Permanent magnets- an overview
Magnetism and magnets are all around us in practical use. Right from the compass needle that was used to guide great explorers discover new continents, to electronic appliances in the modern world!
Properties of magnets
|Magnetic poles||Every magnet has two points- a north pole (north seeking pole) and a south pole (south seeking pole)|
|North and south poles||The north and south poles of a magnet point to the earth’s geographical north and south poles.|
|Law of magnetism||
Like poles (north and north or south and south) repel each other.
Unlike poles (north and south or south and north) attract each other.
Magnetic materials and magnetism
Magnets are made up of magnetic materials. Permanent magnets are often made up of hard steel. They are also made up of another magnetic material called ferrite.
Magnetic materials are made up of magnetic elements. Magnetic elements include:
- Magnetic materials as may be classified as hard and soft
|Type of magnetic material||description||examples||used|
|Hard||Retains magnetism well, but difficult to magnetise .||Hard steel||Permanent magnets, compass needles, loudspeaker magnets|
|Soft||Easy to magnetise but readily looses its magnetism||Soft iron||Cores for electromagnets, transformers and radio aerials.|
Magnetisation and methods of magnetisation
Usually magnetic materials are in an unmagnetised state and they must be magnetised in order to be used. Four methods of magnetisation are listed below:
- A piece of magnetic material can be stroked with a permanent magnet consistently from one end to other (never going in the reverse direction. This causes it to gain magnetism.
- Place a material in a strong magnetic field, as produced by an electromagnet. It becomes magnetised.
- Place a long thin piece of the material so that it lies north-south in the earth’s magnetic field. Heat it. When it cools, it will have been magnetised by the earth’s field.
- Alternatively hammer a piece of the material placed in a north-south direction and it will pick up earth’s field.
Demagnetisation and methods of demagnetisation
Destroying the magnetisation of a magnetic material is known as demagnetisation.
Demagnetisation can be carried out using several methods:
- Hammer the magnet while it is placed in east-west direction. This will cause it to lose its magnetism.
- Place the magnet in the field of an electromagnet with an alternating current supply. Gradually decrease the current to zero. This causes the magnetic field to vary back and forth and finally become demagnetised.
- Heat the magnet; if the magnet’s temperature rises above a certain temperature, it will lose its magnetism.
When the north pole of a bar magnet is brought close to an iron nail, the end of the nail nearest to the North Pole becomes temporarily magnetic South Pole of the pin. This phenomenon is known as induced magnetism.
Induced magnetism happens with all magnetic objects; the point nearest to the poles of the bar magnet automatically become a North or South Pole.
After the bar magnet is withdrawn, the iron nail’s magnetic South Pole disappears and it returns to an unmagnetised state.
A magnet affects any piece of magnetic material that is nearby. We say that there is a magnetic field around magnet.
We represent the magnetic field of a single bar magnet, using magnetic field lines. The pattern tells us two things about the field:
- Direction : If you were to place a tiny compass at a point in the field, it would align itself along the field line at that point. We use a convention that says that field lines come out of north poles and go into south poles.
- Strength lines: The closer the magnetic field lines, the stronger the magnetic field.
Plotting field lines
Iron fillings can show up the pattern of the magnetic field around a magnet. Place the magnet under a stiff sheet of a plain paper or plastic sprinkle filings over the paper or plastic. Tap the paper or plastic to allow the fillings to move slightly so that they line up in the field.
<image of magnetic field pattern here>
Using magnetic materials is not the only way of making a magnet. An alternative method is to use an electromagnet. A typical electromagnet is made from a coil of copper wire. A coil like this is sometimes called a solenoid. When a current flows through the wire, there is a magnetic field around the coil.
Note: it is the electric current that produces the magnetic field.
There are three ways to increase the strength of an electromagnet
- Increase the current flowing through it – the greater the current the greater the strength of the field
- Increase the number of turns of wire on the coil – this does not mean making the coil longer, but packing more turns into the same space to concentrate the field.
- Add the soft iron core.
An iron core becomes strongly magnetised by the field, and this makes the whole magnetic field much stronger.
Electromagnets have a great advantage that they can be switched on and off. When the current is switched off the magnetic field around the coil vanishes.
Electromagnets are also used in electric doorbells, loudspeakers, electric motors, relays and transformers.
The field around a solenoid
When an electric current flows through a solenoid, a magnetic field is produced inside and outside the coin. This field is similar to that around a bar magnet:
- One end of the solenoid is the North Pole other end is the South Pole.
- Field lines emerge from the North Pole and go into the South Pole.
- The field lines are closest together at the poles, showing that this is where the magnetic field is the strongest.
- The lines spread out from the poles, showing that the field is weaker in these regions.
The strength of the field can be increased by increasing the current. The field can be reversed by reversing the direction of the current.
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