Bar Magnet

The lines ofmagnetic field from a bar magnetform closed lines.By convention, thefield direction istaken to be outward from theNorth pole and into the South poleof the magnet. Permanent magnets can be made fromferromagnetic materials.

As can be visualized with the magnetic field lines, the magnetic field is strongest inside the magnetic material. The strongest external magnetic fields are near the poles. A magnetic north pole will attract the south pole of another magnet, and repel a north pole.

The magnetic field lines of a bar magnet can be traced out with the use of a compass. The needle of a compass is itself a permanent magnet and the north indicator of the compass is a magnetic north pole. The north pole of a magnet will tend to line up with the magnetic field, so a suspended compas needle will rotate until it lines up with the magnetic field. Unlike magnetic poles attract, so the north indicator of the compass will point toward the south pole of a magnet. In response to theEarth's magnetic field, the compass will point toward the geographic North Pole of the Earth because it is in fact a magnetic south pole. The magnetic field lines of the Earth enter the Earth near the geographic North Pole.

Electromagnet

Electromagnets are usually in the form ofiron core solenoids. Theferromagnetic property of the iron core causes the internalmagnetic domains of the iron to line up with the smallerdriving magnetic field produced by the current in thesolenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands. Thesolenoid field relationship is

and k is therelative permeability of the iron, shows the magnifying effect of the iron core.