Electromagnetism (pg. 59-64)

• All magnets have two poles - north and south.
• A magnetic field is a region where magnetic materials experience a force.
• Magnetic field lines (or "lines of force") are used to show the size and direction of magnetic fields. They always point from north to south.
• Placing the north and south poles of two permanent bar magnets near each other creates a uniform fied between the two magnets.
• Compasses and iron filings align themselves with magnetic fields.
• You can use multiple compasses to see the magnetic field lines coming out of a bar magnet or between two bar magnets.
• You could use iron filings to see magnetic field patterns. Put the magnet(s) under a piece of paper, scatter the iron filings on top, and tap the paper until the iron filings form a clear pattern.
• Magnets affect magnetic materials and other magnets.
• Like poles repel each other and opposite poles attract.
• Both poles attract magnetic materials.
• When magnetic materials are brought near a magnet (into its magnetic field, that material acts as a magnet. This magnetism has been induced by the original magnet.
• The closer the magnet and the magnetic material get, the stronger the induced magnetism will be.
• A current-carrying wire creates a magnetic field. The larger the electric current, the stronger the magnetic field.
• The direction of the magnetic field depends on the direction of the current.
• The magnetic field around a straight, current-carrying wire is made up of concentric circles with the wire in the centre.
• The magnetic field in the centre of a flat circular coil of wire is similar to that of a bar magnet. There are concentric ellipses of magnetic field lines around the coil.
• The magnetic field inside a current-carrying solenoid (a coil of wire) is strong and uniform. Outside the coil, the field is just like the one around a bar magnet: the ends of the solenoid act like the north and south pole. This type of magnet is called an electromagnet.
• A magnetic material is considered 'soft' if it loses its induced magnetism quickly, or 'hard' if it keeps them permanently.
• Iron is a soft magnetic material.
• Steel is a hard magnetic material.
• Iron is used in transformers because it is a hard material. It needs to magnetise and demagnetise 50 tines a second (50 Hz).
• You can increase the strength of the magnetic field around a solenoid by adding a magnetically 'soft' iron core through the middle of the coil.
• When a current-carrying wire is put between magnetic poles, the two magnetic fields affect one another. The result is a force on the wire, which can cause the wire to move. This is called the motor effect.
• The motor effect happens because charged particles moving through a magnetic field will experience a force, as long as they're not moving parallel to the field lines.
• To experience the full force in the motor effect, the wire has to be at 90º to the magnetic field.
• The motor effect experiment:

  1. Apply a current to a set of rails inside a horseshoe magnet.
  2. Place a bar on the rails to complete the circuit.
  3. This generates a force that rolls the bar along the rails.
  4. Reversing the current or the magnetic field reverses the direction of the force.
• Fleming's left hand rule for motors:

  1. ThuMb. Motion.
  2. First finger. Field.
  3. SeCond finger. Current.
• 4 factors which speed up a simple d.c. electric motor:

  1. More current.
  2. More turns on the coil.
  3. Stronger magnetic field.
  4. A soft iron core in the coil.
• In a simple d.c. motor, the split-ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction.
• The direction of a simple d.c. electric motor can be reversed either by swapping the polarity of the d.c. supply or swapping the magnetic poles over.
• Loudspeakers work because of the motor effect.
• The functioning of loudspeakers:

  1. A.C. electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone.
  2. The coil is surrounded by a permanent magnet, so the A.C. signals cause a force on the coil and make it move back and forth.
  3. These movements make the cone vibrate and this creates sounds.
• Electromagnetic induction is the creation of a voltage (and maybe current) in a wire which is experiencing a change in magnetic field.
• Using electromagnetic induction to generate eletricity using energy from kinetic energy stores is called the dynamo effect.
• There are two different situtations where you get EM induction:

  1. An electrical conductor (a coil of wire) moves through a magnetic field.
  2. The magnetic field through an electrical conductor changes.
• You can test the dynamo effect by connecting an ammeter to a conductor and moving the conductor through a magnetic field (or moving a magnet through the conductor). The ammeter will show the magnitude and direction of the induced current.
• To get a bigger voltage in EM induction, you can increase...

  1. the strength of the magnet.
  2. the number of turns on the coil.
  3. the speed of movement.
• Generators rotate a coil in a magnetic field (or a magnet in a coil).
• How a generator works:

  1. As the coil spins, a current is induced in the coil. This current changes direction every half turn.
  2. A.C. generators have slip rings and brushes so the contacts don't swap every half turn.
  3. Faster revolutions produce not only more peaks but higher overall voltage too.
• Power stations use a.c. generators to produce electricity.