Topic 13 - Electromagnetic induction.

• What is the definition of electromagnetic induction?

  The induction of potential difference (and current if there is a complete circuit) in a wire which is experiencing a change in magnetic field).
• Electromagnetic induction can be represented by a bar magnet passing through a coil - creating a potential difference (voltage).
• Electromagnetic induction can be seen by replacing the moving wire with a spinning coil An a.c (alternating current) generator looks very similar to a motor, but instead of connecting it to a power supply, a coil is spun to produce electricity As the coil rotates, it cuts through the field lines This induces a potential difference between the end of the coil, which also then creates a current The electricity produce is in the form of a.c which can be found in the mains supply of a building This is also called an alternator.
• What are the factors that affect the size and direction of an induced potential difference?

  Factors: Magnetic field strength, velocity of the conductor, length of the conductor, angle between the conductor and magnetic field.
• How do Dynamos generate electric current?

  1)Generators apply a force to rotate a coil in a magnetic field (or a magnet in a coil) -their construction is a lot like a motor. 2) As the coil (or magnet) spins, a current is induced in the coil. This current changes direction every half turn. 3)Dynamos are d.c. generators. They have a split-ring commutator (like a d.c. motor). 4)This swaps the connection every half-turn to keep the current flowing in the same direction.
• How do Alternators generate electric current?

  1)Alternators work in the same way as dynamos, apart from one important difference.2) Instead of a split-ring commutator, a.c. generators have slip rings and brushes so the contacts don't swap every half turn.3) This means an alternator produces an alternating p.d. and therefore an alternating current (a.c.) if the coil is part of a complete circuit.
• How does a microphone generate an electric current?

  • A microphone uses electromagnetic induction to turn sound into an electrical signal.
  • A flexible diaphragm, linked to a wire coil, faces a permanent magnet with one pole inside and the other outside.
  • When sound hits the diaphragm, it moves the coil, inducing an electric current as it interacts with the magnet.
  • Louder sounds make the diaphragm move more, causing variations in the generated current.
  • Microphones translate pressure changes in sound waves into electrical current variations in a circuit.
• How does a loudspeaker work?

  • Loudspeaker: Diaphragm replaced with paper cone.
  • The coil wrapped around one pole of a permanent magnet.
  • A.C. signal causes a force on the coil.
  • Coil movement makes the cone vibrate.
  • Reversing the current changes the force direction.
  • The vibrating cone generates variations in air pressure.
  • Variations in air pressure create sound waves.
• Explain how an alternating current in one circuit can induce a current in another circuit in a transformer:

  • Transformers use induction for changing AC potential differences.
  • Two coils: primary and secondary, are connected by an iron core.
  • Application of AC potential difference across primary produces alternating magnetic field.
  • Iron core easily magnetized and demagnetized, responding to the changing magnetic field.
  • Alternating magnetization in the core induces a potential difference in the secondary coil.
• A transformer can change the size of an alternating voltage.
• What are the types of transformers?

  STEP-UP TRANSFORMERS step the potential difference up (i.e. increase it). They have more turns on the secondary coil than the primary coil. STEP-DOWN TRANSFORMERS step the potential difference down (i.e. decrease it). They have more turns on the primary coil than the secondary.
• What is the equation for transformers to calculate either the missing voltage or the missing number of turns?

  Vp/Vs = Np/Ns.

  
• The National Grid:

  • High Voltage Transmission:
  • Minimizes current in transmission lines.
  • Reduces heat loss according to Ohm's Law (P = I²R).
  • Efficiency Improvement:
  • Lower current decreases resistance-related heat loss.
  • Enhances overall energy transfer efficiency.
  • Power Loss Considerations:
  • Power loss in transmission lines is proportional to I²R.
  • High voltage, and low current minimizes power loss.
  • Voltage Reduction at Localities:
  • Electricity transformed to lower voltages locally.
  • Enhances safety for domestic use.
  • Minimizes the risk of electrical accidents.
• In a transformer, the input power = to the output power:

  Vp * Ip = Vs * Is.