Lesson 9

Faraday’s Law: Introduction to Electricity and Magnetism

  • Faraday’s Experiments
    • Early experiments revealed that while trying to use magnetic fields to generate current, currents were only produced momentarily when the magnetic field was switched on or off.
    • This led to the conclusion that it is the changing magnetic fields that induce currents.

Key Principles of Electromagnetism

  • Motional EMF

    • When a conductor moves in a magnetic field, it experiences a force on its positive charges, similar to the action of a battery that separates charges by chemical means.
    • This separation of charges in a moving conductor results in what is termed motional electromotive force (emf).

  • Direction of Forces

    • A current-carrying wire in a magnetic field experiences a force. To maintain constant speed, an external force must be applied to the conductor.

Magnetic Flux and Faraday’s Law

  • Magnetic Flux

    • The concept of magnetic flux is essential in electromagnetism, defined in a manner similar to electric flux. The unit of magnetic flux is the weber (Wb), where:
    • 1 ext{ Wb} = 1 ext{ Tm}^2

  • To calculate the magnetic flux through a loop, knowing the uniform magnetic field strength is crucial.

    • If the magnetic field is non-uniform, the flux must be calculated at each point and summed up.

  • Lenz’s Law

    • Indicates that an induced current will generate its magnetic field opposing the change in flux through a closed conducting loop.
    • Conditions for induced current:
    • If magnetic flux changes, it allows for the induced current to flow.
    • The direction relies on whether the flux is increasing or decreasing.
      • For decreasing flux, the induced field aligns with the applied field.
      • For increasing flux, the induced field opposes the applied field.

Practical Examples and Applications

  • Example Circuits
    • Problems are presented to find the speed required for a wire in a circuit to light a bulb, as well as determining the force needed to keep the wire moving against magnetic fields, thus integrating motional EMF into practical applications.
  • Current Induction
    • Case Studies:
    • When a switch is opened in a circuit that has been closed for a prolonged period, it discusses what happens in the resulting circuit configurations.
    • Demonstrates how current is induced when magnets move through a conductor (demonstration using copper pipes).

Advanced Concepts in Electricity Generation

  • Induced EMF and Current

    • Discusses the relationship between the induced emf and changing magnetic flux according to Faraday’s law, which is crucial for understanding electromagnetic induction.
    • For multiple loops, the total induced emf is the product of the number of turns and the emf generated per turn.

  • Eddy Currents

    • When a conductor moves through a magnetic field, the lack of wires in a conducting sheet can lead to small circular currents known as eddy currents, which show up as a form of resistive force or energy loss.

  • AC Generators

    • Discussion on the frequency needed to rotate a coil to induce AC voltage using Earth's magnetic field, showcasing practical electrical engineering applications.