Magnetism is a class of physical phenomena mediated by magnetic fields. It arises fundamentally from two sources: the intrinsic spin of electrons and the macroscopic motion of electric charges (electric currents).
Magnetic Poles and Molecular Theory
Every magnet possesses two regions of maximum directive and attractive property called poles.
- North-Seeking Pole (N) and South-Seeking Pole (S): When a bar magnet is suspended freely, it aligns itself along the Earth’s north-south direction. The end pointing toward the geographic north is the North pole, and the end pointing toward the geographic south is the South pole.
- Magnetic Monopoles Do Not Exist: If a bar magnet is broken in half, it does not separate the north and south poles. Instead, two smaller, complete magnets are formed, each with its own north and south poles. Magnetic poles always exist in equal and opposite pairs.
- Law of Magnetic Poles: Like magnetic poles repel each other, whereas unlike magnetic poles attract each other.
Classification of Magnetic Materials
Based on their behavior and atomic structure in an external magnetic field, materials are classified into three primary categories:
- Diamagnetic Materials: Materials that develop a feeble magnetization in a direction opposite to the applied magnetic field. They are weakly repelled by magnets and move from stronger to weaker parts of a magnetic field. Examples include Water, Bismuth, Copper, Gold, and Silicon.
- Paramagnetic Materials: Materials that develop a feeble magnetization in the same direction as the applied magnetic field. They are weakly attracted by magnets. Examples include Aluminum, Platinum, Chromium, and Oxygen.
- Ferromagnetic Materials: Materials that develop strong magnetization in the same direction as the applied magnetic field. They are strongly attracted by magnets and retain their magnetic properties even after the external field is removed. Examples include Iron, Cobalt, Nickel, and alloys like Alnico.
The Concept of a Magnetic Field
A magnetic field is the space around a magnet or a current-carrying conductor within which its magnetic influence can be detected by another magnet or a moving charge.
Vector Nature and Units
- Vector Quantity: The magnetic field is a vector quantity, possessing both magnitude and direction. It is denoted by the symbol B.
- SI Unit: The SI unit of magnetic field intensity is the Tesla (T) or Weber per square meter (Wb/m2).
- CGS Unit: The CGS unit is the Gauss (G).
- Conversion Factor:1 Tesla = 104 Gauss
Magnetic Field Lines (Lines of Force)
Magnetic field lines are imaginary curves used to visually represent the nature, direction, and strength of a magnetic field.
Core Properties of Magnetic Field Lines
- Continuous Closed Loops: Unlike electric field lines (which begin on a positive charge and end on a negative charge), magnetic field lines form continuous closed loops. Outside the magnet, they travel from the North pole to the South pole. Inside the magnet, they run from the South pole to the North pole.
- Tangential Direction: The tangent drawn to a magnetic field line at any point gives the direction of the net magnetic field (B) at that point.
- Crowding Effect: The relative density or crowding of the field lines indicates the strength of the magnetic field. The field is strongest near the poles where the lines are closest together.
- No Intersections: Two magnetic field lines can never intersect each other. If they did intersect at a point, a compass needle placed at that intersection would point in two different directions simultaneously, which is physically impossible.
Magnetic Field Due to Electric Current (Electromagnetism)
In 1820, Hans Christian Oersted discovered that an electric current flowing through a wire deflects a nearby compass needle, proving that moving electric charges generate an associated magnetic field.
1. Straight Current-Carrying Conductor
The magnetic field lines around a straight, current-carrying wire form concentric circles centered on the wire, lying in a plane perpendicular to it.
- Right-Hand Thumb Rule: Used to determine the direction of the magnetic field. If a current-carrying conductor is held in the right hand with the thumb pointing in the direction of the conventional current, then the curled fingers wrap around the conductor in the direction of the magnetic field lines.
2. Circular Loop
When the wire is bent into a circular loop, the magnetic field lines at the center of the loop become straight, parallel lines perpendicular to the plane of the loop. The field strength at the center is directly proportional to the current and inversely proportional to the radius of the loop.
3. Solenoid
A solenoid is a long coil containing a large number of close turns of insulated copper wire wrapped in the shape of a cylinder.
- Field Characteristics: When an electric current passes through a solenoid, it generates a uniform magnetic field inside it running parallel to its axis. The magnetic field pattern produced by a current-carrying solenoid is nearly identical to that of a standard bar magnet, presenting distinct North and South poles at its ends.
- Electromagnets: If a piece of soft iron core is placed inside a current-carrying solenoid, the intense magnetic field strongly magnetizes the core. This temporary magnet is called an electromagnet. Its magnetic strength can be increased by increasing the current or the number of turns in the coil. It loses its magnetism instantly when the current is switched off.
Earth’s Magnetism (Geomagnetism)
The Earth behaves like a giant magnetic dipole, with its magnetic field extending tens of thousands of kilometers into space, forming the magnetosphere.
Origin of Earth’s Magnetic Field
The most widely accepted theory is the Dynamo Effect. The Earth’s core consists of an outer layer of molten iron and nickel. As the Earth rotates, convective currents are set up in this highly conductive liquid metal fluid due to heat escaping from the inner core. This continuous motion of metallic ions sets up circulating electric currents, generating the Earth’s global magnetic field.
Magnetic Axis vs. Geographic Axis
The Earth’s magnetic poles do not coincide with its geographic poles. The magnetic axis is currently tilted at an angle of approximately 11.3° relative to the Earth’s rotational (geographic) axis.
- Geographic North vs. Magnetic North: The Earth’s magnetic pole located in the geographic northern hemisphere is physically a magnetic south pole, which is why it attracts the north-seeking pole of a compass needle.
Elements of Earth’s Magnetic Field
To completely describe the Earth’s magnetic field at any specific location on the surface, three parameters (known as magnetic elements) are measured:
- Magnetic Declination (θ): The angle between the geographic meridian (the plane passing through the true geographic north-south axis) and the magnetic meridian (the plane passing through the magnetic north-south axis) at a given place. It varies with location across the globe.
- Magnetic Inclination or Dip (δ): The angle made by the Earth’s total magnetic field vector (BE) with the horizontal surface of the Earth.
- At the Magnetic Equator, the dip is 0° (the field lines are perfectly horizontal).
- At the Magnetic Poles, the dip is 90° (the field lines point straight down into the Earth).
- Horizontal Component (BH): The component of the Earth’s total magnetic field intensity acting along the horizontal direction. It is expressed asBH = BE cosδ.
Applications and Technological Significance
1. Geomagnetic Shielding and Life on Earth
The Earth’s magnetic field forms a protective shield called the magnetosphere. It deflects harmful solar winds and cosmic rays (highly energetic charged particles emitted by the sun) away from the planet. Without this field, these particles would strip away the atmosphere and destroy organic life. The interaction of these trapped charged particles with atmospheric gases near the poles creates the luminous phenomenon known as Auroras (Aurora Borealis in the North and Aurora Australis in the South).
2. Magnetic Resonance Imaging (MRI)
In medical diagnostics, MRI machines use extremely powerful electromagnets (often utilizing liquid helium-cooled superconductors) to generate fields up to 3 Tesla. This strong field aligns the nuclear spins of hydrogen atoms in the human body, allowing detailed internal imaging without ionizing radiation.
3. Maglev Trains (Magnetic Levitation)
Maglev trains use high-power electromagnets to eliminate mechanical friction. One set of magnets levitates the train above the guide-track through magnetic repulsion, while another set propels it forward using dynamic alternating fields, allowing speeds exceeding 500 km/h.
Last Modified: May 28, 2026