MODULE 2

• Our credit cards, computer hard drives, motors, generators, and speakers all use magnetism
• The earth is one giant magnet, too
• If we use magnets in so many different areas of our lives, we should understand how they work and how we're able to use them in technology
• Lodestones


  Naturally magnetized stones that could attract small pieces of iron in a seemingly magical way, and were found to always point in the same direction when allowed to swing freely, suspended by a piece of string, or floating on water
• Early navigators used these magnets as rudimentary compasses to help them determine their direction while at sea
• Magnet


  The word "magnet" comes from Magnesia, a district in Thessaly, Greece where it is believed that the first lodestone was mined
• Over the years, magnets have evolved into the high-strength materials we have today
• It was discovered that by creating alloys of various materials, one could create similar effects to those found in natural lodestone rocks, and increase the level of magnetism
• Progress in creating stronger magnetic alloys was very slow until the 1920s when alnico magnet materials (an alloy of nickel, aluminum and cobalt) were formulated
• Ferrite magnets were developed in the 1950s and rare-earth magnets in the 1970s
• Since then, the science of magnetism has grown exponentially, and extremely powerful magnetic materials have made miniature and powerful devices possible
• Magnets


  Can be made by placing a magnetic material, such as iron or steel, in a strong magnetic field
• Types of magnets


  • Permanent
  • Temporary
  • Electromagnets
• Permanent magnet


  An object made from a material that is magnetized and creates its own persistent magnetic field. A permanent magnet is one that retains its magnetic properties for a long period of time. Examples are iron, nickel, cobalt and some rare earth alloys
• Temporary magnet


  Made out of soft metals that retain their magnetism only when they are near a permanent magnetic field or an electrical current. In the presence of a magnetic field, they become magnetized. Examples are paperclips, iron nails, and other similar things
• Electromagnet


  A type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil
• Classification of matter according to magnetic property

  • Ferromagnetic
  • Paramagnetic
  • Diamagnetic
• Ferromagnetic


  Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet. These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone
• Paramagnetic


  Substances, such as uranium, platinum, aluminum, sodium, manganese, chromium and oxygen, are weakly attracted to either pole of a magnet. This attraction is hundreds of thousands of times weaker than that of ferromagnetic materials, so it can only be detected by using sensitive instruments or using extremely strong magnets
• Diamagnetic


  Means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances, such as bismuth, mercury, silver, zinc, carbon, copper, water, and plastic, are even more weakly repelled by a magnet
• Magnetic field


  The region surrounding a magnet where the magnetic force can be detected
• Magnetic lines of force


  The imaginary lines that indicate the path taken by an independent n pole in going from the N to the S pole of magnet
• Properties of magnetic lines of force

  • They do not touch each other
  • They do not cross each other
  • They are elliptical closed curves
• How is magnetic field created?

  1. When current flows in a wire, a magnetic field is created around the wire. From this it has been inferred that magnetic fields are produced by the motion of electrical charges. A magnetic field of a bar magnet thus results from the motion of negatively charged electrons in the magnet

  2. Magnetic field lines are defined to have the direction in which a small compass point when placed at a location in the field. The strength of the field is proportional to the closeness (or density) of the lines
• Law of Magnetic Poles

  • Like poles repel
  • Unlike poles attract
• Absolute permeability


  Every medium possesses two permeabilities: Absolute permeability, and Relative permeability
• Relative permeability


  For measuring relative permeability, a vacuum or free space is used as reference medium. It is allotted an absolute permeability of μ0. And the relative permeability of vacuum with reference to itself is unity
• Magnetic flux (Φ) or magnetic lines of force

  The entire group of magnetic field lines, which flow outward from the north pole of the magnet. It is the number of magnetic lines of forces in a magnetic field
• Weber


  SI unit of magnetic flux equal to 1x10^8 lines or Maxwell. Named after the German physicist, Wilhelm Weber
• Maxwell


  CGS unit of magnetic flux equal to one line of force. Named after the Scottish physicist, James Clerk Maxwell
• Magnetic flux density (β)

  The magnetic flux per unit area of a section perpendicular to the direction of flux
• Force on a charge


  The amount of attraction or repulsion between charged objects can be put in quantitative terms by the introduction of the electric force. The simplest case to consider is the force between two points charges (charges with a negligible size)
• Right Hand Rule


  In an open right hand, the direction of the four fingers points to the direction of the magnetic field, the thumb pointing perpendicular to the four fingers points to the direction of the magnetic force in a positive charge is in the direction in which your open palm would push
• Units


  • β - magnetic field (Wb/m^2 (Tesla), Mx/cm^2 (Gauss), lines/in^2)
  • F - force (N, Dynes, lbf)
  • V - velocity (m/s, cm/s, in/s, ft/s)
• Electromagnetism


  A current consists of many small charged particles running through a wire. If immersed in a magnetic field, the particles will be experience a force; they can transmit this force to the wire through which they travel
• Magnetomotive force (mmf)


  A force that sets up or tends to set up magnetic flux in a magnetic circuit. By an electric current through a number of turns of a wire produces it
• Magnetic field strength/force/magnetic intensity (H)

  It should be noted that the field strength is a vector quantity having both magnitude and direction. It is the magnetomotive force (mmf) per unit length of path of the magnetic flux. It is also called as the magnetizing force or the magnetic gradient
• Oersted


  cgs unit of magnetic field strength equal to gilbert per centimeter
• AT/m


  SI unit for H
• Force between two parallel conductors

  When the current is in the same direction, the field strength in the space between the conductors is decreased due to the two fields there being in the opposition to each other, causing the two conductors to be attached towards each other. When the current is in the opposite direction, the field strength is increased in the space between the two conductors due to the two fields being in the same direction there, causing lateral repulsion of the lines of force and the two conductors experiencing a mutual force of repulsion