Magnetism in physics: explanation of magnetic poles, magnetic force, formulas, terrestrial magnetism and the compass.
It is well known that there are bodies that attract iron: when iron filings are thrown onto a lodestone, or magnet, they remain attached “in tufts” to the ends of the magnet. This physical property, called magnetism , therefore characterizes not the entire magnet, but only its extreme zones, called magnetic poles . Experimentally, attractive or repulsive actions are exerted between the magnetic poles . Magnets always appear to be composed of two poles with opposite magnetic properties: the poles of a magnet are therefore denoted as north (or positive) and south (or negative). Homonymous magnetic poles repel each other, poles of opposite charge attract each other. The existence of opposite magnetic poles on the ends of a magnet remains if the magnet is fragmented: each fragment is dipolar.
MAGNETISM PHYSICS: FORMULAS
Study models. Coulomb’s law for magnetism . Experiments carried out with very long magnets (such that the influence of one pole on the other was negligible) led to the hypothesis that magnetic charges (or masses?) were located on the poles.
In fact, by measuring the forces acting between poles of the same or different signs, it was discovered that a Newtonian law was valid, formally analogous to Newton ‘s law of gravitation or to Coulomb’s law for electric charges :
where M 1 and M 2 are the magnetic charges that can be thought of as being located at the poles. Magnetism was therefore born under the hypothesis of the existence of such charges, resulting in a consistent and convincing theory. However, today we know that the magnetic charge is not a physical entity like the electric charge, but only a useful model for interpreting magnetic phenomena : in reality, wherever there are moving electric charges, there is always a magnetic field . Or rather, magnetism is a phenomenon due to the electric current .
TERRESTRIAL MAGNETISM AND COMPASS
If you place a magnetic needle free to rotate on a horizontal plane (compass) at any point on the Earth’s surface , it will point approximately toward geographic north. “Approximately” because south and magnetic north correspond only approximately to geographic north and south, respectively (the angle of deviation is called magnetic declination). If you place the compass on a vertical plane, as long as it is not at the equator, it will tilt. It will tilt to form a positive angle with the horizon in the Northern Hemisphere and a negative angle in the Southern Hemisphere. The maximum inclination, equal to a right angle, is reached at the poles.
The origin of terrestrial magnetism is not very clear: in ancient times it was thought that there were magnetized masses of iron, nickel and cobalt, but this is not possible because the high temperatures inside the Earth would not allow a strong magnetization.
Today it is thought instead that it is the existence of macroscopic electric currents inside the Earth that generates the terrestrial magnetic field .
