Physics assessment 2

• Permanent and Induced Magnets
• Poles of a magnet

  North seeking (or north) and south seeking (or south)
• Magnets produce magnetic fields
• The stronger the magnet, the stronger the magnetic field
• The magnetic field is strongest at the poles of a magnet
• The force between opposite poles of a magnet is attractive, no matter the pole
• If the poles of a magnet are put on each other they will each exert a force on each other
• Like poles will repel each other, opposite poles will attract each other
• Compasses show the directions of magnetic fields

  1. The north pole of the magnet is attracted to the south pole of any other magnet it is near

  2. You can move a compass around a magnet and trace the field lines on some paper to build up a picture of the magnetic field

  3. When they're not near a magnet, compasses always point north, because the Earth generates its own magnetic field
• Permanent magnets


  Produce their own magnetic field
• Induced magnets


  • Magnetic materials that turn into magnets when they're put into a magnetic field
  • The force between permanent and induced magnets is always attractive
  • When you take away the magnetic field, induced magnets quickly lose their magnetism (or most of it) and stop producing a magnetic field
• Charge creates a magnetic field
• The magnetic field produced changes with the current and the distance from the wire
• Solenoid


  A coil of wire that creates a strong and uniform magnetic field
• An iron core in a solenoid becomes an induced magnet whenever current is flowing through the coil
• A solenoid with an iron core (an electromagnet) can be turned on and off with an electric current
• Uses of magnets


  • Alarms
  • Trains
  • Cranes to attract and pick up magnetic materials
  • Electric starters of motors
• Current in a magnetic field experiences a force (the motor effect)
• Fleming's left-hand rule


  Used to determine the direction of the force on a current-carrying conductor in a magnetic field
• Electric motors and loudspeakers work because of the motor effect
• How an electric motor works

  1. Carrying coil of wire rotates in a magnetic field

  2. The commutator reverses the contacts every half turn to keep the motor rotating in the same direction

  3. The direction of the motor can be reversed by reversing the polarity of the supply or swapping the magnetic poles over
• How a loudspeaker works

  1. An alternating current is sent through a coil of wire attached to the base of a paper cone

  2. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the current causes a force on the coil (which causes the cone to move)

  3. When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction too

  4. Variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates sound waves
• Cutting magnetic field lines induces a potential difference (the generator effect)
• The induced current opposes the change that made it
• The size of the induced potential difference can be changed by changing the rate of change of the magnetic field
• How an AC generator works

  1. The coil on the generator is rotated in the magnetic field, inducing an alternating potential difference

  2. The slip rings on the generator reverse the contacts every half turn to produce an alternating current
• How a DC generator (dynamo) works

  1. The coil on the dynamo is rotated in the magnetic field, inducing a potential difference

  2. The split-ring commutator reverses the contacts every half turn to keep the current flowing in the same direction, producing direct current
• An oscilloscope can be used to see the generated potential difference
• Alternating Current


  Current that changes direction periodically
• Alternators generate alternating current
• Dynamos generate direct current
• Dynamo operation


  1. Have split-ring commutator

  2. Commutator reverses current every half turn to keep current flowing in the same direction
• Oscilloscope


  Device used to see the generated potential difference
• Observing AC waveform on oscilloscope

  1. Waveform goes up and down, crossing the horizontal axis

  2. For DC, the waveform stays above the axis
• Increasing revolution frequency


  Increases the overall potential difference but also creates more peaks
• Microphone


  • Basically a loudspeaker in reverse
  • Sound waves hit a flexible diaphragm attached to a coil of wire wrapped around a magnet
  • Coil movement in the magnetic field generates a current
  • Current depends on properties of sound wave
• Transformer


  • Changes the potential difference, but only for alternating current
  • Has primary and secondary coils wrapped around an iron core
  • Turns ratio determines step-up or step-down
• Transformers are not 100% efficient
• High potential difference and low current is more efficient for power transmission in the national grid
• Equation P=IV can be used to explain why high potential difference and low current is more efficient