Magnetism Grobs

• Magnetic field lines are unaffected by nonmagnetic materials such as air, vacuum, paper, glass, wood, or plastics
• magnetic field lines in a magnetic substance


  is concentrated when a magnetic substance such as iron is placed in the field. Inside the iron, the field lines are more dense, compared with the field in air.
• the end of the bar magnet is called

  north seeking pole
• opposite end of the bar magnet is called

  south seeking pole
• the earth's geographic north pole has


  south magnetic polarity
• the earth's geographic south pole has

  north magnetic polarity
• magnetic flux


  The entire group of magnetic field lines, which can be considered flowing outward from the north pole of a magnet
• magnetic flux


  symbol (phi) $\phi$
• stronger the magnetic field has more lines of force and more flux
• One maxwell (Mx) unit equals one magnetic field line
• james clerk maxwell


  proved that electromagnetic phenomena travel in waves at the speed of light.
• the Weber


  the larger unit of magnetic flux
• one weber


  $1\times10^8$lines or maxwells
• one microweber


  100 lines or maxwells
• flux density


  the number of magnetic fi eld lines per unit area of a section perpendicular to the direction of flux
• flux density formula


  $B=$${\frac{\phi}{A}}$
• CGS unit for flux density

  The Gauss
• The SI unit of flux density

  The Tesla
• comparison of flux and flux density


  flux is total area, flux density is specified unit area
• induction 


  The electric or magnetic effect of one body on another without any physical contact between them
• a permanent magnet can induce an unmagnetized iron bar to become a magnet without the two touching. The iron bar then becomes a magnet
• Note that the north pole of the permanent magnet 7 induces an opposite south pole at this end of the iron bar
• permeability 


  This ability to concentrate magnetic flux
• air gap


  the air space between the poles of a magnet
• 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
• The purpose of a short air gap is to concentrate the magnetic fi eld outside the magnet for maximum induction in a magnetic material placed in the gap
• The small part of the fi eld in the air is called leakage flux
• A toroid is made in the form of a doughnut.
• The principle of the closed magnetic ring is used to protect permanent magnets in storage
• keeper for a magnet

  The keeper maintains the strength of the permanent magnet as it becomes magnetized by induction to form a closed loop
• an alternating fi eld can demagnetize the magnet.
• The two broad classes are permanent magnets and electromagnets
• electromagnet needs current from an external source to maintain its magnetic field.
• permanent magnet, not only is its magnetic field present without any external current, but the magnet can also maintain its strength indefinitely
• Sharp mechanical shock as well as extreme heat, however, can cause demagnetization
• Current in a wire conductor has an associated magnetic field
• With the length much greater than its width, the coil is called a solenoid.
• More current and more turns make a stronger magnetic field
• Soft iron is generally used for the core because it is easily magnetized and demagnetized.
• A relay is a switch with contacts that are opened or closed by an electromagnet.