Magnetism: Types, Interactions, and Fields
Magnetism
A magnet is any substance that attracts iron. Compass needles are typically elongated and geometrically regular.
Types of Magnets
- Natural Magnets: Magnetite, a natural magnet, attracts magnetic substances. It’s composed of iron oxide. Magnetic substances are those attracted to magnetite.
- Artificial Permanent Magnets: These substances become magnetized when rubbed with magnetite, retaining their attraction for a long time.
- Artificial Temporary Magnets: These produce a magnetic field only when an electric current flows through them, like an electromagnet.
Interaction Between Magnets
Opposite magnetic poles attract, while like poles repel. If a magnet is broken, each part becomes a new magnet with its own north and south poles. When a compass needle nears a magnet, its south pole points towards the magnet’s north pole due to attraction. Separating a magnet’s poles is impossible.
Magnetic Field
A magnetic field is the space where a force acts on a magnetic needle or magnet. A magnet alters the surrounding space: small compass needles or iron pieces are attracted to the magnet. Magnetic fields are represented by lines of force. The field is strongest near the poles where lines of force are concentrated.
Lorentz Force
When a moving electric charge (Q) with velocity (v) enters a magnetic field (B), it experiences a force (F) expressed as: F = Qv x B. This force is perpendicular to both the velocity and the magnetic field. Because it’s perpendicular to the charge’s velocity, it doesn’t do work on the charge, meaning the charge’s kinetic energy and speed remain constant. The magnetic field only changes the direction of the velocity.
Magnetic Forces
Magnetic Forces on a Current
A moving charge in a magnetic field experiences a magnetic force (Fm) that deflects its path. Since electric current is a continuous flow of charge, a current-carrying conductor in a magnetic field experiences the combined effect of magnetic forces on the moving charges within. For a linear current of length l, the magnetic force is: Fm = IBLsin(θ), where I is the current, B is the field strength, and θ is the angle between the current and the field. This is known as Laplace’s law.
The left-hand rule determines the direction of the magnetic force: the thumb points in the direction of Fm, the index finger in the direction of B, and the middle finger in the direction of the current.
Magnetic Force on a Rectangular Loop
A rectangular loop carrying current in a magnetic field (like that of a horseshoe magnet) experiences forces that cause it to rotate until it aligns with the field direction. Applying Laplace’s law to the loop’s vertical segments gives the expression M = BISsin(A), where S is the loop’s area. When the loop aligns with the field, A becomes zero, and the moment M is zero, indicating equilibrium.
Moving-Coil Galvanometer
A moving-coil galvanometer consists of a coil in a magnetic field. Current flowing through the coil causes it to rotate against a spring. The coil’s final angle depends on the current intensity. Key characteristics include maximum current intensity and internal resistance. It can function as an ammeter (measures current) or a voltmeter (with a series resistance).
Magnetic Field Due to Straight Current
Iron filings around a straight current-carrying wire reveal concentric circles, representing the magnetic field lines. The right-hand rule relates the current direction to the field direction: if the thumb points in the current direction, the curled fingers indicate the field lines’ direction. The field strength (B) depends on the medium’s permeability (μ), current intensity (I), and distance (r) from the wire: B = μI / 2πr, where μ0 (vacuum permeability) is 4π x 10-7 Tm/A.
Magnetic Field Due to a Circular Loop
The magnetic field around a circular current loop resembles that of a bar magnet. The right-hand rule determines the north pole: if the fingers curl in the current direction, the thumb points to the north pole. The field strength (B) inside the loop depends on the medium’s permeability (μ), current intensity (I), and loop radius (R): B = μI / 2R.
Origin of Natural Magnetism
Ampère explained natural magnetism using microscopic electric currents within materials. These currents create magnetic fields, and their combined effect explains the material’s magnetic properties. Electrons in atoms behave like small current loops, contributing to the atom’s and the material’s magnetism.
Phenomena of Magnetic Field
Three phenomena occur when matter is subjected to an external magnetic field:
- Diamagnetism: Changes in electron radius and speed alter the atom’s magnetic moment. This occurs in all atoms but is noticeable when numerous electrons are arranged symmetrically, resulting in a net zero magnetic moment. The internal magnetic field is weaker. Diamagnetic materials are difficult to magnetize.
- Paramagnetism: Occurs in substances with a non-zero atomic magnetic moment. Normally randomly oriented, these moments align with an external field, reinforcing it. This effect depends on temperature. Paramagnetic materials are easily magnetized.
- Ferromagnetism: Strong interatomic forces cause parallel alignment of atomic moments within groups of atoms (domains). When subjected to an external field, these domains align like in paramagnetism.