Which one of the following is a natural magnet ?

Question

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Correct — option (4), lodestone. Lodestone is a naturally occurring piece of the mineral magnetite, whose chemical formula is Fe3O4 and which the paper prints with the 3 and the 4 as subscripts, and it is the original natural magnet — a rock that attracts iron without anything having been done to it. That is exactly what the stem asks for. Magnets are divided into two classes and the division is about origin rather than strength. A natural magnet is one found already magnetised in the earth, and lodestone is essentially the only member of the class. An artificial magnet is one made by a deliberate process, whether by stroking a steel bar with an existing magnet, by placing a piece of ferromagnetic material in a strong magnetic field, or by winding a coil around a soft iron core and passing a current through it to make an electromagnet; bar magnets, horseshoe magnets, compass needles and the magnets in loudspeakers and motors are all artificial. Lodestone has a long history attached to it and the history is worth a line because examiners like it: the property was known in antiquity, the name magnet is generally traced to Magnesia in Asia Minor where the mineral was found, and a suspended sliver of lodestone that settles along the north-south line is the ancestor of the magnetic compass, which is where the English name lode-stone, meaning leading stone, comes from. The physics behind the option set is equally simple once stated. Only a small number of elements are ferromagnetic at ordinary temperatures — iron, cobalt and nickel are the familiar ones — and magnetite is an iron oxide, so its magnetism is unsurprising. Copper, silicon and zinc are not ferromagnetic at all, so none of them can be magnetised into a permanent magnet, let alone be found already magnetised in the ground. That single observation eliminates the other three options together and is more useful than remembering each of them separately. Note that the correct option is the only one that names a compound rather than an element and the only one that carries a chemical formula, which is a printing asymmetry a candidate may notice; here it happens to accompany the answer, but the reason for choosing it is that lodestone is magnetite, not that its option looks different from the others. Option (4) is the answer.

Why the other options are wrong — (a) Copper is not a magnet of any kind, natural or artificial. It is not ferromagnetic, it cannot be permanently magnetised, and a piece of copper brought near a magnet is not attracted to it in any way an ordinary observer would notice; in the precise classification it is diamagnetic, meaning it is very feebly repelled by a magnetic field. Copper's importance in this subject is entirely different and is worth understanding because it is the source of the confusion. Copper is the best cheap conductor of electricity in common use, so the coils that surround the soft iron core of an electromagnet are almost always wound from copper wire, and the copper is therefore always present in any picture of an electromagnet. It carries the current that creates the magnetic field; it is not the thing that becomes magnetic. A candidate who has seen the diagram and not read the caption may well associate copper with magnetism on that basis.  (b) Silicon is not a magnet and is not ferromagnetic. It is a metalloid and a semiconductor, and its overwhelming importance in modern technology lies in electronics rather than in magnetism — it is the material from which diodes, transistors and integrated circuits are made, and from which most photovoltaic cells are made. As with copper there is a real but indirect connection to magnetism that can mislead: silicon is added in small quantities to iron to make the silicon steel used in the cores of transformers and motors, because the alloy has a higher electrical resistivity than pure iron and so loses less energy to circulating eddy currents. The magnetism in that alloy comes from the iron, and the silicon is there to suppress a loss. Silicon on its own is not attracted to a magnet and cannot be made into one.  (c) Zinc is not ferromagnetic and cannot be made into a magnet, so it is neither a natural nor an artificial one; like copper it is properly classified as diamagnetic. Its familiar uses have nothing to do with magnetism: it is the coating applied to iron and steel in galvanising to protect them from rusting, it is one of the two metals in brass, and it forms the container and the negative electrode of the ordinary dry cell. That last use puts zinc in the same chapter of a school textbook as electricity, which is probably why it appears in this option set, and it is a reminder that being associated with a battery is not the same as being magnetic. The general rule that disposes of this option and the two before it is that the common ferromagnetic materials are iron, cobalt and nickel and their alloys and oxides, and zinc is none of these.

Concept: Materials respond to a magnetic field in three broadly different ways, and the classification explains why the answer to a question of this kind is almost always an iron compound. Diamagnetic substances, which include copper, zinc, water and most organic matter, are very feebly repelled by a magnetic field and retain no magnetism when the field is removed. Paramagnetic substances, such as aluminium and platinum, are very feebly attracted and likewise retain nothing. Ferromagnetic substances — iron, cobalt, nickel, their alloys, and certain oxides such as magnetite — are strongly attracted and can retain their magnetism after the field is withdrawn, which is what makes a permanent magnet possible; heating a ferromagnetic material above a characteristic temperature called its Curie point destroys the property, and it returns on cooling. Within the ferromagnetic group a second and practically important distinction is between soft and hard magnetic materials. Soft materials, of which pure or silicon-alloyed iron is the standard example, magnetise easily in a field and lose the magnetism as soon as the field is removed, which is exactly what is wanted in the core of an electromagnet or a transformer. Hard materials such as steel and modern alloys are harder to magnetise but retain their magnetism, which is what is wanted in a permanent magnet. Lodestone sits outside this scheme of manufacture altogether because nobody made it: it is magnetite that acquired its magnetisation naturally, and it is the reason the phenomenon was known thousands of years before any theory of it existed. Its most consequential application was navigational, since a freely suspended piece aligns itself along the local magnetic field and points approximately north and south, and the magnetic compass built on that behaviour changed the practice of sea travel.

State PCS 2022 Q38 — Science & Technology PYQ with Answer

State PCS 2022 Q38 — Science & Technology PYQ with Answer