• Question: How do compasses work?

    Asked by to Aimee, Chris, Dave, Greig, Laurence on 17 Jun 2014. This question was also asked by .
    • Photo: Greig Cowan

      Greig Cowan answered on 17 Jun 2014:

      A compass works by having a magnetised needle that is light enough that it can be moved by the very weak force of the Earth’s own magnetic field. The magnetic field can be thought of as a set of lines that come out of the north pole and travel around the Earth to enter again at the South pole. The compass needle aligns itself so that it is pointing in the same direction as the lines. It does this since this is the position where the force acting on the needle leads to the lowest energy.

    • Photo: Aimee Hopper

      Aimee Hopper answered on 17 Jun 2014:

      Our planet has a magnetic field around it, and so the iron pointer (also magnetic) inside the compass lines up with the magnetic field of the planet.
      As field lines always point from south to north, the pointer points to north.

    • Photo: Dave Jones

      Dave Jones answered on 17 Jun 2014:

      The core of the Earth is like a big magnetic, where the poles of the magnet line up (more or less) with the North and South pole. A compass has a needle which has been magnetised (making it like a small magnet), and the interaction between this little magnet and the big magnet that is the Earth makes it line up. Using the fact that the magnetic poles of the Earth are more or less to the North and the South, we can use this to tell us which way is North and help us navigate.

    • Photo: Laurence Perreault Levasseur

      Laurence Perreault Levasseur answered on 19 Jun 2014:

      The others said it already, the magnetized needle in the compass aligns its north-seeking pole (or north pole) towards the magnetic north pole of the planet, and its south-seeking pole (or south pole) towards the magnetic south pole of the planet.

      Here are few interesting facts 🙂
      – because the north-seeking pole of the magnet points north, the magnetic north pole of the planet is actually a south-seeking pole (!!!) This is because, in magnetism, opposite attract each other! Similarly, the Earth’s magnetic south pole is actually a north-seeking pole!! 😀

      – The Earth’s geographic north and the Earth’s magnetic north are not at all at the same place! The geographic north is place where the planet’s axis of rotation meet the surface, and it is somewhere in the middle of the Arctic ocean, between Canada and Russia (basically it’s the north pole of maps and of terrestrial globes). The Earth’s magnetic north, on the other hand, is somewhere in Canada, almost 700km away from the true geographic north!!

      – even crazier, the magnetic north MOVES every year (!!!) by about 60km! In 1950, it was almost 2000km away from the true north!!

      – the North and South magnetic poles of the Earth are not antipodal, which means they don’t align with a straight line that goes through the centre of the Earth. Rather, the magnetic field of the Earth looks like:

      – On average, every 450,000 years, the north and south magnetic poles of the planet get INTERCHANGED!!! There are cool simulated images here of what happens during the inversion:
      https://www.e-education.psu.edu/earth520/content/l3_p4.html

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