Hello, and welcome to today's lesson on Magnetism. In this chapter, we will explore how magnets interact with materials, understand the invisible magnetic fields that surround them, discover how our Earth itself behaves like a giant magnet, and learn about electromagnets — powerful temporary magnets created using electricity.
Let us begin with the fascinating history of magnets. The first known magnets were pieces of lodestone, a naturally occurring iron oxide with the chemical formula Fe₃O₄. This ore was discovered in Magnesia, a region in Asia Minor, which is where the word "magnet" originates. Lodestone possesses two remarkable properties: it attracts small pieces of iron, and when suspended freely, it aligns itself along a definite direction. The Chinese used these natural magnets for navigation as early as 2500 B.C.
Natural magnets, however, have limitations. They come in irregular shapes and are not magnetically strong enough for practical use. This is why we create artificial magnets from iron and steel in convenient shapes like bar magnets, horseshoe magnets, magnetic needles, and compasses.
When a magnet is suspended freely with a silk thread, it always comes to rest pointing in the geographic north-south direction. The end pointing north is called the north pole, and the end pointing south is called the south pole. Remember this fundamental law: like poles repel each other, while unlike poles attract each other.
Now, let us understand induced magnetism. When an unmagnetised bar of soft iron or steel is placed near or in contact with a magnet, something remarkable happens. The iron bar temporarily becomes a magnet itself — it can attract iron filings. However, the moment the magnet is removed, the iron bar loses its magnetism and the filings fall away.
The temporary magnetism acquired by a magnetic material when it is kept near or in contact with a magnet, is called induced magnetism.
The process in which a piece of magnetic material acquires magnetic properties temporarily in presence of another magnet near it, is called magnetic induction.
Here is a crucial observation about induced magnetism. When a magnet induces magnetism in a nearby iron bar, the near end of the bar develops opposite polarity to the inducing pole, while the far end develops similar polarity. For example, if the north pole of a magnet approaches one end of an iron bar, that near end becomes a south pole, and the far end becomes a north pole.
This leads us to an important principle: induction precedes attraction. When a piece of iron is brought near a magnet, the iron first becomes magnetised by induction — its near end developing opposite polarity. Only then does attraction occur between these unlike poles. The iron does not jump to the magnet magically; it must first become a temporary magnet.
Induced magnetism is purely temporary. Try this experiment: bring a bar magnet near small iron nails, and they will form a chain. Each nail becomes magnetised by induction and attracts the next. But remove the original magnet, and the entire chain collapses as all nails lose their temporary magnetism.
Let us now explore the concept of magnetic fields. A magnetic compass normally points north-south. But place it near a magnet, and the needle swings to a new position. This happens because the magnet creates an invisible influence around itself — a magnetic field.
The space around a magnet in which the needle of a compass rests in a direction other than the geographic north-south direction, is called the magnetic field of the magnet.
Magnetic field is a vector quantity. Its magnitude is measured by the force experienced by a unit magnetic pole placed at that point. Its direction is the direction in which the north pole of a compass needle points when placed at that point.
We can visualise magnetic fields using iron filings. Sprinkle iron filings on a paper placed over a magnet and tap gently. The filings arrange themselves along curved lines called magnetic field lines. Each filing becomes a tiny magnet by induction and aligns with the field.
A magnetic field line is a continuous curve in a magnetic field such that tangent at any point of the curve gives the direction of the magnetic field at that point.
Magnetic field lines have seven important properties you must remember. First, they are closed and continuous curves. Second, outside the magnet, they run from north pole to south pole. Third, the tangent at any point indicates the field direction. Fourth, and this is crucial: field lines never intersect each other. If they did, a compass needle would have to point in two directions simultaneously, which is impossible. Fifth, lines are crowded near poles where the field is strong, and spread out where it is weak. Sixth, parallel and equidistant lines represent a uniform magnetic field. Seventh, field lines behave like stretched elastic strings, always trying to shorten themselves.
