How many of the following materials can be attracted by a magnet ? 1. Plastic 2. Carbon 3. Aluminium 4. Stainless Steel Select the correct answer using the code given below :
- (a)1
- (b)2
- (c)3
- (d)None
Correct — D, None. Only ferromagnetic materials are pulled towards a magnet strongly enough for anyone to notice, and that list is short — iron, cobalt, nickel, most of their alloys, and certain rare-earth metals. Not one of the four items on this list belongs to it. Plastics are organic polymers and are diamagnetic, so a magnet has no hold on them at all. Carbon in its common form, graphite, is also diamagnetic — it is very weakly pushed away by a magnetic field rather than pulled in. Aluminium is paramagnetic, which sounds promising until you look at the size of the effect: paramagnetic attraction normally has to be measured on a sensitive analytical balance, so a bar magnet will not lift an aluminium foil or a soft-drink can. Stainless steel is the one that fools people, because it is a steel and steel means iron — but the everyday grade, Type 304, the 18/8 chromium-nickel austenitic steel used for kitchen vessels and sinks, is essentially non-magnetic. So the count of materials attracted is zero.
- (a)1 — This is what a candidate marks who counts stainless steel alone, reasoning that a steel must behave like iron. The chromium-nickel austenitic grade that gives stainless steel its everyday reputation is essentially non-magnetic, so counting it gives one item too many.
- (b)2 — This usually means aluminium plus stainless steel, on the assumption that any metal answers to a magnet. Aluminium is only paramagnetic, and paramagnetic attraction is so feeble that it takes laboratory apparatus to register, not a magnet in the hand.
- (c)3 — To reach three you have to add carbon to the two metals, but graphite is diamagnetic and is weakly repelled by a magnetic field — pyrolytic carbon in fact has one of the largest diamagnetic constants of any room-temperature material. Repulsion is the opposite of what the question asks about.
Every material responds to a magnetic field, but in three very different ways. Ferromagnetic materials — iron, cobalt, nickel and most of their alloys — have domains that line up with the field and are pulled in hard enough for the effect to be part of daily life. Paramagnetic materials such as aluminium and oxygen are pulled in as well, but so weakly that the force generally has to be measured on a sensitive balance. Diamagnetic materials such as graphite, water, copper and most plastics are pushed away, weakly. When an exam says a material can be attracted by a magnet, it means the first class only.
The trap is the word steel. Stainless steel is not one material but a family, and the family splits on exactly this property. The austenitic grades — Type 304, the familiar 18/8, and Type 316 — are essentially non-magnetic, and those are the grades a textbook has in mind when it names stainless steel. Ferritic and martensitic grades, which carry little or no nickel, do stick to a magnet, which is why a fridge magnet will cling to some steel cutlery in an Indian kitchen and slide off other pieces. The examiner is testing the textbook default, and the key marks None, so that is the reading to carry into the hall. It is worth knowing the real picture anyway, because the magnet test is the standard workshop trick for telling one class of stainless from another.
- Relatively few materials are ferromagnetic; the common ones are iron, cobalt, nickel, most of their alloys, and certain rare-earth metals.
- Aluminium is paramagnetic, and paramagnetic attraction typically requires a sensitive analytical balance to detect, unlike the much stronger ferromagnetic pull.
- Carbon as graphite is diamagnetic, with a magnetic susceptibility of about minus 1.6 in units of ten to the power minus five; most organic compounds and some plastics are diamagnetic too.
- Type 304, the commonest stainless steel and the 18/8 chromium-nickel grade, is essentially non-magnetic, while ferritic and martensitic stainless grades are magnetic.
Plastic and carbon fall in the bottom row, aluminium in the middle, and everyday stainless steel is the non-magnetic austenitic grade — so the count is zero.
- Treating steel as automatically magnetic; the everyday austenitic stainless grade is not.
- Assuming every metal is attracted by a magnet — copper, aluminium, zinc and gold are not.
- Reading paramagnetic as attracted; the force exists but is far too small to show up with an ordinary magnet.
Asked in the count format — how many of four listed materials share a physical property — so one wrong item costs the whole mark.
Which one of the following is paramagnetic in nature?
- (a) Iron
- (b) Hydrogen
- (c) Oxygen
- (d) Nitrogen
Answer(c) Oxygen
The same three-way classification, asked from the middle class instead of the top one. Iron is the planted trap there because it is ferromagnetic rather than merely paramagnetic — exactly the distinction that decides whether aluminium counts here.
The statement "friction force is a contact force while magnetic force is a non-contact force" is
- (a) always true.
- (b) true only at 0°C.
- (c) a false statement.
- (d) either true or false depending upon the temperature of the surroundings.
Answer(a) always true.
The other half of the same idea. That question asks how the magnetic pull reaches a material across a gap; this one asks which materials feel the pull at all.
- practice — not a real PYQ
Which one of the following is attracted strongly by a magnet ?
- (a)Copper
- (b)Cobalt
- (c)Aluminium
- (d)Zinc
Answer(b) Cobalt — it is ferromagnetic along with iron and nickel; copper is diamagnetic, aluminium only paramagnetic and zinc neither.
- practice — not a real PYQ
A material that is weakly repelled when placed in a magnetic field is called
- (a)ferromagnetic
- (b)paramagnetic
- (c)diamagnetic
- (d)ferrimagnetic
Answer(c) diamagnetic — graphite, water and copper behave this way, and pyrolytic carbon shows the effect most strongly of all room-temperature materials.