Asked in Cars & VehiclesMagnetism
Why does a compass align with a magnetic field?
October 28, 2013 12:56AM
The alignment of a compass results from the basic laws of attraction and repulsion between the poles of magnets. The North pole of the compass will be attracted toward the South pole of a magnetic object and, vice versa, the South compass pole will be attracted to the object's North magnetic pole. The resulting orientation of the compass is then said to be in alignment with the magnetic field of the magnetic object.
A compass is a small magnet and it is useful because it aligns with the magnetic field of the Earth, approximately north and south. The North pole of any magnet is attracted to the South pole of any other magnet and, conversely, the South of any magnet is attracted to the North of any other magnet. Likewise, the North pole of any magnet is repelled by the North pole of any other magnet and the South poles repel in an identical fashion.
If we accept the basic laws of attraction and repulsion of magnetic poles, then the alignment of a compass (or any other magnet) is a result of the forces on the North and South poles of the magnet, being such at to orient the North pole of the compass closer to the South pole of the other magnetic object and further from the North pole of the other magnetic object. At the same time, the South pole of the compass is pulled towards the North pole and away from the South pole of the magnetic object creating the field. The resulting alignment is actually the definition of the direction of the magnetic field of the magnetic object.
The alignment of a compass is therefore a consequence of the basic laws of attraction and repulsion between the poles of magnets.
While the Earth has its own magnetic field and the source of that field is deep in the Earth's core, the characteristic of a compass aligning with the Earth's magnetic field is exactly the same characteristic as a compass aligning with the magnetic field of any magnet. Of course, a strong magnetic field creates a stronger twisting force on a compass needle and so the alignment of a compass needle indicates the combined forces of any nearby magnets and the relatively small magnetic field of the Earth.
This all has to be explained in a self consistent fashion. A magnet will align with a magnetic field and that property is used to define both the magnetic field in strength and direction. The explanation of why a magnet aligns in a field can quickly become circular, i.e. if you define the direction of a magnetic according the to alignment of a magnet, then it is useless to say that the magnet aligns because it wants to be in the direction of the magnetic field.
To explain further why a magnet aligns with a magnetic field, one must start by delineating the basic observations and experimental facts which one takes to be true without explanation. (One need never take anything to be true without explanation, but digging deeper is really the answer to questions about the origins of the fundamental forces of nature and thus an answer more complex than this one.)
Here is a list of several caveats being acknowledged which would be addressed in a longer and more complete explanation.
1. Magnetic fields can be created by electricity as well as a permanent magnet but field of an electric current has no North or South pole even though a compass will respond with an orientation that is aligned with the local direction of the field due to the electric current.
2. Magnets are not monolithic objects, but are actually made up of many atoms, each having the properties of a small magnet, so actually the magnetic field of an object is caused by these many small objects and the forces a maggot experiences in a field are the cumulative forces on all the atoms as a result of their magnetic characteristics.
3. Magnets are attracted (or repelled) by nonmagnetic objects, but this is a consequence of the magnet properties of the constituent atoms of the nonmagnetic which are altered by the presence of the field of the magnetic object.
A course on the subject of electricity and magnetism would cover all these topics and give the mathematics needed to produce a completely consistent description.