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HSC Physics

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  1. Module 1: Kinematics
    1.1 Motion in a Straight Line
  2. 1.2 Motion on a Plane
  3. Module 2: Dynamics
    2.1 Forces
  4. 2.2 Forces, Acceleration and Energy
  5. 2.3 Momentum, Energy and Simple Systems
  6. Module 3: Waves and Thermodynamics
    3.1 Wave Properties
  7. 3.2 Wave Behaviour
  8. 3.3 Sound Waves
  9. 3.4 Ray Model of Light
  10. 3.5 Thermodynamics
  11. Module 4: Electricity and Magnetism
    4.1 Electrostatics
  12. 4.2 Electric Circuits
  13. 4.3 Magnetism
  14. Module 5: Advanced Mechanics
    5.1 Projectile Motion
  15. 5.2 Circular Motion
  16. 5.3 Motion in Gravitational Fields
    2 Topics
  17. Module 6: Electromagnetism
    6.1 Charged Particles, Conductors and Electric and Magnetic Fields
  18. 6.2 The Motor Effect
    1 Topic
  19. 6.3 Electromagnetic Induction
  20. 6.4 Applications of the Motor Effect
    1 Topic
  21. Module 7: The Nature of Light
    7.1 Electromagnetic Spectrum
    3 Topics
  22. 7.2 Light: Wave Model
  23. 7.3 Light: Quantum Model
    2 Topics
  24. 7.4 Light and Special Relativity
  25. Module 8: From the Universe to the Atom
    8.1 Origins of the Elements
    5 Topics
  26. 8.2 Structure of the Atom
    3 Topics
  27. 8.3 Quantum Mechanical Nature of the Atom
    2 Topics
  28. 8.4 Properties of the Nucleus
    2 Topics
  29. 8.5 Deep Inside the Atom
    4 Topics
Lesson 20, Topic 1
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Back EMF

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The coil(s) in an electric motor will experience a retarding emf due to the change in flux as they move though the magnetic field. This phenomenon is known as back emf and is an application of the Lenz’s law.

Magnetic flux over time for a simple electric motor
  • The flux through the coil changes over time (see above)
  • Back emf is produced while the motor coil is rotating
  • It always opposes the supply emf, but can never exceed it

Effect on Current

  • Net emf is the difference between the supply and back emf, and is directly proportional to the current in the rotor coil
  • Back emf is directly proportional to the rotational speed of the rotor coil (motor speed), i.e. \text E_{back} = -\frac{\Delta \Phi}{\Delta t}\propto v

The effect of back emf on current and torque is that:

  • There is no back emf when a motor starts (no rotation), so total emf is at maximum. Torque is also at maximum.
  • As the coil rotates faster, the back emf rises, reducing the total current and thus the torque.
  • When rotating at max speed, back emf is at a maximum and total current flowing is at a minimum. Hence the torque is at minimum.