Magnetism

Units

ampere (A): unit of electric current, measuring the rate of flow of electric charge in a circuit; 1 ampere means 1 coulomb of charge passes a point each second

volt (V): unit of potential difference, equal to the energy transferred per unit charge; 1 volt = 1 joule per coulomb

watt (W): unit of power, measuring the rate of energy transfer; 1 watt = 1 joule per second

Magnetism

  • Properties of Magnets

    • Magnets have two poles: north and south.

    • Like poles repel, unlike poles attract.

    • The force between magnets acts at a distance through a magnetic field.

    • Magnets attract magnetic materials such as iron, steel, nickel, and cobalt.

    • The magnetic force is strongest at the poles of a magnet.

  • Magnetically Hard and Soft Materials

    • Magnetically hard materials:

      • Difficult to magnetise but retain magnetism for a long time.

      • Have high coercivity (resist demagnetisation).

      • Used to make permanent magnets (e.g. steel).

    • Magnetically soft materials:

      • Easy to magnetise but lose magnetism quickly when the field is removed.

      • Have low coercivity.

      • Used in electromagnets and transformer cores (e.g. iron).

  • Magnetic Field Lines

    • Magnetic field lines represent the direction and strength of a magnetic field.

    • They point from the north pole to the south pole outside the magnet.

    • Inside the magnet, they go from south to north, forming closed loops.

    • The closer the lines are, the stronger the magnetic field.

    • Field lines never cross, as this would imply two directions at once.

  • Induced Magnetism

    • When a magnetic material is placed in a magnetic field, it can become magnetised.

    • This happens because domains (regions of aligned atoms) align with the field.

    • In soft materials, this effect is temporary.

    • In hard materials, some magnetism may remain after removal of the field.

  • Magnetic Field Patterns (Practical)

    • Iron filings can be sprinkled around a magnet to reveal field patterns.

    • A plotting compass can be used to trace field lines and direction.

    • Around a bar magnet: curved lines from north to south.

    • Between two magnets:

      • Opposite poles: field lines connect, showing attraction and a strong field.

      • Same poles: field lines push apart, showing repulsion and a weaker region between them.

    • The density of filings indicates field strength.

  • Uniform Magnetic Field

    • Produced by placing two flat, parallel magnets with opposite poles facing each other.

    • Field lines are straight, parallel, and evenly spaced.

    • This shows the magnetic field has constant strength and direction throughout the region.

    • Uniform fields are useful in experiments where a constant force is needed.

Electromagnetism

  • Magnetic Field Around a Current

    • A current flowing through a conductor produces a magnetic field around it.

    • The field consists of concentric circular lines centered on the wire.

    • The direction of the field can be determined using the right-hand grip rule (thumb = current, fingers = field direction).

    • Increasing current increases the strength of the magnetic field.

  • Electromagnets

    • Made by wrapping insulated wire into a coil (solenoid) around a soft iron core.

    • When current flows, the coil produces a magnetic field similar to a bar magnet.

    • The soft iron core becomes magnetised, greatly increasing field strength.

    • Strength can be increased by:

      • Increasing current

      • Increasing number of turns

      • Using a better core material (soft iron)

    • Can be switched on and off, unlike permanent magnets.

  • Magnetic Field Patterns

    • Straight wire: circular field lines around the wire, closer near the wire indicating stronger field.

    • Flat circular coil: field lines loop through the coil, resembling a weak bar magnet.

    • Solenoid:

      • Inside: strong, uniform, parallel field lines.

      • Outside: weaker, curved lines similar to a bar magnet.

  • Force on a Charged Particle

    • A charged particle moving through a magnetic field experiences a force.

    • The force acts perpendicular to both the direction of motion and the magnetic field.

    • If the particle moves parallel to the field, no force acts.

    • This can cause circular or curved motion of charged particles.

  • Force on a Current-Carrying Wire

    • A wire carrying current in a magnetic field experiences a force due to interaction between magnetic fields.

    • This is called the motor effect.

    • The force is perpendicular to both current and magnetic field direction.

    • Applications:

      • d.c. motors: convert electrical energy into kinetic energy (rotation).

      • loudspeakers: convert electrical signals into vibrations (sound).

  • Left-Hand Rule

    • Used to determine direction of force on a current-carrying conductor.

    • First finger: direction of magnetic field (north to south).

    • Second finger: direction of current (positive to negative).

    • Thumb: direction of force or motion.

  • Factors Affecting Force

    • Increasing current increases the force.

    • Increasing magnetic field strength increases the force.

    • Increasing length of wire in the field increases the force.

    • Reversing current or field reverses the direction of the force.

Electromagnetic Induction

  • Induced Voltage

    • A voltage is induced when:

      • A conductor moves through a magnetic field

      • A magnetic field changes around a conductor (e.g. moving magnet)

    • This is due to the cutting of magnetic field lines by the conductor.

    • Factors increasing induced voltage:

      • Faster relative motion

      • Stronger magnetic field

      • Greater number of turns in the coil

      • Larger area of the coil

  • Generating Electricity

    • Occurs when mechanical energy is converted into electrical energy.

    • Methods:

      • Rotating a magnet inside a stationary coil

      • Rotating a coil within a magnetic field

    • Produces alternating current (a.c.).

    • Increasing speed of rotation increases frequency and voltage.

  • Transformers

    • Consist of a primary coil, secondary coil, and an iron core.

    • The alternating current in the primary coil produces a changing magnetic field.

    • This induces a voltage in the secondary coil.

    • The size of the voltage depends on the number of turns in each coil.

    • Only work with a.c. because a changing magnetic field is required.

  • Step-Up and Step-Down Transformers

    • Step-up transformer:

      • Increases voltage

      • Secondary coil has more turns than primary

    • Step-down transformer:

      • Decreases voltage

      • Secondary coil has fewer turns than primary

    • National grid use:

      • Step-up transformers increase voltage for transmission to reduce energy loss (less current → less heating).

      • Step-down transformers reduce voltage to safe levels for homes and appliances.

  • Transformer Equation
    input (primary) voltage / output (secondary) voltage = primary turns / secondary turns

  • Power in Transformers
    input power = output power (for 100% efficiency)

    Vp × Ip = Vs × Is

  • Efficiency Considerations

    • In real transformers, some energy is lost as heat and sound.

    • Losses are reduced by using soft iron cores and insulating materials.