P4.1 - Magnets and Magnetic Fields

• Field lines are a way of modelling a magnetic field. They represent the magnetic flux and the number of lines through a particular area (magnetic flux density, also known as magnetic field strength).
• In permanent magnets, there are many small magnetic regions (domains) that all line up. In induced magnets, their domains line up when in a magnetic field.
• Domains in hard magnetic materials stay lined up even after removing the magnetic field. In soft magnetic materials, they return back to their original direction.
• If you hang a magnet, one end would point towards the magnetic north pole.
• The point on the surface of the earth is not the same as the North Pole, which is the point about where the earth rotates. A compass points towards the magnetic north pole, it is a "north-seeking" pole.
• Many compasses are weighted so they lie horizontally. The angle between the field line and the line horizontal to the surface of the earth is the "dip". It is a right angle at the north and sound magnetic poles, and zero at the "magnetic equator".
• Oersted discovered that there is a magnetic field carried in a wire with current. He found that if the current lines were pointing towards you, the field lines were anti-clockwise.
• Magnetic field strength / magnetic flux density depends on:

  • The magnitude of the current, larger current means stronger field.
  • The distance from the wire, nearer to the wire means stronger field.
• The strength of a magnetic field is measured in Teslas (T).
• The strength of the earth's magnetic field is about 0.01mT.
• The strength of 1cm of wire with a current of 5 A is about 0.1mT.
• A very strong permanent magnet would has a magnetic field of 1T near its poles.
• A solenoid is a loop of wire that carries a current. The induced magnetic field travels in straight lines inside it. To strengthen it, add more coils or add a magnetic material inside the core.