What Materials are magnetic
Materials that are attracted by magnets are called magnetic or ferromagnetic materials.
E.g. cobalt, nickel and steel.
Materials that are not attracted by magnets are called non-magnetic materials.
E.g. brass, copper, wood and plastics.
A material that retains its magnetism for a long time is called a permanent magnet.
They are usually made of steel and alloys like alnico and alcomax.
Another type of permanent magnet is made from ferrites.
Its a common misconception that all metallic materials are magnetic. This is not the case. Although almost all metals conduct electricity, not all metals are magnetic. E.g. brass and copper. However these materials can sometime produce(with the help of current) magnetic fields. We will explore this in later chapters.
The two ends of a bar magnet where the magnetic effects are the strongest, are called the poles of the magnets.
The end of a freely suspended bar magnet that points to the northern end of the Earth is called the north-seeking pole (i.e. north pole or N pole). The other end of the magnet that points to the southern end of the Earth is called the south-seeking pole (i.e. south pole or S pole).
Law of Magnetic Poles
Like poles repel, unlike poles attract.
Identifying a magnet
We cannot assume an object is a magnet simply because it attracts a bar magnet. However if repulsion occurs between an object and a known magnet, then we can conclude that the object is indeed a magnet.
Hence, repulsion is the only test to confirm that an object is a magnet.
Magnetic Field Lines…
A bar magnet has two poles; the North seeking pole (labelled N) and the South seeking pole (labelled S). The magnetic force is concentrated at the poles. This can be shown by placing the magnet beneath a thin sheet of card and scattering iron filings over the top. The darker patches of iron filings correspond to stronger magnetic forces. The centre of the bar magnet is where the magnetic force is weakest.
If you do a similar experiment but use little plotting compasses instead of iron filings then you can give the field lines direction too. The pointers on the plotting compass are repelled by the North seeking poles of the magnet but attracted to the South seeking poles.
Combining the two sets of data, you can draw the full field lines around a bar magnet. Note the direction of the arrows, from North to South, and that the lines are closer together at the poles where the forces are stronger.
When a ferromagnetic object is placed near a permanent magnet, not only would it be attracted to the magnet, it would also become a Induced magnet.
This induced magnet would be then be able to attract other magnetic objects (if it is light enough).
Hence magnetic induction is the process of inducing (or bringing about) magnetism in ferromagnetic materials.
Magnetic induction can occur without any contact with the magnet as can be seen from the diagram below.
The diagram on the right shows a bar magnet placed with the north pole on top of a piece of wood. Two nails are seen to be “attracted” to the wood. We cannot conclude that the wood has been induced magnetically by the bar magnet. It is the strength of the magnetic force (field) from the bar magnet that is able to penetrate the wood attracting the two nails. The two iron nails indeed become induced magnets. The pointed tips of these two nails are like poles, north poles to be exact. Thus they would repel each other explaining the reason why they seems to point outwards.
Theory of Magnetism
If a magnet was to be cut in many pieces, that smaller pieces become many smaller individual magnets themselves with their own N poles and S poles.
We can thus see that a bar magnet is made up of many such “tiny magnets” known as magnetic domains.
The orbiting electrons in a magnetic material makes each atom an atomic magnet. A group of atomic magnets pointing in the same direction is called a magnetic domain. The diagram on the left shows the domains of aunmanetised bar. The domains point at random directions. The magnetic effects of the atomic magnets cancel out so there is no resultant magnetic effect, thus the bar is not magnetised. In a permanent magnet, magnetic domains point in the same direction. The atomic magnets at the ends of a bar magnet fan out due to repulsion between the poles.
The maximum strength of a magnet is reached when all the domains are pointing in the same direction. In this state, the magnet is said to be magnetically saturated and cannot be any stronger.
The diagram on the left shows a unmagnetised steel bar being stroke several times by a permanent magnet.
Notice that the bar is stroked every time by the same pole (N pole) of the permanent magnet. The magnet is lifted sufficiently high above the steel bar between successive stoke.
The steel bar would eventually become a permanent magnet by magnetic induction.
Note that the pole produced at the end of the steel bar is opposite to the stroking pole used.
We will explore which magnetic material would make good permanent magnets later.
