It is said that a Cretan shepherd called Magnes first noticed lodestone when the iron-tipped end of his crook was pulled down when he passed a certain rock.
The ancient Greek scientist, Archimedes is believed to have used lodestone to pull nails from enemy ships, which then sank.
Sailors used lodestone to help them navigate. They had found that when a piece of magnetite was suspended from a thread it came to rest in a North-South direction.
Magnetic Materials
Objects such as pins and nails are magnetic.
Screwdriver and hatchet blades can be magnetic.
Objects such as wood, glass and plastic are non-magnetic.
Types of Magnets
There are different types of magnets. Some of these are:
Horsehoe Magnets
Bar Magnets
Ball-ended Magnets
Circular Magnets
Poles of Magnets
The ends of magnets are called poles. Most of the magnetism is concentrated in the poles.
If you hang a bar magnet from a thread it will come to rest in a direction facing North-South
Magnets behave differently depending on which poles you bring together
Bring a North-seeking pole near to a South-seeking pole. The magnets are attracted.
Bring a South-seeking pole near to a South-seeking pole. The magnets repel each other.
The Law of the magnet is:
Like poles repel.
Unlike poles attract.
Magnetic Fields
There is an area round the magnet in which magnetic materials are attracted by the magnet.
This is called the Magnetic Field.
How can you show these lines of force?
Place a bar magnet under a piece of paper. Sprinkle coarse iron filings over the paper and gently tap until they form a pattern.
This will show the lines of force of the magnetic field.
Place two bar magnets under a piece of paper. Place the North-seeking poles opposite each other.Sprinkle coarse iron filings over the paper and gently tap until they form a pattern.
This shows the lines of force of the magnetic field.
In the middle there is a neutral zone where there is no magnetic field.
Place two bar magnets under a piece of paper. Place a North seeking poles near to a South-seeking pole. Sprinkle coarse iron filings over the paper and gently tap until they form a pattern.
This will show the lines of force of the magnetic field.
What are Magnets?
Magnets are usually made of iron or steel.
Powerful magnets can be made of special alloys of aluminium, cobalt, copper, nickel and iron.
Materials can be permanent magnets.
Materials can be induced magnets.
All materials are made of very small particles called molecules. The molecules of iron and steel are magnetic. The Molecular Theory of Magnetism suggests that in a magnet all the molecules are lined up with their own North-seeking and South-seeking poles as in the diagram. This means all the souths are pointing one way and all the norths pointing in the other direction.
In an unmagnetised piece of iron or steel the molecules are arranged at random with the poles pointing in lots of different directions.
You can show this with a large test tube of iron filings. The tube must be almost full and corked securely. the iron filings can be seen to lie in almost every direction. If you bring this tube towards a compass needle, it has almost no effect on the needle. Stroking the test tube, in a horizontal position with the North-seeking pole of a magnet causes the iron filings to line up as in the diagram.
Iron filings lined up after stroking with magnet.
Making Magnets
There is a picture of a smith hammering steel to make a magnet, in a book called De Magnete written by Queen Elizabeth I's physician Dr William Gilbert.
Heating and hammering with a steel bar in a North-South direction can make a magnet.
This causes the molecules to line up in the North-South orientation.
Stroking a piece of unmagnetised iron or steel with a known magnet can make it into a magnet.
This causes the molecules to line up in the North-South orientation.
Placing a magnet alongside a non-magnetised steel knitting needle for some time will magnetise it.
This causes the molecules to line up in the North-South orientation.
Placing an object such as a non-magnetised steel knitting needle in a north-south position will allow it to become a magnet through the influence of the earth's magnetic field.
This causes the molecules to line up in the North-South orientation.
Demagnetizing
A magnet can be destroyed by hammering
This causes the molecules to become arranged at random with the poles pointing in lots of different directions.
Heating a piece of magnetized iron or steel strongly in a flame will demagnetize it.
This causes the molecules to become arranged at random with the poles pointing in lots of different directions.
Magnetism and Electricity
In 1819, Hans Christian Oersted, a Danish scientist was experimenting with electricity.
Oersted noticed that a compass needle moved when a current flowed through a wire nearby.
If you wrap a 10 cm length of bell wire round a pencil in a single layer coil and connect the ends to a 3v battery, you will make a solenoid. Bring a compass needle towards this and watch what happens.
You should find that the compass needle changes direction to line up with the centre of the coil because the coil behaves like a magnet.
If you reverse the battery terminal wires, then the compass needle should reverse. It will still be lined up with the pencil, but in the opposite direction.
Electromagnets
You can make an electromagnet from an iron nail and a length of bell wire.
Wrap the insulated wire round the nail. About 20 turns should make a good magnet.
Connect the wires to a battery. You can put a switch in the circuit.
Close the switch to complete the circuit and see if you can use the magnet to pick up some drawing pins.
Find what happens when you switch off.
The nail should pick up drawing pins when you switch on.
The pins should fall off when you switch off.
Using Electromagnets
An electromagnet can be used for moving scrap steel from one place to another. In a scrapyard, a crane has a large iron disc that is not a permanent magnet.
The crane operator lowers the electromagnet into a pile of scrap steel and then switches on the electricity. This causes the steel disc to become a powerful magnet. Scrap steel is attracted to the disc.
When the operator switches off the electricity, the scrap steel falls off the disc.
In this way, large amounts of scrap steel can be moved easily from one place to another.
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