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A compass is an instrument which is used to find the direction of a magneticfield. A compass consists of a small metal needle which is magnetised itself and which is free to turn in any direction.Therefore, when in the presence of a magnetic field, the needle is able to line up in the same direction as the field.
Lodestone, a magnetised form of iron-oxide, was found to orientate itself in a north-south direction if left free to rotate by suspensionon a string or on a float in water. Lodestone was therefore used as an early navigational compass.
Compasses are mainly used in navigation to find direction on the earth. This works becausethe earth itself has a magnetic field which is similar to that of a bar magnet (see the picture below). The compass needle aligns with the earth's magnetic fielddirection and points north-south. Once you know where north is, you can figure out any other direction. A picture of a compass is shown below:
Some animals can detect magnetic fields, which helps them orientate themselves and navigate. Animals which can do this include pigeons, bees, Monarchbutterflies, sea turtles and certain fish.
In the picture below, you can see a representation of the earth's magnetic field which is very similar to the magnetic field of a giant bar magnet likethe one on the right of the picture. So the earth has two sets of north poles and south poles: geographic poles and magnetic poles .
The earth's magnetic field is thought to be caused by flowing liquid metals in the outer core which causes electric currents and a magnetic field. From the pictureyou can see that the direction of magnetic north and true north are not identical. The geographic north pole , which is the point through which the earth's rotation axis goes, is about 11,5 o away from the direction of the magnetic north pole (which is where a compass will point). However, the magnetic poles shift slightly all the time.
Another interesting thing to note is that if we think of the earth as a big bar magnet, and we know that magnetic field lines always point from north to south , then the compass tells us that what we call the magnetic north pole is actually the south pole of the bar magnet!
The direction of the earth's magnetic field flips direction about once every 200 000 years! You can picture this as a bar magnet whose north and south poleperiodically switch sides. The reason for this is still not fully understood.
The earth's magnetic field is very important for humans and other animals on earth because it stops electrically charged particles emitted by the sun from hitting the earth and us. Charged particles can also damage and cause interference with telecommunications (such as cell phones). The stream of charged particles (mainly protons and electrons) coming from the sun is called the solar wind. These particles spiralin the earth's magnetic field towards the poles. If they collide with particles in the earth's atmosphere they sometimes cause red or green lights or a glow in the sky which is called the aurora. This happens close to the northand south pole and so we cannot see the aurora from South Africa.
This simulation shows you the Earth's magnetic field and a compass.
This video provides a summary of the work covered in this chapter.
Khan academy video on magnets
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