Much like an electric field around an electric charge, there is a
magnetic field around a magnet. Anything that enters this force
(and is magnetically inclined) will experience a force.
A man named
Oerstead once discovered that wires that conduct electric charge
also creat a magnetic field. (This is called electromagnetism.) If you want to
find the direction of this field, it is much easier than the electric field method.
All you need to do is grasp the wire in your
left hand, with your thumb in the direction
of the flow of charge (negative). Your curved fingers will be pointing in the direction of
the field. This is called the Left Hand Rule.
Magnetic Fields have a circular direction
around a wire. This only deals with straight wires. However, sometimes we make what
is called a coil. If you want to make a coil, wrap some wire around and around a
pencil, but don't let the wire cross itself. Then remove the pencil.
You have a simple coil!
There are actually
three left hand rules. The second left hand rule states
that if your wrap your left hand around a coil of wire, with your fingers in
the direction of the flow of charge, your thumb points to the end of the coil
that is the north pole. That's right- coils of current conducting wire are
also magnets!
The
presence of a magnetic field is detected by the forces
of attraction or repulsion which these exert on similar objects
within the range of this field. This magnetic field is characterized
by the
presence of magnetic lines which seem to be non-intersecting
and oriented from one pole of the magnet to the other. A magnetic
field is a
vector quantity, one which is described by both magnitude
and direction. The magnetic field lines are not physical lines but
imaginary ones, which help in the understanding of the magnetic field.
In short, the orientation and the working range of the field is
represented and more easily understood by representing this field
in the form of magnetic lines.
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A current carrying conductor experiences a
force when placed in a magnetic field.
The
direction of force is reversed when the direction of current in the conductor is reversed.
The force acting on the current-carrying conductor can be changed by
changing
the direction of the magnetic field.
According to this rule, extend the thumb, forefinger, and the middle
finger of the left hand in such a way that all the three are mutually
perpendicular to each another. If the
forefinger points in the direction
of the magnetic field and the
middle finger in the direction of the
current, then, the
thumb points in the direction of the force exerted
on the conductor.
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Consider a small piece of wire of length ds carrying a current I. This defines a vector ds that points in the direction of the current. The magnetic field dB set up by this piece of current-carrying wire at a point a distance r away is:
Proportional to
1/r^2 .
Proportional to
I, the current, and ds, the length of the wire .
In a direction
perpendicular to both ds and r, the vector from the wire to the point
Proportional to
sinx , where x is the angle between ds and r
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