Stroking method with 2 permanent magnets
A steel bar can also be magnetised by using two permanent magnets. This would speed up the magnetising process. Take note of the poles of the two magnets stroking the steel bar. They are of different polarity. It is important that this procedure is taken to ensure the proper magnetisation of the steel bar.
Electrical Method using a direct current (D.C.)
In Figure A above, a steel bar is wrapped with a cylindrical coil of insulated copper wire known as a solenoid. Solenoids usually have several hundred turns of the copper wire. When a direct current (D.C.) is passed through the solenoid, the steel bar would become a magnet after a while.
Wires with current passing through them would produce a strong magnetic effect by the current. This would in turn magnetise the steel bar in the case mentioned in Figure A. We will explore the magnetic effect of a current later.
The poles of the magnet can be determined by a simple method known as the right-hand grip as seen in Figure B.
Note that this method is not a concept but a tool to help determine poles of induced magnets by electricity.
Hammering a magnet would cause the magnetic domains to alter their alignment. This would cause a magnet to lose its magnetism.
Like hammering, heating a magnet would also result in it losing its magnetism. Energy from the heat from external source would cause the atoms of a magnet to vibrate more rigorously and hence would cause the magnet’s domains to lose their alignment.
Electrical method using an alternating current (A.C.)
The most effective method of demagnetisation. Magnet is placed inside a solenoid connected to an alternating current supply as seen in the diagram of the right.
An alternating current is an electric current which varies its direction many times per second (more will be covered later).
The magnet is then slowly withdrawn in the East-West direction with the alternating current still flowing in the solenoid.
Storage of magnets using soft iron keepers
Permanent magnets would eventually lose their magnetism one way or another as the magnetic domains would be moved out of their alignments by external factors. Storing them side by side would further reduce the time taken for these magnets to lose their magnetism.
To prevent magnets from losing their magnetism too quickly, we store magnets in pairs by using soft iron keepers across the ends of the bar magnets (see diagram on the left). The poles of the magnets are in closed loops ‘locking’ the alignment of the magnetic domains.
We know that when 2 magnets with like poles facing each other will tend to repel or unlike poles facing each other would attract each other. From this, we can see that magnets produce magnetic force fields around them. These magnetic force fields are not visible but we can prove they exist by simply sprinkling iron filings onto a bar magnet as can be seen on the left. The iron filings would reveal the pattern of the magnetic field around the bar magnet.
A magnetic field is a region in which a magnetic object placed within the influence of the field, experiences a magnetic force.
Plotting Magnetic Field Lines
We can plot the field lines of a bar magnet using a small plotting compass.
1. Place the magnet on a piece of paper so that its N pole faces North and its S pole faces South. This is to prevent the compass from being affected by the earth’s magnetic field.
2. Starting near one pole, the positions of the ends, S and N, of the compass needle are marked by a pencil dots X and Y respectively. The compass is then moved so that the S end is at Y and the new position of the N end is marked with a third dot Z.
3. Repeat process of marking the dots. Join the dots and this will give the plot of the magnetic field lines.
Magnetic field lines between two magnets
Magnetic field lines tend to pass through magnetic materials (like iron). The figures on the left and above are two examples. The figure on the left shows the effect a thin sheet of iron has on a magnetic field. If a magnet is placed on one side of the sheet, the other side would be free from magnetic field. Hence this is a very efficient way of enclosing sensitive equipment and shielding it from surrounding magnetic fields like the figure above.
Temporary and Permanent Magnets
Different magnetic materials have different properties. Some make better permanent magnets than other and the same can be said for temporary magnets (magnetism only last a short time). In this section, we will explore two such materials; Iron and Steel.
When the above experiment was conducted with iron, the following was observed:
1. Iron makes stronger induced magnets.
2. Soft magnetic materials like Iron do not retain their magnetism but are easily magnetised.
Hence iron makes good temporary magnets.
Applications of temporary magnets include electric bell, electromagnets and magnetic relays.
When the above experiment was performed with steel, the following was observed:
1. Steel makes weak induced magnets.
2. Hard magnetic materials like steel retain their magnetism longer but are harder to magnetised.
Hence steel makes good permanent magnets.
Applications of permanent magnets include, moving-coil ammeter, magnetic door catch and moving-coil loudspeakers.