Magnetism and Electromagnetism

• A permanent magnet


  produces its own magnetic field
• An induced magnet 


  an object that becomes a magnet when placed in a magnetic field
• Poles of a magnet is where the magnetic forces are…


  strongest
• What is attraction and repulsion

  • attraction - two unlike poles attract each other (NORTH AND SOUTH)
  • repulsion - two like poles repel each other (NORTH AND NORTH)

  these are examples of non contact forces
• How to plot the magnetic field pattern of a magnet using a compass

  1. place a compass near the north pole of magnet
  2. move compass so south pole of the compass is on the cross. again draw another cross at north pole of compass.
  3. continue doing this until there’s a complete magnetic field line. connect the dots with a line.
  4. draw an arrow to show direction of field (NORTH TO SOUTH)
  5. repeat starting at different points around the north pole of magnet
• How is the behaviour of a magnetic compass related to evidence that the core of the earth must be magnetic

  • the core is magnetic and creates a magnetic field around the earth
  • if we had a compass away from any magnets, the needle would always point North-South direction
  • this tells us that the earth has its own magnetic field, due to its core
• Ferromagnetic materials

  • iron
  • steel
  • cobalt
  • nickel
• Properties of a solenoid

  strong and uniform

  (solenoid is a coil of wire)
• Explain how shaping a wire into a solenoid affects the strength of the magnetic field produced around the wire

  it increases the size of the magnetic field
• How can we increase strength of magnetic field produced by a solenoid

  • increase turns of coil
  • increase current
  • place a block of iron inside the magnet
• What is the Right Hand Rule


  shows relationship between direction of current in a wire and direction of magnetic field lines. only works in DC current
• Describe how a magnetic field is produced around a wire

  • when a current flows through a conducting wire, a magnetic field is produced
  • the strength of the magnetic field depends on current through the wire and distance from the wire
• What is the motor effect

  when a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor, exert a force on each other
• Flemings Left Hand Rule

  • place your thumb, index and middle finger at right angles
  • point index finger in direction of magnetic field from North to South
  • point middle finger in direction of current (+ to -)
  • direction of your thumb is the direction of the force
• Factors that affect the force on a conductor

  • current
  • length
  • magnetic flux density

  F=B x I x L is an easier of remembering
• Force equation


  F = B x I x L

  force= magnetic flux density x current x length

  newtons= tesla x amps x metres
• How does the force on the conductor in a magnetic field cause the rotation of the coil in an electric motor

  when a current is passed through the coil, the coil spins because the force on the left hand side acts upwards and the force on the right hand side acts downwards
• How do motors work

  1. the coil turns clockwise because of the forces on the two halves
  2. as the coil turns, the wires slip around of the commutator
  3. when the coil is in a vertical position, there’s no force on it, but its momentum carries it forward a little
  4. the wires make contact with the other halves of the ring which causes the current to flip
  5. the forces therefore flip causing the motion of the wire to continue in the same direction
  6. the cycle repeats when the coil is vertical once more
• Equation for weight (force)

  W= M x G

  weight = mass x gravitational field strength
• Explain why a coil rotates when there is a current in the coil

  • there is a magnetic field due to the permanent magnet and current in the wire causes a magnetic field
  • current is in opposite directions in each side of coil
  • so forces act in opposite directions on either side of the coil
  • the split ring ensures that the current in left hand side is always in same direction