Introduction to Magnetism Revision Notes

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16.1Introduction to Magnetism

In these revision notes for Introduction to Magnetism, we cover the following key points:

  • What are magnets?
  • How many types of magnets are there?
  • What is magnetic field? How do we represent the magnetic field?
  • What are magnetic poles? What do they have in common with Earth Poles and where do they differ?
  • What is magnetic force? What happens when we bring like and unlike poles close to each other?
  • How a magnet is produced?
  • How to remove magnetism from a magnet?
  • What magnetic tool do we use to orient ourselves when no direction is known?
  • What is magnetosphere? How does it protect us?
  • How magnetism is used by animals?
  • How do we use magnetism in electrical equipment? Medicine? Industry?

Introduction to Magnetism Revision Notes

Magnets are materials that are able to attract or repel metal objects made from iron, steel, cobalt, nickel and iron oxides.

Magnets that exist in natural form and which are obtained through mining are known as natural magnets, while those produced in factory are known as artificial magnets. Most magnets we use today are artificial.

Materials attracted by magnets are known as magnetic materials. For example, iron is a magnetic material as we can attract iron objects using magnets while copper, paper, plastics, gold, aluminium etc., are non-magnetic materials as they are not attracted by magnets.

By definition, magnetic field is the space around a magnet in which its attractive or repulsive effect is observed.

The two extremities in which the magnetic field is the strongest, are known as magnetic poles. To make distinction between poles, we call them North and South poles respectively. The North pole is coloured in red while the South pole in blue

The general law governing the interaction of magnets is similar to that of electric charges, i.e.:

"Like poles repel each other while unlike poles attract each other."

The interaction between magnets is an indicator for the existence of the magnetic force - a force that causes attraction or repulsion between poles.

Magnetic field lines are imaginary lines (similar to those of electric field) used to represent the direction of magnetic field. Magnetic field lines have the following properties:

  1. Magnetic field lines (unlike electric field ones) are closed lines that start from one pole (North) and end to the other pole (South).
  2. The magnetic field lines are denser at the poles. This means that the magnetic field is the strongest at the poles.

We represent the magnetic field lines through arrows starting from the North Pole and ending to the South Pole of magnet. Magnetic field lines never cross each other.

Other properties of magnets include:

  • All magnets have two poles; unipolar magnets do not exist.
  • When a magnet is cut in two pieces, two new magnets with two poles each are obtained.

A single proton-electron pair is the smallest magnet possible. Such pairs are known as dipoles. Dipoles are assumed as enclosed within small rooms called domains where each domain contains a single dipole. In normal conditions, all materials are neutral and their dipoles are randomly oriented. When we allow some electric current flow through the material, the dipoles are oriented according the direction of electric current. In this way, a bipolar magnetic material is obtained. This method of magnetisation is known as the electrical method.

Other ways to obtain a magnet include:

  • Contact method consisting in touching a magnet with a metal that possesses magnetic properties. In this way, the metal is magnetised after a certain period due to the regular orientation of its dipoles;
  • Induction method, i.e. bringing a non-magnetized metal near a magnet. After a while, the metal is magnetised, i.e. its dipoles are oriented according the magnetic field lines;
  • Stroking method. If we strike gently a steel needle by a magnet, the needle is magnetised after a number of strokes.

Demagnetisation, i.e. the process of magnetism removal from a magnetised object is carried out in one of the three following ways:

  1. By striking the magnet with a hammer. Strong strokes distort the regular orientation of magnetic dipoles, turning the metal in the state is has been prior to becoming a magnet.
  2. By dropping the magnet from a height. This method works in a very similar way to the previous one.
  3. By heating the magnet. Heating an object results in a higher kinetic energy of its molecules. This brings a less regular orientation of dipoles and consequently, a decrease in the magnetic properties of the object.

We classify magnets in permanent and temporary based on their ability to keep the magnetism.

Permanent magnets are magnets able to keep their magnetism for a long time. Cast-iron is an alloy that is used to produce permanents magnets. Most of magnets we use today are made from cast-iron. Other materials used to obtain permanent magnets include steel, brass, iron oxide etc.

Temporary magnets are magnets that lose their magnetism very quickly; almost immediately after the electricity supply is removed from them. Iron is an example of a temporary magnetic material.

Temporary magnets are also known as soft magnetic materials.

Earth is considered as a giant magnet containing its own magnetic field. Magnetic and geographic poles do not lie exactly in the same direction; they diverge from each other by an angle of about 220. That's why there exist two distinct North Poles: one geographic and the other magnetic.

The device used from travellers to orient themselves is called a compass. It consists on a small magnetic needle placed on a thin pin, parallel to a circular case in which the main directions are written.

Magnetosphere is the region of space surrounding Earth where the dominant magnetic field is the magnetic field of Earth, rather than the magnetic field of interplanetary space. Magnetosphere is formed by the interaction of the solar wind with Earth's magnetic field. It shields the surface of the Earth from the charged particles of the solar wind and is generated by electric currents located in many different parts of the Earth.

The average distance of magnetosphere from the Earth is about 65000 km. It extends well beyond the other layers of atmosphere, which extend up to 1000 km above the Earth surface.

Some animals such as pigeons, magnetostatic bacteria, tuna fish, butterflies, whales, dolphins etc., use the Earth magnetic field to find the direction of motion.

Magnets have a wide range of use in today's world. Thus, magnets are used in many home appliances such as in refrigerators, computers, sound systems, etc.

Magnetic resonance is one of the most marvelous applications of magnetism in medicine. NdFeB (neodymium) is a high-end rare earth magnet. In terms of medical treatment, NdFeB has been widely used in physical magnetic therapy. NdFeB is often used for the physical treatment of a variety of diseases.

Magnetism in industry is mainly used in metal processing technology. Permanent magnets are used to separate metals from non-metals.

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