Magnetic fields

• Which two sides of a magnet repel?

  N-N or S-S
• Which two sides of a magnet attract?

  N-S or S-N
• Are magnetic forces contact or non-contact forces?

  Non-contact
• Name some materials that are magnetic materials:

  • Iron
  • Steel
  • Nickel
  • Cobalt
• What is a permanent magnet?

  • Made from magnetic material, such as iron
  • Always causes a force on other magnetics or on magnetic materials
• State 2 key features of a permanent magnet:

  • Produces it's own magnetic field
  • The magnetic field cannot be turned on and off - it is there all the time
• State two examples of permanent magnets:

  • Bar magnets
  • Horseshoe magnets
• What is an induced magnet?

  • Only becomes a magnet when it is placed in a magnetic field
  • The induced magnetism is quickly lost when the magnet is removed from the magnetic field
• The iron filings become induced magnets when..

  They are near the bar magnet
• State 2 key features of induced magnets:

  • They are only attracted by other magnets, they are not repelled
  • They lost most or all of their magnetism when they are removed from the magnetic field
• A permanent magnet can:

  • Attract or repel another permanent magnet
  • Attract a magnetic material but cannot repel it
• A magnetic field can be detected by..

  • Using a magnetic compass
• The needle points in the magnetic compass to...

  • The direction of the Earth's magnetic field
  • The magnetic field of a magnet
• The closer the lines on a magnetic diagram...

  • The stronger the magnetic field
• On a magnetic diagram, the lines have arrowheads to show...

  • The direction of the force exerted by a magnetic north pole
• When current flows in a wire, it creates...

  A circular magnetic field
• The magnetic field can deflect..

  The needle of a magnetic compass
• The strength of the magnetic field is greater:

  • Closer to the wire
  • If the current is increased
• What is a solenoid?

  A straight coil of wire which can carry an electric current to create a magnetic field
• A solenoid consists of...

  A wire coiled up into a spiral shape
• When electric current flows in a solenoid, the shape of the magnetic field is..

  Similar to the magnetic field of a bar magnet
• The magnetic field inside a solenoid is...

  Strong and uniform
• What is a solenoid with an iron core called?

  An electromagnet
• The iron core increases the soldenoids...

  Magnetic field strength
• Where are electromagnets used?

  • Electric bells
  • Door locks (remotely controlled)
• What is the motor effect?

  • When a wire carrying a current carrying a magnetic field interacts with another magnetic field, causing a force that pushes the wire at right angles
• Describe how a coil turns anticlockwise in the motor effect

  • Current in the left hand part of the coil causes a downward force
  • The current in the right hand part of the coil causes an upwards force
  • This causes the coil to rotate anticlockwise
• What happens when the coil is vertical?

  • Moves parallel to the magnetic field, producing no force
  • This would tend to make the motor to stop, but the momentum of the motor carries carries it on a little and a split ring commutator changes the current direction in every half turn
• Once the conducting brushes reconnect with the commutator after a half turn:

  • Current flows in the opposite direction through the wire in the coil
  • Each side of the coil is now near the opposite magnetic pole
• What is a transformer?

  A device that can change the potential difference or voltage of an alternating current
• What does a step up transformer do?

  Increases the voltage
• What does a step down transformer do?

  Reduces the voltage
• What is a basic transformer made up of?

  Two coils of wire:

  • A primary coil from the alternating current input
  • A secondary coil leading to the alternating current output
• The coils are not electrically connected, but instead

  Wound around an iron core which is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil
• When a transformer is working:

  • Primary voltage drives an alternating current through the primary coil
  • The primary coil produces a magnetic field which changes as the current changes
  • The iron core increases the strength of the magnetic field
  • The changing magnetic field induces a changing potential difference in the secondary coil
  • The induced potential difference produces an alternating current in the external circuit
• In a dc motor, what is the function of the split ring commutator?

  It reverses the current direction every half turn to keep the motor spinning in the same direction all the time