CDS (I) 2021 General Knowledge · Question 105 of 120

Correct — C, Human body. The human body conducts electricity because it is mostly water carrying dissolved salts — sodium, potassium, chloride and other ions in blood, sweat and tissue fluid. Free ions can move, and moving charge is current, so the body offers a path for current rather than blocking it. This is not a laboratory curiosity: it is the reason an electric shock is dangerous, the reason electricians wear rubber gloves and stand on dry wooden platforms, and the reason wet skin is far more hazardous than dry skin, since sweat lowers the body's resistance sharply. The other three options are the standard insulators of the syllabus — plastic, dry wood and, less obviously, pure water.

• (a)A plastic sheet — Plastic is the insulator of everyday life. Its electrons are locked into covalent bonds and none are free to drift, which is why cables are sheathed in it and switch casings made of it.
• (b)Distilled water — The interesting wrong answer. Water conducts only through the ions dissolved in it, and distillation removes almost all of them — pure water is a very poor conductor. Tap water, sea water and salt solution conduct well for exactly that reason.
• (d)A wooden thin sheet — Dry wood is an insulator, which is why it is used for the handles of tools meant to be used near live wires. Thinness makes no difference to whether charge can move through the material.

A material conducts electricity when it holds charges free to move. In metals those charges are loose electrons from the outer shells; in solutions and in living tissue they are ions from dissolved salts. Insulators such as plastic, glass, dry wood and rubber have no such mobile charges, so an applied voltage produces almost no current.

The point being tested is that conduction is not a metals-only affair. Three of the options are picture-book insulators, and the fourth is a living body most candidates do not think of as a conductor at all. The route in is to ask what could carry the charge in each case, and the answer is only in the body, where dissolved salts supply ions. Distilled water is the option that separates a memorised answer from an understood one, because 'water conducts electricity' is such a common half-truth. What conducts is the salt content, and distillation is what takes it away.

• The human body conducts because its fluids are solutions of salts, and the dissolved ions are free to move.
• Sweat lowers the body's electrical resistance, so wet skin makes a shock more dangerous than dry skin does.
• Distilled water is a poor conductor; tap water and salt solution conduct because of dissolved ions.
• Plastic, dry wood, glass and rubber are insulators — they hold no charges free to move.
• Graphite is the standard non-metallic solid conductor, and it works through free electrons rather than ions.

• Believing all water conducts; distilled water barely does, and the salts are what matter.
• Assuming only metals conduct, and so rejecting the body.
• Thinking a thinner insulating sheet somehow conducts better — the material decides, not the thickness.

As a pick-the-conductor item with distilled water planted among the options, or as a safety-based question on why rubber gloves and dry wooden platforms are used.

• practice — not a real PYQ

  Distilled water is a poor conductor of electricity because it

  • (a)has no water molecules
  • (b)contains very few free ions
  • (c)is too dense
  • (d)has a high boiling point

  Answer(b) contains very few free ions — distillation removes the dissolved salts, and it is those ions, not the water itself, that carry the current.

• practice — not a real PYQ

  Electricians use tools with rubber-covered handles chiefly because rubber

  • (a)is a good conductor
  • (b)is an insulator
  • (c)is a semiconductor
  • (d)is magnetic

  Answer(b) is an insulator — it has no free charges to carry current, so it keeps the current from passing into the user's body.