Magnetism

• Magnetic phenomena were first observed at least 2500 years ago in fragments of magnetized iron ore found near the ancient city of Magnesia (now Manisa, in western Turkey)
• Permanent magnets


  • Fragments of magnetized iron ore
  • Magnets on refrigerator door
• Permanent magnets were found to exert forces on each other as well as on pieces of iron that were not magnetized
• When an iron rod is brought in contact with a natural magnet, the rod also becomes magnetized
• A magnetized rod that is floated on water or suspended by a string from its center tends to line itself up in a north-south direction
• The needle of an ordinary compass is a piece of magnetized iron
• North pole (N pole)


  One end of a bar-shaped permanent magnet that points north
• South pole (S pole)


  The other end of a bar-shaped permanent magnet
• Opposite poles (N and S) attract each other, and like poles (N and N or S and S) repel each other
• An object that contains iron but is not itself magnetized is attracted by either pole of a permanent magnet
• The earth itself is a magnet, with its north geographic pole close to a magnetic south pole
• The earth's magnetic axis is not quite parallel to its geographic axis, so a compass reading deviates somewhat from geographic north
• The angle of the earth's magnetic field up or down is called magnetic inclination, and at the magnetic poles the magnetic field is vertical
• Magnetic field lines show the direction that a compass would point at each location, and the direction of the field at any point can be defined as the direction of the force that the field would exert on a magnetic north pole
• While isolated positive and negative electric charges exist, there is no experimental evidence that one isolated magnetic pole (magnetic monopole) exists; poles always appear in pairs
• Magnetic field


  The force that a charged particle experiences moving in this field, after accounting for gravitational and electric forces
• Tesla (T)


  SI unit for magnetic field strength
• Gauss (G)


  Smaller unit sometimes used for magnetic field strength
• Magnetic field lines


  • They form closed loops
  • The direction of the magnetic field is tangent to the field line
  • The strength of the field is proportional to the closeness of the lines
  • Magnetic field lines can never cross
  • Magnetic field lines are continuous, forming closed loops without a beginning or end
• If isolated magnetic charges (magnetic monopoles) existed, then magnetic field lines would begin and end on them
• Motion of a charged particle in a uniform magnetic field

  1. Particle follows a curved path

  2. Particle continues to follow this curved path until it forms a complete circle

  3. Magnetic force is perpendicular to velocity, so it does no work on the charged particle

  4. Particle's kinetic energy and speed remain constant, only the direction of motion is affected
• Moving charges in a wire (current) experience a force in a magnetic field
• A current-carrying wire produces circular loops of magnetic field
• Right-hand rule 2 (RHR-2)


  Thumb points in the direction of the current, fingers wrap around the wire, pointing in the direction of the magnetic field produced