Get ready for better learning with NCERT & CBSE Magnetism and Matter Notes Class 12 Physics. No need to roam anywhere we will provide you with all the study content you need. Let’s study smartly. PDF view is also there for your suitability.
Electromagnetic Induction Notes
Magnets and magnetism. A magnet is a material that has both attractive and directive properties. It attracts small pieces of iron, nickel, cobalt, etc. This property of attraction is called magnetism. When suspended freely, a long thin piece of magnet comes to rest nearly in the geographical north-south direction.
As early as the 6th century B.C., the Greeks had some knowledge of natural magnets. Thales of Miletus knew that pieces of naturally occurring iron ore, lodestone or magnetite or black iron oxide Fe3O4 had the property of attracting small pieces of iron. The word magnetism originates from the place – Magnesia – a province in the upper part of Greece – where this ore was found. Later on the Chinese discovered that thin long pieces of lodestone, if suspended horizontally and freely with a string, would naturally orient themselves roughly in the geographical north-south direction. In fact, the word lodestone means a leading stone which expresses this directional property, valuable to travellers. By about A.D. 1000, the Chinese were using the magnetic compass for navigation. It was William Gilbert who, in his book ‘De Magnete’ of 1600, first suggested that the earth itself was a huge magnet, causing the alignment of compass needles.
In the early part of the nineteenth century, Oersted discovered that moving charges or currents are the sources of magnetic fields. However, the science of magnetism was known long before the nineteenth century.
Magnetism and Matter Notes Class 12 Physics
ARTIFICIAL MAGNETS
Artificial magnets Generally, the natural magnets are not strong enough magnetically and have
inconvenient shapes. The pieces of iron and other magnetic materials can be made to acquire the properties of natural magnets. Such magnets are called artificial magnets. The main advantage of these magnets is that they can be made much stronger than natural magnets and also of any convenient shape and size. They are generally available in the following forms :
1. Bar magnet. It is a bar of circular or rectangular cross-section.
2. Magnetic needle. It is a thin magnetised steel needle having pointed ends and is pivoted at its centre so that it is free to rotate in a horizontal plane.
3. Horseshoe magnet. It has the shape of a horseshoe and thus it has been named so.
4. Ball-ended magnet. It is a thin bar of circular cross-section ending in two spherical balls.
BASIC PROPERTIES OF MAGNETS
3. State the important properties of magnets.
Basic properties of magnets :
1. Attractive property. A magnet attracts small pieces of iron, cobalt, nickel, etc. When a magnet is brought near a heap of iron filings, the ends of the magnet show the greatest attraction. These ends, where the magnetic attraction is the maximum, are called poles of the magnet. Thus every magnet has two poles.
2. Directive property. When a magnet is suspended or pivoted freely, it aligns itself in the geographical north-south direction. The pole of the magnet which points towards the geographical north is called the north-seeking or north (N) pole. The other pole which points towards the geographical south is called the south-seeking or south (S) pole of the magnet.
3. Like poles repel and unlike poles attract. If the N-pole of a magnet is brought near the N-pole of a suspended magnet, the poles are found to repel each other. Two S-poles also repel each other. In contrast, N- and S- poles always attract each other. This action can be described by the law of magnetic poles which states that like magnetic poles repel, and unlike magnetic poles attract each other.
4. Magnetic poles always exist in pairs. If we try to isolate the two poles of a magnet from each other by breaking the magnet in the middle, each broken part is found to be a magnet with N- and S-poles at its ends. If we break these parts further, each part again is found to be a magnet. So unlike electric charges, magnetic monopoles do not exist. Every magnet exists as a dipole.
5. Magnetic induction. A magnet induces magnetism in a magnetic substance placed near it. This phenomenon is called magnetic induction. When N-pole of a powerful magnet is placed close to a soft iron bar, the closer end of the bar becomes S-pole and the farther end N-pole. As a result, the magnet attracts the iron bar. Thus induction precedes attraction.
For Your Knowledge
Repulsion is the surer test of magnetism. A magnet can attract another magnet. Also it can attract magnetic substances like iron, nickel, cobalt, etc. However, a magnet can repel another magnet only. So repulsion is the surer test of magnetism.
Magnetism and Matter Notes Class 12 Physics
SOME IMPORTANT DEFINITIONS CONNECTED WITH MAGNETISM
1. Magnetic field. The space around a magnet within which its influence can be experienced is called its magnetic field.
2. Uniform magnetic field. A magnetic field in a region is said to be uniform if it has same magnitude and direction at all points of that region. At a given place, the magnetic field of the earth can be considered uniform. The field due to a bar magnet is not uniform.
A uniform magnetic field acting in the plane of paper is represented by equidistant parallel lines [Fig. 5.6(a)]. A uniform magnetic field acting perpendicular to the paper and directed outwards is represented by dots [Fig. 5.6(b)]. A uniform magnetic field acting perpendicular to the plane of paper and directed inwards is represented by crosses [Fig. 5.6(c)].
3. Magnetic poles. These are the regions of apparently concentrated magnetic strength in a magnet where the magnetic attraction is maximum. The poles of a magnet lie somewhat inside the magnet and not at its geometrical ends.
4. Magnetic axis. The line passing through the poles of a magnet is called the magnetic axis of the magnet.
5. Magnetic equator. The line passing through the centre of the magnet and at right angles to the magnetic axis is called the magnetic equator of the magnet.
6. Magnetic length. The distance between the two poles of a magnet is called the magnetic length of the magnet. It is slightly less than the geometrical length of the magnet.
If is found that magnetic length / Geometric length = 0.84
Magnetism and Matter Notes Class 12 Physics
COULOMB’S LAW OF MAGNETIC FORCE
Coulomb’s law of magnetic force. This law states that the force of attraction or repulsion between two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.
If qm and qm are the pole strengths of the two magnetic poles which are distance r apart, then the force between them is given by
F ∝ qm1qm2 / r2
or F = k.qm1qm2 / r2
where k is a proportionality constant which depends on the nature of the medium as well as on the system of units chosen. For SI units and for poles in vacuum,
F = μ 0/ 4π .qm1qm2 / r2
where μ 0 is the permeability of free space and is equal to 4π × 10-7 henry/metre. We can define unit magnetic pole from Coulomb’s law :
If qm2 = qm2 = 1 unit; r = 1 m, then
F = μ 0/ 4π = 10-7N
Hence a unit magnetic pole may be defined as that pole which when placed in a vacuum at a distance of one metre from an identical pole repels it with a force of 10-7 newton.