NDA (I) 2017 General Ability Test · Question 109 of 149

Correct — B, Electrons do not flow easily through them. Every material is full of electrons; what separates a conductor from an insulator is whether those electrons are free to move. In a metal the outermost electrons are shared across the whole lattice and drift the moment a potential difference is applied, which is what an electric current is. In rubber, glass, dry wood, mica and the plastics used for cable sheathing, the electrons are locked into covalent bonds around their own atoms, and a very large energy gap has to be crossed before any of them can move. So charge does not travel through the material, and that single property is what makes it an insulator.

• (a)They contain no electrons — Every atom of every substance contains electrons, and an insulator is made of atoms just as a conductor is. The difference lies entirely in whether those electrons are free to move, not in whether they exist.
• (c)They are crystals — Some insulators are crystalline — mica and quartz, for instance — but many are not. Rubber, most plastics and ordinary glass are amorphous, with no long-range order at all, and they insulate perfectly well. Crystal structure is neither necessary nor sufficient; diamond is crystalline and insulates, while copper is crystalline and conducts.
• (d)They have more number of electrons than the protons on their surface — That describes a body carrying a negative charge, not an insulating material. An uncharged insulator has exactly as many electrons as protons and is electrically neutral. An insulator can certainly be given a surplus of electrons by rubbing, but so can many other things, and having that surplus is a state the object is in rather than a property of the material.

Materials are classified electrically by how readily charge moves through them. Conductors such as metals have a sea of delocalised electrons and very low resistivity. Insulators, also called dielectrics, hold their electrons in tight covalent bonds separated from the empty conduction states by a large energy gap, and their resistivity is enormous. Semiconductors such as silicon and germanium sit in between, with a small gap that heat, light or added impurities can bridge — which is why their conductivity rises with temperature while a metal's falls.

Three of the four options here are statements about an object rather than about a material, and spotting that is the shortcut. 'Contains no electrons' and 'more electrons than protons' both describe charge, which is a condition an object can be put into and taken out of. 'They are crystals' describes structure, which varies wildly among insulators. Only one option speaks about how charge behaves inside the material, and that is what the word insulator means in the first place. Note also that a good electrical insulator is often, though not always, a poor conductor of heat as well, since in metals the same free electrons carry both.

• Metals conduct because their outermost electrons are delocalised and free to drift under an applied potential difference.
• In insulators the electrons are bound in covalent bonds and a large energy gap separates them from conducting states.
• Rubber, glass, dry wood, mica, porcelain and most plastics are common insulators; the amorphous ones show that crystallinity is not required.
• Semiconductors have a small energy gap, so their conductivity increases with temperature while that of a metal decreases.

• Believing an insulator has no electrons; it has as many as any other material.
• Equating insulation with crystal structure — rubber and glass are amorphous insulators.
• Mixing up a charged body with an insulating material; charge is a state, insulation is a property.

NDA asks which statement about insulators or conductors is correct, or gives a list of materials and asks which one does not conduct.

• practice — not a real PYQ

  The essential difference between a conductor and an insulator is that in a conductor

  • (a)the atoms contain more electrons
  • (b)a large number of electrons are free to move
  • (c)the protons are free to move
  • (d)the atoms are always arranged in a crystal

  Answer(b) a large number of electrons are free to move — conduction depends on mobile charge carriers, not on how many electrons the atoms contain.

• practice — not a real PYQ

  As the temperature is raised, the electrical conductivity of a pure semiconductor

  • (a)increases
  • (b)decreases
  • (c)remains unchanged
  • (d)first decreases and then becomes zero

  Answer(a) increases — heat lifts electrons across the small energy gap and creates more carriers, the opposite of what happens in a metal.