electromagneTISM

• Magnets = any material or object that produces a magnetic field 
• All magnets have two poles: north and south. They’re surrounded by a magnetic field. You can find the north pole by using a magnet, which will always be attracted to the north pole because it always lines up with whichever magnetic field it was put in. 
• The only 3 magnetic elements:

  • Iron 
  • Cobalt 
  • Nickel 
• Induced Magnets = only have a magnetic field temporarily

  This happens when a permanent magnet lines up all the material’s domains to be pointing the same way  
• Nickel and Iron lose their magnetism really quickly - they’re called ‘magnetically soft’ 

  Steel is the opposite, know as ‘magnetically hard’ 
• Electric currents produce their own magnetic field 
• A straight wire with a current running through it would have concentric circle lines around the wire, closest together near the wire 
• The magnetic field of a solenoid is strong and uniform 

  A solenoid is an electromagnet - they’re only magnetic as long as you keep the current flowing, so they can be turned off. 

  You can reverse the direction of the magnetic field in a solenoid by reversing the direction that the current is flowing.
• The Motor Effect = a current carrying wire in the presence of a magnetic field will experience a force
• How to increase the strength of the magnetic field of an electromagnet:

  • Increase current that flows through the solenoid 
  • Increase number of coils in a solenoid whilst keeping the length the same
  •  Decrease length whilst keeping the same amount of coils 
  • Add an iron core to the inside of the solenoid 
• Fleming’s left-hand rule:

  • Take your left hand, point your thumb up, 1st finger forwards, 2nd finger to the side 
  • 1st finger represents direction of the magnetic field, so point it from the north to south pole 
  • Place 2nd finger in the direction of the current 
  • Whichever way your thumb then points, will be the direction of the force 
• When you put a current-carrying wire in between the north and south poles of two magnets (with their own magnetic field), then the two magnetic fields will interact.   This results in a force on the wire and will push it out of the field.

  To experience the full force, the wire has to be perpendicular to the magnetic field. 
• The left hand rule can be applied to a horseshoe magnet 
• Force = Magnetic flux density x current x length of wire 

  Newtons = Teslas x amps x metres 
• When you place a coil in between two north and south poles, it will rotate due to the direction of the force at the positive terminal is opposite to the direction of the fore at the negative terminal - this only turns 180 degrees then it flips 
• To make the coil turn 36 degrees, you need to change the direction of the current every half turn, so the direction of the forces will always be acting clockwise.
• To achieve a 360 degree coil turn, you use a split-ring commutator - it swaps the positive and negative connections every half turn, so the direction of the current swaps every half turn too. Therefore the forces acting on the coil will always be acting in the same direction 
• How to increase speed of rotation in an electric motor:

  • Increase current 
  • Add more turns to coil
  • Increase magnetic flux density (use more powerful magnets)
• Generator effect = electromagnetic induction 
• When you move a metal coil in between a magnetic field, it’ll induce a potential difference in the wire.

  Whenever the wire stops moving, the potential difference disappears 

  Direction of the potential difference swaps each time the coil changes direction

  To generate a current, you induce a potential difference 
• How to change the size of an induced potential difference:

  • Change strength of magnetic field (stronger magnets), produces larger potential difference
  • Move wire more quickly,makes a faster magnetic field change 
  • Shape coil into it having more turns 
• The generator effect will still work if you have a coil and just move one magnetic in and out of it. 
• Generators = generate electricity from rotational motion (e.g rotating a wire) 

  Alternators and dynamos are different types of generators 
• Dynamos have a split-ring commutator

  Alternators have slip rings and brushes 
• Dynamos produce a Direct current

  Alternators produce an Alternating current 
• How alternators work:

  • Coil of the wire rotates relative to the magnets, inducing a magnetic field in the coil, then a voltage and current 
  • The slip ring and brushes mean that the contacts don't swap every half turn (like a motor or dynamo)
  • This means they produce an alternating current 
  • As the coil rotates faster, the oscillations get larger and more frequent 
• How dynamos work:

  • Coil of the wire spins relative to the magnets, inducing a magnetic field, voltage and current 
  • The split ring commutator means that the contacts swap every half turn 
  • This means they produce a direct current because the current will always be flowing in the same direction 
  • As the coil rotates faster, the oscillations get larger and more frequent 
• Loudspeakers take electrical signals and use the motor effect to convert them into vibration, then generate sound waves. 

  Microphones detect vibrations caused by sound waves and use the generator effect to convert them into electrical signals.

  They both involve alternating currents. 
• Loudspeakers and microphones both convert between sound waves and electrical signals 
• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet and connected to a cone. The coil of wire is permanently attached to the cone, but the coil and cone can slide back and forth along the magnet. 
• How loudspeakers work:

  • Alternating current flows through the wire, generating a magnetic field in the coil 
  • The magnetic field of the coil interacts with the magnetic field of the permanent magnet  
  • The magnetic fields will exert a force on the coil, causing it to move back and forth along the magnet 
  • This causes the cone to change shape 
  • As the current is alternating, the magnetic field and its force will also alternate - they vibrate 
  • The vibrations cause pressure variations in the air (sound)
• Microphones have a similar structure to loudspeakers, but they detect sound. 
• How microphones work:

  • Sound waves hit the diaphragm, causing it and the coil of the wire to move 
  • As the wire is moving within the magnetic field of the permanent magnet, it’ll generate a current 
  • Frequency and amplitudes of the sound waves determine how much the diaphragm vibrates, therefore determining the frequency + amplitude of the current 
• How transformers work:

  • An alternating potential difference is produce along the primary coil, causing a current to flow (generating a magnetic field around it, which will also alternate) 
  • The alternating magnetic field (primary coil) induces an alternating magnetic field in the iron core 
  • This induces a potential difference in the secondary coil, if its a part of a full circuit, a current will flow 
• The way transformers work is to allow a change in size of the potential difference between the two coils (step up and down transformers) 
• Voltage of primary coil / voltage or secondary coil = number of turns in primary coil / numbers of turns in secondary coil 
• Voltage of primary coil x current of primary coil = voltage of secondary coil x current of secondary coil