Magnetism

1. Introduction to Magnets

• The earliest known magnets were pieces of lodestone, an ore of iron oxide found naturally. These natural magnets are irregular in shape and possess weak magnetic strength.
• Artificial magnets (like bar magnets, horseshoe magnets, and magnetic needles) are manufactured from iron into various convenient shapes and sizes for practical applications.
• A freely suspended magnet aligns itself in the geographic north-south direction.
• Basic rule of magnetism: Like poles (North-North or South-South) repel each other, while unlike poles (North-South) attract each other.

2. Induced Magnetism

• Magnetic Induction: The process by which an unmagnetised magnetic material (like soft iron or steel) temporarily acquires magnetic properties when placed near or in contact with a magnet.
• During induction, the inducing magnet creates an opposite polarity on the near end of the magnetic material and a similar polarity on the farther end.
• Induction precedes attraction: A piece of ordinary iron is first magnetized by induction (creating opposite poles that attract), which explains why magnets attract unmagnetised iron pieces.
• Induced magnetism is purely temporary. The material loses its magnetism as soon as the inducing magnet is removed.

3. Magnetic Field and Field Lines

• The magnetic field is the space around a magnet in which a compass needle rests in a direction other than the standard geographic north-south. It is a vector quantity, meaning it has both magnitude and direction.
• A magnetic field line is a continuous curve showing the path along which a compass needle aligns. The tangent at any point on the curve gives the direction of the magnetic field at that point.
• Properties of Magnetic Field Lines:
  • They form closed and continuous curves.
  • Outside the magnet, they are directed from the North Pole towards the South Pole.
  • Two field lines never intersect. If they did, it would imply two different directions of the magnetic field at a single point, which is impossible.
  • Lines are crowded near the poles (indicating a strong magnetic field) and spread further apart near the middle (indicating a weaker field).
  • Parallel and equidistant field lines represent a uniform magnetic field (e.g., Earth's magnetic field in a limited space).
  • They behave like stretched elastic rubber strings.

4. Magnetic Field of the Earth

• The Earth behaves like a giant bar magnet. Evidence for this includes: freely suspended magnets pointing north-south, buried iron rods becoming weak magnets over time, and the existence of neutral points.
• The Earth's magnetic South pole is located near the geographic North pole, and its magnetic North pole is near the geographic South pole.
• The Earth's magnetic axis is slightly tilted, making an angle of about 17° with its axis of rotation.
• A magnetic needle becomes completely vertical at the Earth's magnetic poles and horizontal at the magnetic equator.
• In a limited area, the Earth's magnetic field is uniform. The field lines are parallel, equidistant, and directed from geographic South to geographic North.

5. Plotting Fields and Neutral Points

• Magnetic fields around artificial magnets (like bar and horseshoe magnets) are non-uniform, featuring curved, converging, or diverging lines.
• Neutral Points are specific locations where two magnetic fields (usually the field of a magnet and the Earth's magnetic field) are equal in magnitude but opposite in direction. The net magnetic field at these points is zero.
• A compass placed at a neutral point remains unaffected and can rest in any direction.
• Positions of Neutral Points:
  • When a bar magnet is placed with its North pole pointing towards geographic North, two neutral points are located on either side of the magnet in the east and west directions.
  • When a bar magnet is placed with its South pole pointing towards geographic North, two neutral points are located along the axis of the magnet in the north and south directions.