16.1 - Magnets & Magnetic Fields

Magnets, poles and the magnetic field

• Every magnet has two poles: a north pole (north-seeking) and a south pole (south-seeking).
• The poles are at the ends of the magnet, where its magnetic effect is strongest.
• A magnet produces a magnetic field in the space around it - the region where another magnet, or a magnetic material, experiences a force.
• Because magnetic materials feel a force without being touched, magnetism is a non-contact force.

Magnetic and non-magnetic materials

• Magnetic materials feel a force in a magnetic field - e.g. iron, steel, nickel and cobalt.
• Most everyday materials are non-magnetic - e.g. plastic, wood, copper and aluminium.

Magnetic flux density

• The strength of a magnetic field at a point is called the magnetic flux density.
• It's measured in teslas (T).

Forces between poles - Like repel, unlike attract

• Like poles repel - Two north poles, or two south poles, push apart.
• Unlike poles attract - A north pole and a south pole pull together.

Magnet and a magnetic material

The force between a magnet and a magnetic material (such as unmagnetised iron) is always attractive, whichever pole is brought near it.

Uniform field between two magnets

• Placing the north and south poles of two bar magnets near each other forms a uniform magnetic field in the gap.
• A uniform field has the same strength and direction everywhere.
• It's drawn as parallel, equally spaced field lines running from the north pole across to the south pole.

Drawing magnetic field lines

• A magnetic field can be represented using magnetic field lines (lines of force).
• Field lines show both the direction and the strength of the field:
  • Direction - Field lines always point from the north pole to the south pole outside the magnet. This is the direction of the force on a north pole placed in the field.
  • Strength - The closer together the lines, the stronger the field. The field is strongest at the poles, where the lines are most crowded.

Rules for field lines

• Field lines never touch or cross - at any point the field can only act in one direction.
• A point where the opposing fields cancel out to zero is called a neutral point.