What is Magnetism?
Magnetism is a physical phenomenon by which materials exert attractive or repulsive forces on each other, primarily due to the motion of electric charges within them.
This force manifests most strongly in magnetic materials such as iron, nickel, and cobalt, and it is governed by the laws of electromagnetism.
Magnetic Field (MF)
Magnetic fields are invisible regions around magnets where these forces act. They are produced by moving electric charges (current) and by the intrinsic magnetic moments of elementary particles.
Magnetic field is measured in units of tesla (T) or gauss.
The strength of a magnetic field decreases with distance.
Magnets
A magnet is a material or object that produces a magnetic field.
Magnets have two poles: north and south.
Unlike poles attract, while like poles repel each other.
All magnets are dipoles—if you split one, each piece still has both a north and south pole.
Types of Magnets
- Permanent magnets: retain their magnetic properties even without an external field (e.g. fridge magnets).
- Temporary magnets: only display magnetism when in a magnetic field (paper clips, iron nails).
- Electromagnets: magnets generated by electric current; turn off when the current stops (e.g. MRI machines).
Types of Magnetism
There are five types of Magnetism.
- Ferromagnetism: Strong attraction and can be permanently magnetized. (e.g. iron, cobalt, nickel).
- Para magnetism: Weak attraction to magnetic fields (e.g. aluminum, oxygen).
- Diamagnetism: Weak repulsion from magnetic fields (e.g. copper, carbon).
- Antiferromagnetism: Adjacent magnetic moments align in opposite directions, canceling each other out. (e.g. used in magnetic sensors and memory devices).
- Ferrimagnetism: Similar to Antiferromagnetism, but with unequal moments. (e.g. used in electronic devices).
Important Laws and Principles
Biot–Savart Law: Describes the magnetic field produced by a current element.
Ampère’s Circuital Law: Relates magnetic field around a closed loop to current passing through it.
Faraday’s Law of Induction: A changing magnetic field induces an electric current.
Lenz’s Law: Induced current opposes the change that produces it.
Gauss’s Law for Magnetism: Magnetic monopoles do not exist; net magnetic flux through a closed surface is zero.
Key Equations
Magnetic Force on a Moving Charge (Lorentz Force):
Force on a Current-Carrying Conductor:
Biot–Savart Law:Magnetic Field Due to a Long Straight Wire:
Magnetic Field at Center of Circular Loop:Magnetic Field Inside a Long Solenoid:
Gauss’s Law for Magnetism:
Magnetic Field Measurement Techniques
Magnetic field measurement techniques include a variety of methods and instruments depending on the application, required accuracy, and field strength range. Common techniques are:
- Hall Probe (Gaussmeter): Uses the Hall effect, where a voltage generated in a conductor or semiconductor is proportional to the magnetic field perpendicular to the current flow. It provides instant digital readout and is widely used for point measurements and field mapping.
- Fluxmeter (Induction coil method): Based on Faraday’s law, measures induced voltage in a coil when the magnetic flux changes, useful for measuring magnetic flux and fields in accelerator magnets and industrial applications.
- Nuclear Magnetic Resonance (NMR): Measures magnetic field strength with very high accuracy (down to 0.1 ppm) by detecting precession frequency of nuclei in a sample in a magnetic field. Requires stable and homogeneous fields.
- Vibrating Sample Magnetometer (VSM): Measures magnetic moment by vibrating a sample in a uniform magnetic field and detecting induced voltage in pick-up coils. It gives sensitivity suitable for weak to strong magnetic materials.
- Magnetic Field Mapper: Moves sensors in a defined spatial area to map the magnitude and direction of the field in 3D, often using 3-axis sensors for detailed distribution studies.
- Field Indicators: Mechanical devices with iron vanes that physically move in response to a magnetic field, mostly used for qualitative or residual field measurements.
These techniques are chosen based on required accuracy, field range (from microtesla to several teslas), and whether the measurement is absolute (NMR) or relative (Hall probe, fluxmeter). Hall probes and fluxmeters are common for industrial and accelerator magnet measurements, while NMR is the standard for absolute precision in stable fields.
Magnetic Field Sources in Nature
The main natural sources of magnetic fields include the Earth’s core (producing the geomagnetic field), magnetized rocks in the crust, electric currents in the ionosphere and magnetosphere, and—on smaller scales—certain materials and oceanic circulation.
