20 magnetism


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20 magnetism

  1. 1. MAGNETISM A Strangely Attractive Topic
  2. 2. HISTORY  Term comes from the ancient Greek city of Magnesia, at which many natural magnets were found.  We now refer to these natural magnets as lodestones (lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.
  3. 3. HISTORY  Pliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron.
  4. 4. HISTORY  Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic property, and that such a rod when suspended from a string would align itself in a north-south direction.  Use of magnets to aid in navigation can be traced back to at least the eleventh century.
  5. 5. HISTORY Basically, we knew the phenomenon existed and we learned useful applications for it. We did not understand it. Not until its connection to electrical charges and currents was discovered.
  6. 6. What is magnetism?  Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.
  7. 7. What causes magnetism?  Atoms themselves have magnetic properties due to the spin of the atom‟s electrons.  Groups of atoms join so that their magnetic fields are all going in the same direction.  These areas of atoms are called “domains”.
  8. 8. What causes magnetism?  When an unmagnetized substance is placed in a magnetic field, the substance can become magnetized.  This happens when the spinning electrons line up in the same direction.
  9. 9. What causes magnetism?  An unmagnetized substance looks like this…  While a magnetized substance looks like this…
  10. 10. How to “break” a magnet?  Drop it.  Heat it.  This causes the domains to become random again!
  11. 11. A big natural magnet..  It exerts magnetic forces and is surrounded by a magnetic field that is strongest near the North and South magnetic poles. Magnetic South Pole Geographic South Pole Geographic North Pole Magnetic North Pole
  12. 12. PROPERTIES OF MAGNETS  There are north poles and south poles.  Magnets set up a magnetic field around it.  Like poles repel, unlike poles attract.  Magnetic forces attract only magnetic materials.  Magnetic forces act at a distance.
  13. 13. 1. North and South Poles  Every magnet has at least one north pole and one south pole.  If you take a bar magnet and break it into two pieces, each piece will again have a north pole and a south pole.  No matter how small the pieces of the magnet become, each piece will have a north pole and a south pole.
  14. 14. 1. North and South Poles, cont.  It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).
  15. 15. 2. Magnetic Fields  Michael Faraday realized that a magnet has a „magnetic field‟ distributed throughout the surrounding space.  This field exerts a force on any charge/magnetic material on it.
  16. 16. 2. Magnetic Fields, cont.  Field lines converge where the magnetic force is strong, and spread out where it is weak. In a compact bar magnet or dipole, field lines spread out from one pole and converge towards the other.
  17. 17. 3. Like poles, unlike poles  LAW of MAGNETISM: Like poles repel, unlike poles attract.
  18. 18. 4. Attract only magnetic materials  Magnets only attract certain types of metals, other materials such as glass, plastic and wood aren't attracted.  Metals such as iron, nickel and cobalt are attracted to magnets.  Most metals however are not attracted to magnets, these include copper, silver, gold, magnesium, platinum, aluminium and more. They may however magnetize a small amount while placed in a magnetic field.  Magnetism can attract magnetic objects or push them away.
  19. 19. 5. Action at a Distance  Although two magnets may not be touching, they still interact through their magnetic fields.  This explains the „action at a distance‟, say of a compass.
  20. 20. Electricity to Magnetism Hans Oersted   first observed that a current in a wire affects a nearby compass needle Implication: an electric current creates a magnetic field
  21. 21. Electromagnets - Magnets created using a current Magnetic strength increases with:  more current in the wire  more coils of wire  bigger ferromagnetic core
  22. 22. Uses of Electromagnetism 1. Electric motors  Converts electrical energy to mechanical energy  Anytime electricity is converted into a motion its through an electric motor
  23. 23. Uses of Electromagnetism 2. Current meters  Galvanometer – used to measure small currents  Ammeter – used to measure currents  Voltmeter – used to measure voltage
  24. 24. Magnetism to Electricity Michael Faraday   if an electric current can create a magnetic field, then maybe a magnetic field can create a electric current this led to his Law of Electromagnetic Induction
  25. 25. Law of Electromagnetic Induction  Any change in the magnetic field of a coil of wire will cause a voltage to be induced in the coil, called the induced voltage.  If the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current. How to increase induced voltage/current  Increase number of coils  Increase magnetic field  Increase speed of relative motion between coil and magnetic field
  26. 26. Uses of Electromagnetic Induction 1. Generator  Converts mechanical energy to electrical energy  Used in most power plants to create electricity  In coal, oil, natural gas, nuclear or geothermal power plants, the heat is used to boil water into steam which is then used to rotate the turbines  In wind, hydroelectric (dam/falls) and tidal power plants, the turbines are rotated by the wind/water  The only sources of electricity that does not use generators are solar, chemical (batteries), piezoelectric (crystals) and thermoelectric (heat)
  27. 27. Uses of Electromagnetic Induction 2. Transformer  Used to increase or decrease voltage/current  Step-up or Step-down  High voltage electricity is used in long distance transmission  Low voltage is used in homes