Magnetism

• produced by current from a battery

  Electromagnet (EM)
• without any external source of current

  Permanent magnet (PM)
• north and south poles of a magnet are the points of concentration of magnetic strength
• The magnet can be considered the generator of an external magnetic field, provided by the two opposite magnetic poles at the ends.
• Magnetic fi eld lines are unaffected by nonmagnetic materials such as air, vacuum, paper, glass, wood, or plastics.
• Magnetic field lines never cross each other but instead push apart

  from each other.
• The entire group of magnetic fi eld lines, which can be considered flowing outward from the north pole of a magnet, is called magnetic flux.
• A strong magnetic field has more lines of force and more flux than a

  weak magnetic field.
• One maxwell (Mx) unit equals one magnetic fi eld line
• To convert microwebers to lines or maxwells

  A) 10^-6

  B) 10^8

  C) 10^2

  D) 100

  
• James Clerk Maxwell proved that electromagnetic phenomena travel in waves at the speed of light.
• 1 Wb = 1 x 10^8 Mx
• The SI unit for magnetic flux is weber (Wb)
• flux density is the number of magnetic fi eld lines per unit

  area of a section perpendicular to the direction of flux.
• Flux Density Formula B=Φ/A
• Tesla is the SI unit for flux density
• Magnetic field strength is measured by a magnetometer
• With a flux of 10,000 Mx through a perpendicular area of 5 cm^2 , what is the flux density in gauss?

  2000 G
• In the cgs system, Gauss unit is one line per square centimeter, or 1 Mx/cm 2 .
• In SI, the unit of flux density B is webers per square meter (Wb/m^2 )
• One weber per square meter is called a tesla
• abbreviated T is named for Nikola Tesla
• Make the following conversions: (a) 0.003 T to G; (b) 15,000 G to T.

  #G = 30 G

  #T = 1.5 T
• Remember that the flux Φ includes total area, whereas the flux density B is for a specified unit area.
• The electric or magnetic effect of one body on another without any physical contact between them is called induction.
• For instance, a permanent magnet can induce an unmagnetized iron bar to become a magnet without the two touching.
• More generally, the magnetic fi eld between magnetic poles and the electric field between electric charges form the basis for wireless radio transmission and reception
• If the permanent magnet were reversed, its south pole would induce a north pole.
• No matter which pole is used, it will induce an opposite pole, and opposite poles are attracted.
• Soft iron, is very effective in concentrating magnetic field lines by induction in the iron.
• This ability to concentrate magnetic flux is called permeability
• The shorter the air gap, the stronger the field in the gap for a given pole strength.
• Since air is not magnetic and cannot concentrate magnetic lines, a larger air gap provides additional space for the magnetic lines to spread out.
• toroid or ring magnet in made in the form of a doughnut.
• The small part of the field in the air is called leakage flux.
• The principle of the closed magnetic ring is used to protect permanent magnets in storage.
• The keeper maintains the strength of the permanent magnet as it becomes magnetized by induction to form a closed loop.
• The two broad classes are permanent magnets and electromagnets.
• An electromagnet needs current from an external source to maintain its magnetic fi eld.
• With a permanent magnet, not only is its magnetic field present without any external current, but the magnet can maintain its strength indefinitely.