As seen, the magnetic field lines produced by a straight current-carrying wire are circles centered on the wire. If we imagine bending the wire in order to form a single-loop, all the circular field lines become oriented in the same direction. They add giving a stronger field in the center of the loop; the form of such a magnetic field is the one shown in the figure below.
If we want a still stronger field, we can use a longer wire and coil it into a lot of loops instead of just one. Such a coil of wire is called "solenoid". (See the figure below)
It’s easy to recognize that the magnetic field of such a solenoid is similar to the one produced by a bar magnet. (figure below)
This similarity is due to a multitude of microscopic currents inside the magnet. In effect each atom can be regarded as a single loop of current because of the orbiting electrons in it. In the no-magnetic materials these micro-loops of current are random-oriented and their effects cancel each other; in the magnets they are oriented along a preferential direction and their effects sum each other to form a macroscopic magnetic field. (See the figure below)
no magnetic material
magnet
This model explains how the magnetic field produced by a cylindrical magnet is identical to the one produced by a solenoid.
atomic
currents inside a magnet
As a matter of fact, in a magnet, the atomic currents have opposite directions in each internal point of the material, hence their magnetic effects cancel each other. On the contrary, on the surface these currents don’t compensate one another, the result is a superficial current flowing on the surface, similar to the one in the coil of wire. (see the figure above). This superficial current is responsible for the magnetic field produced by the magnet.
From the previous arguments we can deduce one of two
fundamental aspects of electromagnetism:
Electric currents produce magnetic fields.
Electric currents are made of moving charged particles,
Hence we can states that:
A moving charged particle produces a magnetic field.
If the charged particle is at rest it produces only a static electric field in
the surrounding space; if the particle moves it produces not only a varying
electric field but a magnetic field too.
In the field language, this statement assumes a more general and surprising
form:
A VARYING ELECTRIC FIELD PRODUCES A VARYING MAGNETIC
FIELD. next...