Section 6- Magnetism and Electromagnetism

Magnets and Magnetic Fields
1. Repel
2. Attract
  1. Placing the north and south poles of 2 permanent bar magnets creates a uniform field.
    1. When a magnet is moved closer to a magnetic material, the material becomes induced by the magnet and gets induced poles.
3. Flux lines
  1. Field direction+strength
  2. Always NORTH to SOUTH
  3. Experiments
Electromagnetism
1. An electric current in a material produces a magnetic field around it.
2. Larger current= stronger magnetic field
3. Direction of field depends on current direction
4. Solenoid= current-carrying coil of wire
5. Magnetically soft= material that loses magnetism quickly
  1. Magnetically hard= material that can permanently hold magnetism
6. Right hand rule
  1. Flat coil of wire
  2. Wire
The Motor Effect
1. Flemming's Left Hand Rule
2. A current carrying wire between magnetic poles experiences a force because of the two magnetic fields interacting. This can cause the wire to move.
  1. This is because charged particles moving through a magnetic field will experience a force so long as they're not moving parallel to the field lines.
3. wire 90 degrees to magnetic field= full force experience
  1. Parallel= no force
    1. In between= some force
4. Force strength increases with magnetic field strength and amount of current
  1. Reverse current direction or magnetic field to reverse force direction
5. Loudspeakers use the motor effect
  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. The coil is surrounded by a permanent magnet so the a.c signals cause a force on the coil making it move back and forth. These movements make the cone vibrate and creates sound.
Electric Motors
1. Speeding up a DC motor
  1. Increase current
  2. More turns in coil
  3. Stronger magnetic field
  4. Soft iron core in the coil
2. Forces act on the sides or the coil (usual forces from current in magnetic field). The coil is on a spindle and one arm receives up force, one down therefore it spins. However, it can only rotate halfway without a split-ring commutator which swaps the contacts every half turn to ensure the motor rotates in the same direction continually.
3. Reverse direction by swapping polarity of the d.c. supply or swap magnetic poles over.
4. Can use left hand rule to work out which way coil will turn.
Electromagnetic Induction
1. The creation of a voltage (and so maybe a current) in a wire which is experiencing a change in magnetic field.
2. The dynamo effect- move the wire or the magnet (kinetic energy-->electrical energy)
  1. Electrical conductor moves through a magnetic field (usually a coil of wire)
  2. The magnetic field through an electrical conductor changes (bigger, smaller or reverses).
3. Can test using ammeter
4. Direction of movement reversed= induced voltage/current reversed too.
5. To increase voltage you can increase the magnet strength, number of turns in the coil or speed of the movement.
6. A.c. generators
  1. A motor has a current in the wire and a magnetic field which causes movement. Whereas a generator has a magnetic field and movement which induces a current.
    1. As the coil spins a current is induced in the coil, this current changes direction every half turn. Instead of a split-ring commutator, a.c. generators have slip rings and brushes so the contacts don't swap every half turn. This means they produce a.c. voltage- faster revolutions= more peaks and higher voltage overall.
Transformers
1. Change the voltage size of an alternating current
2. They all have 2 coils, the primary and the secondary, joined with an iron core.
  1. when an alternating voltage goes across the primary coil, the iron becomes magnetised then demagnetised quickly to induce an alternating voltage in the secondary coil. The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils.
3. Step-up transformers step the voltage up by having more turns on the secondary coil than the primary coil.
  1. Step-down transformers step the voltage down by having more turns on the primary coil than the secondary coil.
4. Nearly 100% efficient so power in= power out

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Section 6- Magnetism and Electromagnetism

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