A magnetic material is any material which feels a force when placed in a magnetic field e.g. cobalt, nickel, and a few alloys such as steel.
A non-magnetic material is any material which does not feel a force when placed in a magnetic field e.g. brass, copper, wood and plastics.
A magnetised-material is any material which has become magnetised by a permanent magnet.
Electrical magnetisation:
The unmagnetised object is placed inside a cylindrical coil, called a solenoid, wound round a cardboard roll. The solenoid must have several hundred turns of insulated copper wire. When a direct current (d.c.) flows through the solenoid it becomes a magnet. By switching the current on for a short while and then off again, the unmagnetised object becomes magnetised when removed from the solenoid. The polarity of this magnet can be found in two ways:
i. looking at the direction of the flow of current i.e. clockwise or anticlockwise, with anticlockwise being a N-pole and clockwise a S-pole; ii. gripping the solenoid with the right hand so that the fingers are in the direction of the current, the end of the magnetised steel bar that the thumb points to is a N-pole.
A magnet can be demagnetised by placing it inside a solenoid connected to an alternating current (a.c).
Permanent Magnets
Any material e.g. magnetite that is able to keep its magnetism for a long time is called a permanent magnet. Steel is a permanent magnet:
harder to magnetise and demagnetise then steel;
requires a strong magnetic field to magnetise.
Magnetic materials such as steel are hard to magnetise and retain their magnetism longer. They are called hard magnetic materials. Steel is used to make permanent magnets and bar magnets.
Uses:
Moving Coil Galvanometer: this is a current detector which is made up of a core suspended in the magnetic field of a permanent magnet. When a current flows through the coil, a turning effect is produced in the coil. A pointer attached to the coil will then detect a deflection on the scale.
Magnetic Door Catch: magnetic strips are used for the doors of freezers and refrigerators to keep the doors air-locked.
Other Uses: operation in machines e.g. d.c. motors and a.c. generators; ring magnets as a magnetic memory in computers; magnets are used to remove magnetic materials form eyes; to reset the metal index in a maximum and minimum thermometer; moving coil loudspeaker.
Temporary Magnet
Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field, but lose their magnetism when the magnetic field disappears. Soft iron is a temporary magnet:
easily magnetised and demagnetised;
can be magnetised by a weak magnetic field.
Magnetic materials like iron which are easily magnetised but do not maintain their magnetism are called soft magnetic materials. Iron is used in electromagnetics, transformer cores and magnetic shields.
Uses:
Electromagnetic cranes: current is switched on to energise the magnet and pick up the scrap metal. When it is moved to the correct position, the electromagnet is switched off and the metal drops.
Magnetic Induction
When an unmagnetised magnetic material is brought near or touches the poles of a permanent magnet, it is attracted or becomes a magnet. Magnetic induction is the process of inducing magnetism into a an unmagnetised ferromagnetic material without any contact with the magnet. E.g. when a permanent magnet is brought near to a soft iron bar, the north pole of the permanent magnet induces a south pole into the end of the iron bar whilst the far end of the iron becomes a north pole.
Magnetic Field and Plotting Compass
A magnetic field is the region where a magnetic force is exerted on any magnetic object placed within the influence of the field. The earth’s magnetic field is most likely caused by circulating electric currents within the earth’s core. These currents may be generated by convection in the earth’s liquid core. Magnetic fields are also present in the interior of atoms and in stars.
Experiment:
Apparatus: Bar magnet, plotting compass, plain paper
1) Place the magnet at the centre of the piece of paper, so that the south pole faces south and the north pole face north. 2) Starting near one pole of the magnet, the positions of the ends, N and S, of the compass needle are marked by pencil dots X and Y. The compass is then moved until one end is exactly over Y and the position of the other end is marked with a new dot. 3) Repeat the process of marking the dot. Join the series of dots and this will give the plot of the field lines of the magnetic field.
Precautions:
Check that the compass is in good working condition.
Ensure that there is no other magnetic field except the earths’ around the plotting compass.
Magnetic Screening
Sometimes objects or places need to be screened for magnetism and therefore external magnetic fields must be excluded e.g. the earth’s magnetic field. E.g. In a MRI scan where a patients’ body is screened for tiny changes of magnetic fields.
Audio and video tape
These are synthetic tapes with a layer of ferromagnetic material. When recording the tape is magnetised with an electromagnet . The recorded information is represented in electric forms as varying currents. The currents will magnetise the tape according to the information.