magnetic
poles. In a single bar magnet as shown to the right, they attempt
to form closed loops from pole to pole.
- They never cross one another.
- They all have the same strength.
- Their density decreases (they spread out) when they move from
an area of higher permeability to an area of lower permeability.
- Their density decreases with increasing distance from the
- They are considered to have direction as if flowing, though
no actual movement occurs.
- They flow from the south pole to
the north pole within a material and north pole to south pole