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Magnetic Fields
Magnets
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Magnets are polarized
– It has two distinct and opposite ends
North-seeking pole
 South-seeking pole
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– Every magnet will have both north and
south poles
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Like magnetic poles repel
Opposite magnetic poles attract
– North will repel north, but attract the
south pole of a magnet
– South will repel south, but attract the
north pole of a magnet
Magnetic Fields
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Magnetic fields are vector quantities that
exist in a region in space where a magnetic
force occurs.
The presence of a magnetic field can be
seen using iron filings
These iron filings would make up a twodimensional plot of the field
– This makes visualizing the magnetic field lines
possible
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Magnetic field lines, like electric field
lines are imaginary.
They are used to help us visualize a
field
Magnetic Flux
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The number of magnetic field lines passing
through a surface is called the magnetic
flux
The flux per unit area is proportional to the
strength of the magnetic field
The magnetic flux is most concentrated at
the poles, thus this is where the magnetic
field strength is the greatest
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The direction of the magnetic field line
is the direction in which the north pole
of a compass points when it is placed
in the magnetic field.
– So, field lines emerge from the magnet at
its north pole and enter at its south pole
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There are no isolated poles in the
magnet so the field lines even travel
through the magnet itself to go from
the south pole to the north pole and
form closed loops
Creating a Magnet
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You can create a magnet by having an
electric current travel through a wire coil
The current carrying coil will repel the north
end of a magnet on one side and attract on
the other
– This means that that coil has a north and south
pole and is itself a magnet
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This is called an electromagnet
You can increase the strength of the
electromagnet by placing an iron rod or core
within the coil of wire.
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Magnetic field strength  B
– This is also known as the flux density
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The weber, Wb, is a derived SI unit for
measuring the number of lines of flux.
The tesla, T, is the derived SI unit of flux
density or magnetic field strength
– One tesla is equal to one weber per square
meter.
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Therefore, magnetic field strengths (B) are
measured in units of teslas (T)