›› Magnetic Fields
Although we cannot
see or feel them, magnetic fields are all around us. Any
moving electric charge produces both electric and magnetic
fields. Just like an electric field, a magnetic field exerts
a force on other charges as well, so long as they are moving in a
direction that is not parallel to the field itself.
Electric current flowing through a wire produces a predictable magnetic
field. The direction of the field can be determined by using
what is commonly known as the 'right hand rule'. By imagining
that you are grasping the wire with your right hand and pointing your
thumb along the wire in the same direction that the current is moving,
the way in which your fingers curl is the same way the resulting
magnetic field is oriented. From this, its clear that
magnetic fields resulting from a current carrying wire occur in a
circular form, spinning around the wire. The field is
strongest close to the wire, and diminishes with distance.
Magnetic fields are measure in the SI unit the Tesla (T), named in
honor of the inventor and engineer Nikola Tesla. In cgs
units, the fields are measured in gauss (G), after the famous German
scientist Carl Friedrich Gauss. Ten thousand gauss equal one
tesla.
Another way to define magnetic fields is by the amount of force they
will exert on a given charge. The force is determined using
the equation F = qvB, where F is the force in newtons, q is the charge
the force is acting on measured in coulombs, v is the velocity of
charge q in meters per second, and B is the magnetic density of the
field in teslas.
Due to the electronic alignment of certain substances, some objects
themselves can generate magnetic fields. These objects are
usually metals (metals have the required valence electrons) and are
commonly referred to as magnets. The magnetic fields of these
objects orient in a dipole manner, one pole referred to as positive,
the other as negative, much like designations used in electric
fields. Sometimes these poles are referred to as north and
south.
For reasons that aren't entirely understood, the Earth itself has a
magnetic field. It is this magnetic field that is the basis
for the compass, which is essentially a sensitive magnetic pointer that
will align itself along the Earth's magnetic field. Since the
Earth's field is oriented from magnetic south to magnetic north, a
compass gives a good indication of which way is north. Even
though magnetic north is not the same as the actual north pole, from
most locations on the planet it is close enough to make compasses an
accurate guide.
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