P2 magnetism and electromagnetism

• What is the shape of the magnetic field around a straight wire with current?

  Circular

  
• What rule determines the direction of current or magnetic field around a wire?

  The right hand grip rule

  
• What does the right hand grip rule indicate?

  • Thumb shows direction of current
  • Fist represents direction of magnetic field

  
• What is a solenoid?

  A coil of wire that becomes an electromagnet

  
• How can you strengthen the magnetic field in a solenoid?

  • Increase the current
  • Use more turns of wire
  • Use an iron core
• What is a transformer?

  A device with primary and secondary coils
• How does an alternating current in one circuit cause an alternating current in another in a transformer?

  • Two coils placed by an iron core
  • One coil produces a changing magnetic field
  • Magnetic field passes through the iron core
  • Induces current in the second coil
• In a step-down transformer, does the secondary coil have more or less turns?

  Fewer turns
• What is an advantage of a step-down transformer in the national grid?

  Reduces voltage to safe levels for homes
• Why are step-up transformers important in the national grid?

  They increase efficiency by reducing heat loss
• How does a step-up transformer work?

  • Primary coil has fewer turns than secondary
  • A.C. current connected to primary coil
  • Produces a magnetic field
  • Iron core transmits magnetic field to secondary coil
  • Induces higher voltage in secondary coil
• What equation links primary voltage, secondary voltage, and number of turns in coils?

  V_p/V_s = N_p/N_s

  
• Where are step-up and step-down transformers used in the national grid?

  • Step-up transformers at power stations
  • Step-down transformers used locally
• What is the power equation for transformers with 100% efficiency?

  P_p = P_s
• What are advantages of transmitting power in high voltage cables?

  • Lower current reduces energy loss
  • Increases efficiency due to P=I²R