Magnets and Currents Study Notes

Simple Phenomena of Magnetism

  • Magnetic Poles: Forces exist between magnetic poles; like poles repel, while opposite poles (north pole and south pole) attract.

  • Magnetism Induction: Magnetic forces result from interactions between magnetic fields; magnetism can be induced in unmagnetised materials.

  • Material Properties:

    • Temporary magnets: Made of soft iron; easily magnetised and demagnetised.

    • Permanent magnets: Made of steel; retain magnetism.

  • Magnetic Fields: A region where a magnetic pole experiences a force.

    • Field Lines: Direction flows from North to South; spacing represents relative field strength.

    • Plotting: Can be determined using a compass or iron filings.

Magnetic Effect of Current

  • Field Patterns: Current flowing through straight wires and solenoids creates magnetic fields.

  • Strength Factors: Field strength varies with the magnitude of current and the direction of flow.

  • Applications: Used in the operation of relays and loudspeakers.

Electromagnetic Induction

  • Induced e.m.f.: Produced when a conductor moves across a magnetic field or when a magnetic field linking a conductor changes.

  • Magnitude Factors: Affected by the speed of movement, strength of the field, and number of turns in a coil.

  • Lenz's Law: The direction of an induced e.m.f. opposes the change causing it.

  • A.C. Generator: Utilizes a rotating coil or magnet with slip rings and brushes to produce alternating current.

Force on a Current-Carrying Conductor

  • The Motor Effect: A force acts on a current-carrying conductor in a magnetic field; reversing current or field direction reverses the potential force.

  • Charged Particles: Beams of charged particles also experience force within magnetic fields.

  • D.C. Motor: Features a current-carrying coil that experiences a turning effect.

    • Turning Effect Increase: Increased by current, number of turns, or magnetic field strength.

    • Split-ring Commutator: Used with brushes to maintain continuous rotation direction.

Transformers and Power Transmission

  • Construction: Consists of primary and secondary coils wrapped around a soft iron core.

  • Voltage Transformation:

    • Equation: VpVs=NpNs\frac{V_p}{V_s} = \frac{N_p}{N_s}

    • For 100%100\% efficiency: Vp×Ip=Vs×IsV_p \times I_p = V_s \times I_s

  • Step-up and Step-down: Transformers increase or decrease voltage for efficient transmission.

  • High-Voltage Transmission: Used to minimize power losses in cables as higher voltage allows for lower current, reducing heat loss.