Physics - magnetism and electromagnetism

• Fleming's left hand rule can be used to find the direction of either the field, current or movement as long as the other two are known
• Fleming's left hand rule

  1. Hold the left hand so that the thumb, first finger and second finger are all at right-angles to one another

  2. Align the first finger in the direction of the magnetic field, from north to south

  3. Rotate the wrist so that the second finger points along the wire in the direction the current is flowing

  4. The thumb will be pointing in the direction of the force, i.e. the direction the conductor would move
• A current carrying coil in a magnetic field will rotate because the current going up one side of the coil is in the opposite direction to the current coming back down the other side
• The brush contacts at the commutator ensure that the current direction in the coil is always in the same direction
• This ensures that the motor continues rotating and does not simply stop in the upright position
• Increasing the current or the magnetic field will make the motor rotate faster
• Reversing the current or the magnetic field will make the motor rotate in the opposite direction
• When a current carrying conductor is placed in a magnetic field, it experiences a force known as the motor effect
• The motor effect is caused by the field created by the current interacting with the magnetic field
• The force can be increased by increasing the size of the current, the length of conductor in the magnetic field, or the flux density
• Reversing the direction of either the current or the magnetic field will cause the direction of the force to reverse
• Permanent magnets produce their own magnetic field
• Induced magnets become a magnet when placed in a magnetic field, but lose their magnetism quickly when removed
• The force between a permanent magnet and a magnetic material or an induced magnet is always one of attraction
• The strength of the magnetic field depends on the distance from the magnet and is strongest at the poles
• Like poles repel, while unlike poles attract
• Induced magnets become a magnet when placed in a magnetic field
• When removed from the field, induced magnets lose their magnetism quickly
• The arrows on field lines run from the north pole to the south pole and show the direction of the force that would act on a north pole placed at that point
• Flux density


  The density of the field lines indicating the strength of the field at that point - the closer together the lines, the higher the flux density
• The higher the flux density


  The stronger the field and the greater the force that would be felt by another magnet
• Plotting Fields


  Using a magnetic compass to plot the field around a bar magnet
• A magnetic compass aligns with the Earth's magnetic field and always points to the magnetic north, providing evidence that the Earth's core must be magnetic
• Plotting Fields with a magnetic compass

  Place the bar magnet on a piece of paper, trace the compass at one end of the magnet, mark where the point of the compass needle is, move the compass so the tail of the needle is at the marked point, repeat and connect the marks until the full field is plotted
• Electromagnetism


  A magnetic field is produced around a wire when a current flows through it
• Right Hand Grip Method


  Method to determine the direction of the magnetic field lines around a wire by holding the wire in the right hand with the thumb pointing in the direction of the current
• Solenoid


  A coil of wire that forms a magnetic field when a current flows through it
• Shaping the wire into a solenoid


  To increase the magnetic field strength, the wire is shaped into a coil, creating an electromagnet
• The strength of the magnetic field around a solenoid is similar to that around a bar magnet
• Identifying the north pole of a solenoid

  Using the right hand grip method to determine the direction of the current flow in the solenoid to find the north pole
• Many devices use electromagnets, and interpreting diagrams can help understand how they work
• Operation of an electromagnet in an electric bell

  When the switch is pushed, the electromagnet is magnetized, attracting the armature which strikes the gong, breaking the circuit, and repeating the cycle
• Quick Test: The field lines around a magnet being close together indicates a strong magnet, sketching field lines around a current-carrying wire, and increasing the strength of an electromagnet