Magnetism - Lycée Blaise Pascal Rouen


, by C. Reymond, DORE

Magnetism is a force of attraction or replusion that acts at a distance. It is due to a magnetic field*, which is caused by moving electrically charged particles. It is brought about by magnetic objects such as a magnet or the Earth.

A magnetic field consists of lines of flux which are called magnetic field lines* coming from moving electrically charged particles. The lines of magnetic flux extend* from one end of the object to the other. By convention, we call one end of a magnetic object N or North-seeking pole* and the other S or South-seeking pole*, as related to the Earth’s North and South magnetic poles. The magnetic flux is defined as moving from North-seeking to South-seeking.

A magnetic field can be defined as the Lorentz force law. The Lorentz force* is the force that a charged particle suffers from a magnetic field. This law is a relationship between the movement of the particle through the magnetic field, the strength of that magnetic field and the force of the magnetic field on the particle. The following equation is F = qvB. F is the force on the particle in Newton, q is the electric charge of the particle in Coulombs, v is the speed of the charge in meters per second and B is the strength of the magnetic field in tesla.

The magnetic field of an object creates a magnetic force on other objects which have magnetic fields too. That force is what we call magnetism.

When two magnets or magnetic objects are close to each other, there is a force that attracts the opposite poles together ; and when the same poles face each other, the magnetic force pushes them apart.

Individual particles such as electrons can have magnetic fields therefore larger objects can also have a magnetic field, which comes from a sum of the fields of its particles. When the force of this magnetic field is important, these objects are called magnets. A magnet of the size of one coin has a magnetic field with a force of 1,25 teslas, it can attract ferromagnetic materials such as iron, nickel or cobalt and can lift an object which weighs 9kg.

The Earth is a giant magnet but the magnetic pole that is in the northern hemisphere is a south-seeking pole because it attracts the north-seeking pole of a magnet. As a result, its magnetic field travels frome the South pole (North-seeking pole) to the North pole (South-seeking pole). The force of its magnetic field is on average 5.10-5 T (=0,00005 T) or 0,5 G because 1 tesla is equal to 10 000 gauss.

Today magnetism has many uses in modern life for example in medicine with Magnetic Resonance Imaging* (MRI). It’s a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of Nuclear Magnetic Resonance (NMR) to image nuclei* of atoms inside the body.

An MRI scanner is a device in which a patient is lied down* in a large and powerful magnet. The magnetic field of the magnet is used to align the magnetization of some atomic nuclei in the body, and radio frequency magnetic fields are applied to change the alignment of this magnetization. As a result the scanner detects a rotating magnetic field produces by the nuclei and it uses these information to make an image of the scanned area of the body.
The force of the magnetic field in MRI varies from 0,2 to 3 teslas (it depends on the scanner) and there are research scanners that study higher field strenghts such as 7 teslas.

In conclusion, magnetism is a force that acts at a distance and is caused by a magnetic field. A magnetic field is composed of magnetic field lines that are orientated from North-seeking pole to South-seeking pole of a magnetic object like a magnet or the Earth. A magnetic field is derived from the Lorentz force law : F=qvB ; this is the force that a charged particle puts up with a magnetic field. The unit of the strength of the magnetic field is in tesla and today this strenght is used in medical imaging technique* in radiology to visualize internal structures of the body in detail.

Sources :

Vocabulary :

Magnetic field : Champ magnétique
Magnetic field lines : Lignes de champs
to extend : s’étendre
North-seeking pole : Pôle Nord magnétique
South-seeking pole : Pôle Sud magnétique
The Lorentz force : La force de Lorentz ou force électromagnétique
Magnetic Resonance Imaging : Imagerie par Résonance Magnétique (IRM)
Nucleus (sg), nuclei (pl) : noyau, noyaux
to be lied down : être allonger
Medical imaging technique : technique d’imagerie médicale