Electromagnetic Induction Notes

Magnets, Magnetic Fields, and Electromagnetic Induction

Magnets

  • A magnet is an object exhibiting magnetic properties, including:
    • Exerting an attractive force on iron or other ferromagnetic materials.
    • Exerting both attractive and repulsive forces on other magnets.

Magnetic Poles

  • Magnets exert forces on one another, attracting or repelling based on proximity.
  • Electric charges produce electrical forces, while magnetic poles produce magnetic forces.
  • Types of magnetic poles:
    • North Pole
    • South Pole
  • Like poles repel each other; opposite poles attract.

Isolation of Magnetic Poles

  • Electric charges can be isolated, but magnetic poles cannot.
  • A north pole cannot exist without a south pole.

Magnetic Field

  • Magnetic Field: The space around a magnet where a magnetic force is exerted.
  • The shape of the magnetic field is revealed by magnetic field lines.

Magnetic Field Lines

  • Magnetic field lines run from the north pole to the south pole.
  • Unlike poles attract, and like poles repel.

What Creates a Magnetic Field?

  • Albert Einstein explained that moving charges have both an electric and magnetic field in his theory of special relativity (1905).
  • Electrons in atoms are in constant motion around the nuclei and spin around their axis.
  • Every spinning electron acts as a tiny magnet.
  • Electrons spinning in the same direction create a stronger magnet.
  • Electrons spinning in opposite directions work against each other.
  • Most atoms have electrons that cancel each other out.
  • Iron (Fe) has four electrons that spin in the same direction, making it a strong magnetic atom.

Magnetic Domains

  • Magnetic domains are clusters of aligned atoms.
  • In bulk material, domains usually cancel, leaving the material unmagnetized.
  • When an external magnetic field is applied, iron will become magnetized in the direction of the applied field.
  • This magnetization produces a magnetic pole in the iron opposite the pole nearest to it, causing attraction.

Earth's Magnetic Field

  • Earth's magnetic field is crucial for life on Earth.
  • It protects us from the sun's solar winds.

Creation of Earth’s Magnetic Field

  • Flowing metals (iron and nickel) in the outer core create large magnetized domains that generate a magnetic field.
  • This can be considered an electric current.

Electric Currents and Magnetic Fields

  • A moving charge produces a magnetic field.
  • Many moving charges (electric current) produce a stronger magnetic field.

Curl Right Hand Rule (Right-hand Thumb Rule)

  • Thumb points along the direction of the current.
  • Other fingers give the direction of the magnetic field.

Electromagnet

  • Looping the wire concentrates the magnetic field.
  • Increasing the number of loops increases the magnetic field intensity.
  • Electromagnet: A current-carrying coil of wire with many loops.

Tips for Drawing Currents and Field Lines

  • Current coming out of the page (toward you): Represented by a dot, symbolizing the arrow tip.
  • Current going into the page (away from you): Represented by a cross, symbolizing the arrow's tail-feathers.
  • The first circle drawn represents the wire's rim.
  • Flux pattern around a current-carrying wire.

Magnetic Forces on Moving Charged Particles

  • A charged particle at rest will NOT interact with a static magnetic field.
  • A moving charged particle in a magnetic field will interact and experience a deflecting force as it crosses the magnetic field.
  • This effect deflects charged particles from outer space around Earth’s magnetic field.

Magnetic Forces on Current-Carrying Wires

  • If a charged particle experiences a force when passing across a magnetic field, a current will experience the same force.
  • The direction of the force depends on the direction of the current and the magnetic field.

Other Right Hand Rule

  • Thumb: Direction of Current
  • Fingers: Direction of the Magnetic Field
  • Palm: Direction of the Force

Parallel Current-Carrying Wires

  • Two parallel current-carrying wires exert forces on each other.

Motors

  • Electric motors use rotating coils of wire driven by the magnetic force exerted by a magnetic field on an electric current.
  • They transform electrical energy into mechanical energy.
  • An electric current in a magnetic field will experience a force.
  • If the current-carrying wire is bent into a loop, the two sides at right angles to the magnetic field will experience forces in opposite directions.
  • The pair of forces creates a turning influence or torque to rotate the coil.
  • Practical motors have several loops on an armature to provide a more uniform torque, and the magnetic field is produced by an electromagnet arrangement called the field coils.

Electromagnetic Induction

  • The discovery that an electric current in a wire produced magnetism was a turning point in physics and technology.
  • The question arose whether magnetism could produce an electric current in a wire.
  • Before 1831, voltaic cells were the only current-producing devices, generating small currents by dissolving expensive metals in acid.
  • In 1831, Michael Faraday and Joseph Henry discovered electromagnetic induction, which led to electricity becoming commonplace.

Faraday’s Law

  • The induced voltage in a coil is related to:
    • The number of loops
    • Magnetic field changes within those loops
  • The current within the loops depends on:
    • The voltage produced
    • The resistance of the coil and the circuit it is attached to

Generators and Alternating Current

  • If one end of the magnet is moved in and out of the coil, the induced voltage alternates in direction.
    • Magnet enters coil: magnetic field strength increases, voltage is induced in one direction.
    • Magnet leaves coil: magnetic field strength decreases, voltage is induced in the opposite direction.
  • Moving the coil instead of the large magnet is more practical.
  • A generator is when the coil is rotated inside a stationary magnet.

Generators and Alternating Current

  • As the coil spins, the area affected by the magnetic field lines changes, producing an induced voltage that varies over time.
  • This creates an alternating current.

Generators

  • Mechanical energy input to a generator turns the coil in the magnetic field.
  • A voltage proportional to the rate of change of the area facing the magnetic field is generated in the coil. This is an example of Faraday's law.
  • An energy source of some kind is required to operate a generator.

Motors vs. Generators

  • Moving charges experience a force perpendicular to both their motion and the magnetic field they traverse.
    • When a current moves to the right, there is a force on the electrons, and the wire is tugged upward.
    • When a wire with no current is moved downward, the electrons in the wire experience a force, creating a current.