Electromagnetism

• When electric current flows through a long straight wire, a magnetic field is produced which rotates in circles around the wire.

  This can be demonstrated with plotting compasses, or with the right hand grip
• When electric current flows through a long straight wire, a magnetic field is produced which rotates in circles around the wire.

  • The larger the current, the stronger the field.
  • The closer you go to the wire, the stronger the field will be.
• Describe an investigation to find the shape and direction of the magnetic field around a long straight wire.

  Place a wire through a piece of paper

  Run a current through the wire

  Place a plotting compass around the wire, and mark the direction it points

  Repeat this step, until all plotting compass lines connect
• Most electromagnets are made by passing current through a coil of wire, called a solenoid.
• Outside the solenoid, the fields cancel out to make a shape similar to a bar magnet.

  Inside the solenoid the fields add up to make a strong uniform magnetic field.
• To make an electromagnet:

  1. Wrap a coil of wire
  2. around an iron core
  3. Pass a current through the wire
• A moving charged particle produces a circular magnetic field.

  If this field is placed perpendicular in the uniform magnetic field of a permanent magnet, the two fields interact, putting a sideway force on the wire and the magnet.
• If a current carrying wire is placed at 90 degrees to the field between two magnets, the wire will experience a force.

  The force will be at 90 degrees to the current and 90 degrees to the field of the magnets.

  The direction of the force can be found using 'Flemming's left hand rule'.
• Current carrying wire in a magnetic field experiences a force.

  • Increase magnitude of current, increase magnetic field strength of wire, increase magnitude of force on wire.
  • Reverse direction of current, reverse direction field of wire, reverse direction of force on wire.
• A wire carrying current produces a circular magnetic field.

  If this field is placed in the uniform magnetic field of a permanent magnet, the two fields interact.

  There is a force on the opposite sides of the coil in opposite directions.

  The coil starts to rotate.
• Speaker

  1. current in coil produces magnetic field.
  2. field of coil interacts with field of magnet.
  3. producing a force on cone.
  4. direction of current is constantly changing because current is a.c.
  5. direction of this magnetic field is (continuously) changing;
  6. direction of the force changes;
  7. cone vibrates;
  8. air particles (next to the cone) are made to vibrate;
• Describe the construction of an electromagnet.

  Wrap a coil of wire around an iron core.

  Pass a current through the wire.
• Explain why a force is exerted on the current-carrying wire.

  A moving charged particle produces a circular magnetic field.

  If this field is placed perpendicular in the uniform magnetic field of a permanent magnet, the two fields interact, putting a sideway force on the wire and the magnet.
• Explain the motion of the coil of wire

  A wire carrying current produces a circular magnetic field.

  If this field is placed in the uniform magnetic field of a permanent magnet, the two fields interact.

  There is a force on the opposite sides of the coil in opposite directions.

  The coil starts to rotate.

  
• Explain how the loudspeaker produces a sound wave.

  Current in coil produces magnetic field.

  Field of coil interacts with field of magnet.

  Producing a force on cone.

  Direction of current is constantly changing because current is AC.

  Direction of this magnetic field in continuously changing.

  Direction of the force changes.

  Cone vibrates.

  Air particles next to the cone are made to vibrate.
• Describe an investigation that could be used to determine the shape of the magnetic field around a bar magnet. [3]

  Plotting compass

  Repeated use

  Joining up dots
• The north pole of one magnet will repel the north pole of another magnet.

  There is attraction between steel and magnets.

  Materials that are difficult to magnetise are called hard magnetic materials.

  The direction of the magnetic field lines for a magnet is from north to south.

  Iron is a soft magnetic material.
• There are two identical metal rods placed inside a coil of wire.

  When a direct current is supplied to the coil, a magnetic field forms and the metal rods move apart. Explain why. [2]

  rods magnetised

  and repel
• When the current is switched off, the metal rods return to their starting places. Suggest what material the metal rods are made from. Explain your answer. [3]

  iron

  because the material is capable of being magnetised

  does not retain its magnetism
• The direct current in the coil is replaced by an alternating current. Explain what will happen to the magnetic field. [2]

  field switches direction

  field weaker

  field alternates with current at 50 Hz

  rods may not have time to become fully magnetised
• Describe a method that the student could use to show that a magnetic field is produced by the current in the wire. [2]

  plotting compass

  moved to new position
• Describe the construction of an electromagnet. [3]

  current carrying wire

  wrapped into coil

  wrapped on iron core
• Describe the changes that allow the electromagnet to release the door when the fire alarm sounds. [2]

  current/voltage reduces

  magnetic field reduces

  force holding the iron plate to the magnet no longer present
• Two coils are suspended vertically.

  When DC passes, the coils move together.

  When the current in the right-hand coil is reversed, the two coils move apart.

  Explain why the coils move in this way. [3]

  magnetic fields interacting

  attraction/repulsion

  reversing current reverses direction of magnetic field/force

  like poles repel, unlike poles attract
• A solenoid with a steel core can be used as an electromagnet.

  When the current is switched on, the electromagnet picks up some steel paper clips.

  Explain why the steel paper slips remain attached to the steel core when the current is switched off. [2]

  Steel is magnetically hard

  Steel becomes permanently magnetised

  Steel remains magnetised/paper clips remain attracted to steel
• Describe the difference between a.c. and d.c. [2]

  a.c. current changes direction frequently/continuously/repeatedly/50 Hz

  d.c. current is in one direction only
• Explain why the loudspeaker does not work when the coil is connected to a battery.

  battery supplies constant current

  no vibration