Compasses

D. Crowley, 2008

Compasses To understand how compasses work

Ocean Imagine you are stuck out at sea – every direction you look,

you see nothing but deep blue sea and it is overcast so you cannot tell the position of the Sun…

How could you work out which way to travel?

Compass No matter where you stand on Earth, you can hold a compass

in your hand and it will point toward the North pole

A magnetic compass consists of a small, lightweight magnet balanced on a nearly frictionless pivot point

The Earth does not contain a giant bar magnet, but it behaves as if it does - the Earth has a magnetic north pole and a magnetic south pole

Compasses work because bar magnets can line up in the Earth's magnetic field and point north

Magnetic The North pole of a bar

magnet is actually called the ‘North-seeking pole', and it points to the Earth's magnetic North pole

The South pole of a bar magnet is actually called the ‘South-seeking pole', and it points to the Earth's magnetic South pole

Compasses point towards the Earth’s magnetic North pole

Plotting Compasses Small compasses called plotting compasses can be used to

show the magnetic field around a bar magnet, instead of using iron filings

The needle in each compass turns and lines up along the field lines

Plotting Compasses If we arrange some plotting compasses around a bar magnet,

we see that the needles point away from the magnet's north pole and towards its south pole

Magnet How could you design you own compass if you were stranded

on the boat in the middle of the ocean?

You have luckily brought your favourite magnet with you!

Making Magnet Remember, a magnet can be made by magnetising a material

which is attracted to a magnet, e.g. a paper clip

There are three methods that can be used to make a magnet: -• Stroke a magnet along the paperclip from one end to the

other and then starting from the same place, repeat the movement - the more times this is done, the more magnetic the clip becomes

• Hold a nail in a magnetic field and hit it with a hammer• Put a magnetic material in a strong magnetic field

Magnet To make the magnet you will need: -

A needle or some other wire-like piece of steel (e.g. a straightened paper clip)

Something small that floats such as a piece of cork, the bottom of a Styrofoam coffee cup, a piece of plastic or the cap from a milk jug

A dish, preferably a pie plate 23 - 30 cm in diameter, with about 2.5 cm of water in it

Step 1 Turn the needle into a magnet – stroke a magnet along the

needle 20-30 times in the same direction

Step 2 Place you float in the middle of the dish – the float on water

creates a nearly frictionless bearing, vital for a compass to work as the magnetic field of the Earth is quite weak at the surface

Compasses Complete the compasses

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Compasses

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