Bar Magnet

The lines of magnetic field from a bar magnet form closed lines. By convention, the field direction is taken to be outward from the North pole and in to the South pole of the magnet. Permanent magnets can be made from ferromagnetic 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 the Earth'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.

Comparison of magnetic and electric fieldsCompare to solenoid magnetic field
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Magnetic field concepts
 
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Electric and Magnetic Sources

The electric field of a point charge is radially outward from a positive charge. The magnetic field of a bar magnet.
Electric sources are inherently "monopole" or point charge sources. Magnetic sources are inherently dipole sources - you can't isolate North or South "monopoles".
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Magnetic force

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Bar Magnet and Solenoid

The magnetic field produced by electric current in a solenoid coil is similar to that of a bar magnet.

Add iron core to solenoid
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Iron Core Solenoid

An iron core has the effect of multiplying greatly the magnetic field of a solenoid compared to the air core solenoid on the left.

Properties of iron core solenoid
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Magnetic field concepts
 
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Electromagnet

Electromagnets are usually in the form of iron core solenoids. The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands. The solenoid field relationship is

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

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Magnetic field concepts
 
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