- Earth’s core: The dominant source is the geodynamo process in the fluid outer core, where convection of molten iron and nickel generates electrical currents, creating the Earth’s magnetic field.
- Magnetized rocks (lithosphere): Some rocks in Earth’s crust, notably those containing magnetic minerals like magnetite, retain a permanent or induced magnetization, contributing to localized magnetic anomalies.
- Ionosphere and magnetosphere: Electric currents driven by solar radiation or thermospheric winds in the ionosphere and magnetosphere also generate magnetic fields, especially affecting surface measurements and causing daily variations.
- External and induced fields: Variations can be further affected by fields originating outside Earth (such as from solar activity), and by weak contributions from ocean currents.
- Beyond Earth, cosmic magnetic fields are present throughout the universe, including those produced by other planets, stars, and galaxies.
The geomagnetic field is strongest and most stable due to the core, but all contributions—including crustal and atmospheric sources—combine to form the total field measured at the surface.
Glossary of Key Terms
Recap of the Key Terms in Magnetism
- Air Gap: The space between the poles of a magnet, across which the magnetic field acts.
- Anisotropic Magnet: A magnet with a preferred direction of magnetic orientation for optimal properties.
- Coercive Force (Hc): The intensity of a magnetic field required to reduce a magnetized material’s magnetism to zero.
- Curie Temperature (Tc): The temperature at which a magnetic material loses its magnetic properties.
- Demagnetizing Force: A force applied to reduce the field in a magnetized material.
- Electromagnet: Temporary magnet created when current flows in a coil.
- Gauss: A unit of magnetic induction or magnetic flux density.
- Isotropic Magnet: A magnet whose properties are the same in all directions and can be magnetized along any axis.
- Keeper: A piece of iron placed between magnet poles to reduce flux leakage.
- Lorentz Force: Force experienced by a moving charge in electric and magnetic fields.
- Magnet: An object or material possessing a magnetic field, attracting ferrous materials, with distinct north and south poles.
- Magnetic Field: The region around a magnet where magnetic forces are observed.
- Magnetic Flux: The total magnetic induction over a given area; a measure of the amount of magnetic field passing through that area.
- Magnetic Induction (B): The process of producing magnetism in a substance by putting it in a magnetic field; measured in gauss.
- Magnetic Orientation: The direction in which the poles of a magnet are aligned.
- Magnetic Polarity: The orientation of a magnet’s north and south poles.
- Magnetic Saturation: The point at which a magnetic material cannot absorb any more magnetism.
- Magnetizing Force (H): Force per unit length at any point in a magnetic circuit; measured in Oersteds.
- Magnetomotive Force (F): The magnetic potential difference between two points, analogous to voltage in electrical circuits.
- Maximum Energy Product (BHmax): The point on the magnet’s curve where the product of magnetic flux and field strength is greatest (expressed in Mega Gauss Oersteds, MGOe).
- Oersted (Oe): A unit used to measure magnetic field intensity.
- Permeability (μ): A measure of how easily a medium allows magnetic field lines.
- Residual Magnetism: Tiny amounts of magnetism left in a material after it has been exposed to a magnetic force.
- Right-hand Rule: Determines direction of field or force using thumb and fingers orientation.
- Solenoid: A long coil of wire; produces a nearly uniform magnetic field inside.
Quiz
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is magnetism?
Magnetism is a physical phenomenon by which materials exert attractive or repulsive forces on each other due to the motion of electric charges.
What are the two poles of a magnet called?
The two poles are called north and south.
What happens when like poles of two magnets are brought together?
Like poles repel each other.
Which materials are commonly magnetic?
Iron, nickel, and cobalt are commonly magnetic.
What is a magnetic field?
A magnetic field is an invisible region around a magnet where magnetic forces are exerted.
Name one example of a natural source of magnetic field.
The Earth’s core generates the planet’s geomagnetic field through the geodynamo process.
What is a permanent magnet?
A permanent magnet retains its magnetism even without an external magnetic field.
What type of magnet turns off when the electric current is stopped?
An electromagnet loses magnetism when the electric current stops.
Which unit is used to measure magnetic field strength?
Magnetic field strength is measured in tesla (T) or gauss.
What technology uses magnets to produce images of the inside of the body?
MRI (Magnetic Resonance Imaging) machines use powerful magnets for medical imaging.