separate.
Like electric fields, magnetic fields can be diagrammed with field lines. We have
superimposed a drawing of magnetic field lines on the illustrations to the right. You can
see how the orientations of bits of iron filings reflect the direction of the field at a number
of places around the magnet.
As the diagram shows, the field lines run outside the magnet from the north to the south
pole, and they always form closed loops. As with an electric field, the closer the lines,
the stronger the field. The strength of the field decreases with distance from the
magnet.
The table in Equation 2 shows approximate magnetic field strengths near some
magnetic objects. The magnetic fields you encounter every day are much less than one
tesla. The most powerful lab magnets have field strengths of tens of teslas. However,
human technology cannot create magnetic fields anywhere near the field strengths
exhibited by a class of highly magnetized neutron stars called magnetars.
Magnetic field
Region where magnet exerts force
External lines point from north to southB is symbol for magnetic field
Units: teslas (T)
Magnetic field strength near
magnetic objects
28.3 - Physics at work: lodestones
Lodestone: A naturally occurring magnetic
rock found in various regions of the world.
The Chinese first used lodestone magnets in their navigational compasses in the twelfth
century. Later explorers, including Columbus, carried lodestones to “recharge” their
iron-based compasses. To do so, they would pass the needle of a compass near the
lodestone to strengthen its magnetic properties.
A lodestone consists of two materials, one with a high saturation magnitude. Saturation
magnitude is the measure of the maximum strength of the magnetic field that can be
induced in a substance. Iron, for example, has a high saturation magnitude. The other
material has high coercivity, which is the measure of how well a substance retains its
magnetic field. Pure iron loses its magnetic properties relatively easily, while a
substance such as magnetite does not.
The combination of these two materials results in a strong and durable magnetic
source. One substance supplies the strength, the other supplies durability.
Lodestones
Natural magnetic rock combines
materials with
·Strong magnetic field
·Long-lasting magnetic field