Electromagnetism and Electromagnetic Induction Notes

Electromagnetism

  • Hans Oersted: Discovered that an electric current creates a magnetic field, evident in compass needle deflection.
    • Observation: Compass points to the wire carrying current, not magnetic north.

Direction of Magnetic Flux

  • Right-Hand Rule:
    • Thumb indicates the direction of current flow.
    • Fingers indicate the direction of magnetic flux around the wire.
  • Electron Movement: Electrons flow opposite to conventional current.
  • Strengthening the Magnetic Field:
    • Looping the current-carrying wire increases magnetic field strength.
    • Multiple loops (solenoids) lead to stronger fields.
    • Adding an iron core further concentrates and strengthens the magnetic field, forming an electromagnet.

Direction of Magnetic Field from Solenoid

  • Variation of Right-Hand Rule for a solenoid:
    • Fingers curve in the direction of current; thumb indicates magnetic field direction out from the north pole.
  • Magnetic Flux Lines:
    • Emerge from the north pole, curve back to the south pole.
    • Field too weak at distances to detect force without proximity to ferromagnetic materials.

Electromagnetic Induction

  • Michael Faraday: Discovered that moving a conductor through a magnetic field induces a current.
    • Induced current occurs with relative motion between the wire and magnetic field (wire moving through or vice-versa).
    • Example: Moving a magnet into a wire coil induces current.

Number of Coils in the Conductor

  • Increasing the number of coils in a moving magnetic field increases the induced voltage.
    • Proportional Relationship:
    • Doubling coils from 2 to 4 doubles voltage.
    • Tripling coils from 2 to 6 triples voltage.

Angle of Conductor

  • Voltage induced depends on the angle of motion between the wire and magnetic field.
    • Perpendicular motion induces more voltage than oblique motion (cuts more magnetic flux lines).

Magnetic Field in a Conductor

  • Alternating Current (AC): Magnetic fields expand and collapse with current direction change.
  • Polarity: As current alternates, magnetic north-south poles switch, creating a fluctuating magnetic field.

Mutual Induction

  • Induction of current in one coil by another coil's moving magnetic field.
    • Primary Coil: Generates magnetic field.
    • Secondary Coil: Current induced due to primary field.
    • Resulting current in secondary coil is also alternating due to fluctuating polarity.

Lenz’s Law

  • Induced current flows in a direction opposing its causing action (changes in magnetic field).
  • Current induces its magnetic field that opposes primary current direction (self-induction).

Self-Induction

  • With direct current (DC), induced action is brief.
  • With alternating current (AC), fluctuating magnetic fields induce continual secondary currents opposing the primary current.
  • This principle is used in x-ray circuits within autotransformer designs.

Review Questions

  1. Who established the principle of electromagnetism?
    • Answer: Oersted
  2. What occurs when a primary coil induces current in a secondary coil?
    • Answer: Mutual induction
  3. What is the induction of ‘back’ current called?
    • Answer: Self-induction
  4. What does the right-hand rule determine?
    • Answer: Direction of magnetic flux
  5. Effect of doubling speed of moving conductor?
    • Answer: Doubles resulting voltage
  6. Which coil creates the greatest voltage when moved through a magnetic field?
    • Answer: The coil with the most turns (40 turns).
  7. Thumb indication when using the right-hand rule with a coil?
    • Answer: Direction of magnetic field.