PHYSICS: MAGNETISM

PHYSICS: MAGNETISM

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Overview

• The term magnetism describes how materials respond to an applied magnetic field
• All materials are influenced to a greater or lesser extent by the presence of a magnetic field. Some are attracted (paramagnetism) while some are repulsed (diamagnetism)
  
• Substances that are negligibly attracted by magnetic fields are called non-magnetic materials. Eg: copper, aluminium, water, glass
• The magnetic state of a material depends on its temperature, with the result that a substance may exhibit different magnetic characteristics depending on its temperature
• Magnetism can arise from either intrinsic magnetic moments contained in particles, or by electric currents applied to the substance
• Magnet is a material that produces a magnetic field
• Permanent magnet is a material that retain its magnetic field

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Types of magnetism

• Diamagnetism
  • Diamagnetism is the tendency of a material to oppose a magnetic field
  • It appears in all materials. However, in a material with paramagnetic properties, the paramagnetic behaviour dominates
    
  • Diamagnetic materials do not have unpaired electrons
  • Superconductors are diamagnetic materials
• Paramagnetism
  • Paramagnetism is the tendency of a material to be attracted to an applied magnetic field
  • Paramagnetism only occurs in the presence of an externally applied magnetic field. When the external field is removed, the magnetisation will drop to zero
    
  • Paramagnetic materials have one unpaired electron, allowing it to orient in the direction of the magnetic field
    
  • Oxygen, myoglobin are examples of paramagnets
• Ferromagnetism
  • Ferromagnetism is the only type of magnetism that can produce forces strong enough to be felt, and is responsible for the magnetic phenomena in everyday life
  • Ferromagnetic materials have unpaired electron, but unlike paramagnets, they remain oriented even after the external magnetic field has been removed
  • Ferromagnetic materials remain magnetized even after the external applied magnetic field has been removed
  • All permanent magnets are either ferromagnets or ferrimagnets
  • Eg: refrigerator magnets
• Antiferromagnetism
  • Magnetic moments of electrons point in opposite directions
  • Anitferromagnets have zero net magnetic field
  • They are not very common and usually occur only low temperatures
    
  • Antiferromagnetism disappears above the Neel Temperature and the material becomes paramagnetic
  • Examples include hematite, chromium, iron manganese
    
• Ferrimagnetism
  • Neighbouring pairs of electrons point in opposite direction
  • However, ferromagnetic materials retain their magnetisation in the absence of the magnetic field
  • Example is magnetite

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Electromagnets

• Electromagnet is a magnet whose magnetic field is produced by the flow of electric current
• The magnetic field disappears when the current ceases
• The electromagnet was invented by William Sturgeon (Britain) in 1824
• Electromagnets are widely used in electrical devices such as motors, generators, loudspeakers, particle accelerators
• Magnetic Levitation (MAGLEV) trains run on electromagnetic suspension produced by electromagnets

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Earth’s magnetic field

• The Earth’s magnetic field, which extends several tens of thousands of km into space is called the magnetosphere
• The earth’s magnetic field is explained by dynamo theory. The theory explains the mechanism by which celestial bodies like the earth, or a star generate magnetic fields. According to the theory, earth’s magnetic field is produced by electric currents in the liquid outer core
  
• The magnetic north pole of the Earth is located near the geographic south pole, and the magnetic south pole is located near the geographic north pole. This can be explained by understanding that the north pole of a suspended magnet points towards the north, indicating that the geographic north pole should have south polarity
  
• The earth’s magnetic poles move with time due to magnetic changes in the earth’s core. Currently, the magnetic north pole lies near Ellesmore Island in northern Canada, while the south pole is near Wilkes Land, Antarctica. The north pole is moving northwest by about 64 km/year and the south pole is moving northwest by 10-15 km/year

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