Lesson Notes By Weeks and Term v3 - Junior Secondary 3
Magnetism
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Magnetism
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Basic Science
Class: Junior Secondary 3
Term: 3rd Term
Week: 2
Theme: You And Energy
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
describe loadstone as a naturally occurring magnet; state laws of magnetism; illustrate magnetic poles and fields; explain how to care for a magnet.
Definition: Loadstone (also spelled lodestone) is a naturally magnetized mineral, specifically a form of magnetite (an iron ore). It is the only known natural magnet.
Composition: Chemically, loadstone is an iron oxide with the formula Fe3O
4. Its magnetic properties are due to its crystal structure and the arrangement of its iron atoms, which align to create a permanent magnetic field.
Discovery and Historical Significance: Loadstone was discovered in ancient times, particularly in Magnesia (hence the term "magnetism"), an area in ancient Greece (now part of Turkey). Early civilizations, including the Chinese and Greeks, observed its ability to attract small pieces of iron. Its most significant historical application was as an early form of compass. Small pieces of loadstone, when suspended freely, would consistently align themselves in a North-South direction, making them invaluable for navigation across land and sea, aiding trade and exploration in regions like the ancient Trans-Saharan trade routes.
Properties: Like artificial magnets, loadstone has North and South poles and can attract ferromagnetic materials (iron, nickel, cobalt). It also exhibits a magnetic field around it. There are two fundamental laws that govern the interaction between magnets or magnetic poles: Law of Attraction: Opposite (unlike) magnetic poles attract each other.
Explanation: When a North pole is brought near a South pole, a force of attraction is experienced, pulling the two poles together.
Example: If the North pole of one bar magnet is brought close to the South pole of another bar magnet, they will pull towards each other.
Law of Repulsion: Similar (like) magnetic poles repel each other.
Explanation: When a North pole is brought near another North pole, or a South pole near another South pole, a force of repulsion is experienced, pushing the two poles apart.
Example: If the North pole of one bar magnet is brought close to the North pole of another bar magnet, they will push away from each other. The same applies to two South poles.
Key Principle: Repulsion is the sure test of magnetism, as a magnet can attract both magnetic and non-magnetic materials (through induction), but only magnets will repel each other.
Magnetic Poles: Every magnet, regardless of its shape or size (even loadstone), has two regions where its magnetic force is strongest. These regions are called magnetic poles.
There are always two poles: a North-seeking pole (or simply North pole, N) and a South-seeking pole (or simply South pole, S). These poles cannot be isolated. If a magnet is broken into smaller pieces, each piece will still have its own North and South poles. This indicates that magnetism resides in the elementary particles (domains) within the material. The North pole is defined as the end of a freely suspended magnet that points towards the Earth's geographical North.
Magnetic Field: Definition: A magnetic field is the region around a magnet or a current-carrying conductor where its magnetic influence can be detected. Any magnetic material or another magnet placed within this field will experience a magnetic force.
Illustration (Magnetic Field Lines): Magnetic fields are conventionally represented by imaginary lines called magnetic field lines or lines of force.
Properties of Magnetic Field Lines: Direction: They emerge from the North pole and enter the South pole outside the magnet, forming continuous closed loops within the magnet itself (from South to North).
Density: The density of the field lines (how close they are to each other) indicates the strength of the magnetic field. Where lines are denser (e.g., at the poles), the field is stronger.
Non-Intersection: Magnetic field lines never intersect each other. If they did, it would imply two directions for the magnetic force at a single point, which is impossible.
Repulsion/Attraction: Field lines originating from like poles diverge (push away), illustrating repulsion. Field lines originating from unlike poles converge (join), illustrating attraction.
Visualizing Magnetic Fields: The pattern of magnetic field lines around a magnet can be visualized by sprinkling iron filings on a sheet of paper placed over the magnet. The filings align themselves along the field lines. Magnets can lose their magnetism (demagnetize) if not properly handled and stored. It is crucial to care for them to preserve their magnetic properties.
Causes of Demagnetization: Heating: Heating a magnet above a certain temperature (known as the Curie temperature, which varies for different materials) causes the magnetic domains within the material to become disoriented, leading to a loss of magnetism.
Hammering/Dropping: Repeatedly striking or dropping a magnet causes mechanical shocks that disorient the magnetic domains, thus weakening or destroying its magnetism.
Improper Storage: Storing magnets incorrectly, especially leaving them exposed without keepers or near other strong magnetic fields, can lead to demagnetization over time.
Alternating Current (AC): Exposing a magnet to a strong alternating current can disrupt its magnetic alignment.
Methods of Caring for Magnets: Use of Keepers (Armatures): Bar magnets should be stored in pairs with opposite poles facing each other, and two soft iron pieces (keepers) placed across their ends. This provides a closed loop for the magnetic field lines, preventing demagnetization and keeping the domains aligned. Horseshoe magnets typically use a single soft iron keeper across their poles.
Avoid Heating: Magnets should not be heated, especially to high temperatures.
Avoid Dropping/Hammering: Magnets should be handled carefully to prevent mechanical shocks.
Store Away from Other Magnets: Unless stored with keepers, magnets should ideally be kept away from strong magnetic fields that could interfere with their own.
Proper Orientation: When storing multiple magnets without keepers, ensure they are oriented such that like poles are together (e.g., N-N, S-S) and separated by non-magnetic material to minimize interaction and demagnetization.
Navigation and Exploration in Nigeria: Compasses, which operate based on the Earth's magnetic field and the principles of magnetism, are crucial tools. Nigerian fishermen navigating the Atlantic Ocean, oil prospectors in the Niger Delta, or land surveyors mapping new developments across the country rely on compasses to determine direction. Understanding magnetism helps them appreciate how these tools function to guide them accurately.
Waste Sorting and Recycling: In Nigerian urban centres like Lagos or Port Harcourt, managing waste is a significant challenge. Magnetic separation is a practical method used in recycling plants (or even informally by scavengers) to separate ferrous metals (iron, steel) from non-magnetic waste. Large electromagnets lift and sort metallic items from mixed refuse, aiding in resource recovery and reducing landfill burden. Domestic Appliances and Electrical Systems: Magnets are integral components in many devices found in Nigerian homes and businesses. For example, the speakers in radios and phones use magnets to convert electrical signals into sound waves. Electric motors in blenders, fans, and borehole pumps rely on the interaction between magnetic fields and electric currents to produce motion. Generators that provide electricity in homes and businesses (especially prevalent due to power fluctuations) also use strong magnets to induce electric currents.