NDA (I) 2017 General Ability Test · Question 119 of 149

Correct — D, The negative charges are transferred from wool to rod. Rubbing does not create charge; it moves it, and in solids the only charges that can move are electrons. Protons sit locked inside nuclei and never travel from one body to another by friction. So a rod that ends up negatively charged must have gained electrons, and the only place they can have come from is the wool it was rubbed against. The wool, having lost those electrons, is left with an equal positive charge — the two bodies always end up with equal and opposite charges, because the total charge of the pair is conserved throughout.

• (a)The positive charges are transferred from rod to wool — Positive charge in a solid resides in the nuclei, which are bound in the lattice and do not move during rubbing. The end state described would be right — the rod negative and the wool positive — but the mechanism is impossible.
• (b)The positive charges are transferred from wool to rod — Besides invoking a transfer of protons, which does not happen, this would leave the rod positively charged and the wool negative, which is the opposite of what the question describes.
• (c)The negative charges are transferred from rod to wool — The carrier is right this time but the direction is reversed. A rod that loses electrons is left positively charged, whereas the question states that the rod becomes negative.

Charging by friction, one of the three ways of charging a body along with conduction and induction, works by the transfer of electrons between two surfaces in close contact. Which way the electrons go depends on the pair of materials, an ordering captured in the triboelectric series — glass rubbed with silk loses electrons and turns positive, while ebonite or a plastic comb rubbed with wool or hair gains electrons and turns negative. Charge is conserved in every such process and comes in whole multiples of the electronic charge, so the two bodies carry equal and opposite amounts.

The four options are built from two independent choices, the carrier and the direction, and the examiner has laid out all four combinations. Settle the carrier first — only electrons move — which kills half the list at a stroke. Then settle the direction from the end state given in the question: the rod is negative, so the rod gained. Everyday experience backs this up, since it is always the insulating body being rubbed that holds the charge; a metal rod held in the hand would leak its charge to earth through you before you noticed anything.

• In solids only electrons are transferred during charging by friction; protons remain bound in the nuclei.
• The body that gains electrons becomes negatively charged and the body that loses them becomes positively charged.
• Charge is conserved, so the two bodies carry equal and opposite charges after rubbing.
• Charge is quantised as q = ne, where e is about 1.6 × 10⁻¹⁹ coulomb.

• Talking about a transfer of positive charge; protons do not move between bodies by rubbing.
• Getting the direction backwards — the body that becomes negative is the one that gained electrons.
• Thinking that rubbing creates charge, when it only separates charge that was already there.

NDA asks which particle is transferred and in which direction, or which of two rubbed bodies ends up with which sign.

• practice — not a real PYQ

  When a glass rod is rubbed with silk, the glass rod becomes positively charged because

  • (a)protons move from the silk to the glass
  • (b)electrons move from the glass to the silk
  • (c)electrons move from the silk to the glass
  • (d)protons move from the glass to the silk

  Answer(b) electrons move from the glass to the silk — losing electrons leaves the glass with a surplus of positive charge.

• practice — not a real PYQ

  Two neutral bodies are rubbed together. After rubbing, the total charge on the pair is

  • (a)positive
  • (b)negative
  • (c)zero
  • (d)double what it was

  Answer(c) zero — charge is conserved, so the equal and opposite charges on the two bodies still add up to nothing.