Magnets and Magnetic Fields

Magnets

  • An iron ore called lodestone, we say it is magnetised.

Properties of Magnets

  • Attract ferromagnetic materials.

    • e.g. iron, steel, nickel, cobalt, and their alloys.

  • If a bar magnet is suspended by thread it will roughly line up North-South.

  • Magnetic poles exist in pairs (i.e. north-south) the strength of each pole in the pair is equal.

  • Like poles repel and unlike poles attract.

  • The force of attraction/ repulsion is proportional to the distance between them.

  • Ferromagnetic materials bought near/touching a magnet will become magnetised (induced magnetism).

Magnetic Fields

  • A magnetic field is any region of space where magnetic force can be felt.

  • The direction of the magnetic field at a point is the direction of the force on the north pole if it were placed at that point.

  • A compass should line up north-south but if another magnetic field nearby it will deflect the compass and, if strong enough, the compass will line up parallel to the nearby field instead.

Magnetic Field Lines

  • A line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point is called a magnetic field line.

Magnetic Effect of Electric Current

  • An current-carrying conductor has a magnetic field around it as long as the current is flowing.

Right Hand Grip Rule

  • The Right Hand Grip Rule states that if the right-hand clasps a conductor with the thumb pointing in the direction of the current, then the fingers give the direction of the magnetic field around the conductor.

Magnetic Field: Circular Loop/Coil

Magnetic Field: Solenoid

  • A coil whose length is much longer than its radius.

South vs North Pole of Coil/Loop/Solenoid

  • Clockwise Current = South facing you

  • Anti-clockwise Current = North facing you

Electromagnets

  • When a soft iron core is placed in a solenoid and current passed through it.

Uses:

  • Lifting scrap iron and steel

  • In electric motors

  • In electromagnetic relays

Earths Magnetic Field

  • Magnetic compasses do not point to true north, but to magnetic north.

  • The difference between the two is called magnetic declination or magnetic variation.

  • The size of the declination varies from place to place and also with time - marine maps need to be very detailed to allow travellers to allow for these variations.