Basics of Magnetism

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We have all experienced magnets in some form or another in our day to day life. Magnets are all around us and are required by many electronic devices for functioning such as speakers.

The question to be asked is What exactly is magnetism?

Magnetism is a force of attraction or repulsion that acts at a distance and a magnet is an object or material that can attract certain metals. For e.g. ferromagnets are magnets of iron which are the oldest known magnets (around 12th century).

Till 1821, magnetism was thought to be only produced by special materials such as ferromagnets. But this changed when the Danish scientist, Hans Christian Oersted observed a phenomenon in which the flow of electric current through a wire which was close to a compass needle caused the needle to move.

This phenomenon was further studied by Andre-Marie Ampere who deduced the nature of magnetism. He defined magnetism as the force between electric currents. Thus two parallel currents in the same direction attract, while two parallel currents in the opposite direction would repel.

What did this mean? This mean’t that electric currents (or moving charges) could produce magnetism and in-turn magnetic forces could be used to produce electricity. This phenomenon is the basis of Nuclear Magnetic Resonance.

What is a Magnetic Field?

Magnetic Field Lines
The image shows the arrangement of iron filings when a strong magnet is placed in it. You can see clearly the magnetic field lines emerging from the poles. (Image Reference – 1)

A magnetic field is a field or area of force produced by a magnetic object (either a substance or due to passing electric current) and is detected by the force it exerts on other magnetic materials and moving electric charges. Magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. For e.g. when a magnetic bar is placed in iron filings you can see that the iron filings arrange themselves in groups along magnetic field lines.

The strength of the magnetic field (or the force experienced due to a magnet) is measured by a value known as B-field value (or in certain instances a modification of this is the H-field value). The B-field is defined as the following: a particle having an electric charge of q, and moving with a velocity v, experiences a magnetic force of F, then the B-field is given by the equation

F = q (v B)

Thus the magnetic field is the force experienced (F) by a charge (q) which is moving (v). If the charge would not be moving then the velocity v = 0 and there would be no field as B would also = 0.

The SI units of B are Tesla and the cgs units are gauss.

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So now the question to be asked is how do we translate these physical properties of magnetism to identify chemical structures or detect atoms?

A charged particle in motion defines an electric current- and an electric current produces a magnetic field. Atoms contain charged particles such as electrons and protons. Protons are positively charged particles in the center of the atoms which is known as the nucleus. While electrons are negatively charged particles moving around the nucleus. Apart from these there are neutral particles known as neutrons which are also present in the nucleus. Every element has a specific number of protons, electrons and neutrons. Thus the nucleus of each element is unique.

Atom
A diagram of an atom showing the protons and neutrons in the nucleus and the electrons orbiting around it. (Image Reference – 2)

Since the nucleus of the atoms is positively charged and it spins around its own axis, the nucleus of the atom produces a very tiny magnetic field. Every element has a specific number of protons, electrons and neutrons. Thus the nucleus of each element produces a magnetic field which is different from another elements. Not only that, but the magnetic field produced by the nuclei is also affected by the environment in which it is. Thus nuclei in different magnetic environments (e.g. atoms which are crowded together, compared to atoms which are further apart) would produce magnetic fields of different strengths. By detecting the magnetic field produced by the nuclei of atoms within molecules, nuclear magnetic resonance can identify the type of elements within the molecule. Thus physical property of magnetism can be transferred and utilized to give structural information.

Atom (1)
A diagram showing the nucleus in the center (yellow) which spins about its axis (just like the earth) and from it is emitted the magnetic field lines (indicated by the black lines). (Image Reference – 3)

Image References:

  1. Wikipedia – article on Magnetic Field - http://en.wikipedia.org/wiki/Magnetic_field
  2. Units-of-measure.com - http://www.units-of-measure.com/
  3. Ourhollowearth.com - http://www.ourhollowearth.com/

Suggested Readings:

  1. Wikipedia – article on Magnetic Field - http://en.wikipedia.org/wiki/Magnetic_field
  2. NASA website - http://www-istp.gsfc.nasa.gov/Education/Imagnet.html
  3. Basics of Magnetism - http://www.school-for-champions.com/science/magnetism.htm