Now, consider this remarkable fact: our Earth itself behaves like a huge magnet. Several observations prove this.
First, a freely suspended magnetic needle always aligns north-south. This happens because Earth's magnetic south pole lies near geographic north, attracting the needle's north pole. Second, an iron rod buried along the north-south direction becomes weakly magnetised over time. Third, when plotting field lines of a bar magnet, we find neutral points where the net field is zero — this occurs because Earth's field cancels the magnet's field. Fourth, a magnetic needle suspended freely in a vertical plane tilts at different angles at different locations, becoming vertical at magnetic poles and horizontal at the magnetic equator.
Earth's magnetic field is uniform in limited spaces. Its field lines are parallel and equidistant, running from geographic south to geographic north. They are horizontal at the magnetic equator and vertical at the magnetic poles.
When we plot field lines of a bar magnet placed with its north pole facing geographic north, we obtain a non-uniform field pattern. Near the magnet, lines curve due to the strong magnetic field. Far away, lines become parallel due to Earth's dominant field. Most importantly, we find two neutral points on either side of the magnet in the east-west direction. At these points, the magnet's field exactly cancels Earth's horizontal field, resulting in zero net field.
If we reverse the magnet — placing its south pole toward geographic north — the neutral points shift to the north-south direction on either side of the magnet. The position of neutral points depends entirely on how the magnet is oriented in Earth's field.
Neutral points are the points at which two magnetic fields are equal in magnitude, but opposite in direction.
The net magnetic field at a neutral point is zero.
Let us now turn to electromagnets — one of the most useful applications of magnetism. An electromagnet is a temporary strong magnet made from a piece of soft iron by passing current through a coil wound around it.
To make an I-shaped electromagnet, wind insulated copper wire around a soft iron bar and connect it to a battery through a switch. When current flows, the end where current flows clockwise becomes a south pole, and the end where current flows anticlockwise becomes a north pole. The bar magnetises only while current flows, and loses magnetism immediately when switched off.
A U-shaped or horseshoe electromagnet is made by winding wire on both arms of a U-shaped soft iron core, with windings in opposite directions. This creates a strong, nearly uniform magnetic field in the gap between the poles.
The strength of an electromagnet can be increased in two ways: by increasing the number of turns in the coil, or by increasing the current through it. Always use direct current, not alternating current, if you want stable polarity. With AC, the polarity reverses 50 times per second, though the iron still magnetises.
How does an electromagnet compare to a permanent magnet? Electromagnets are made of soft iron; permanent magnets of steel. Electromagnets produce temporary fields only while current flows; permanent magnets retain their field indefinitely. Electromagnet strength can be varied and reversed; permanent magnet strength is fixed. Electromagnets can produce much stronger fields and are easily demagnetised by switching off.
Electromagnets have countless applications. They lift heavy iron scrap in industries — up to 20000 kilograms in a single lift. They load furnaces, separate iron from ores, and even remove iron fragments from wounds. In electric bells, electromagnets create the ringing mechanism through repeated make-and-break circuits. They are essential in telegraphs, relays, motors, generators, microphones, and loudspeakers.
Let us quickly recap the key takeaways from this chapter.
First, induced magnetism is temporary magnetism acquired by magnetic materials when placed near a magnet; induction always precedes attraction. Second, magnetic field lines are continuous curves that never intersect; outside the magnet, they run from north pole to south pole. Third, Earth's magnetic field is evidenced by compass behaviour, magnetisation of buried iron rods, neutral points, and the varying angle of magnetic inclination. Fourth, neutral points are locations where two opposing magnetic fields of equal magnitude cancel each other completely. Fifth, electromagnets are temporary magnets made by passing current through a coil wound on soft iron. Sixth, electromagnet strength depends on number of turns and current magnitude.
Magnetism surrounds us — in the compass that guides explorers, in the MRI machines that save lives, and in the very planet beneath our feet. Understanding these principles opens doors to countless technological marvels. Keep curious, keep questioning, and I look forward to our next exploration together.