NDA (II) 2017 General Ability Test · Question 71 of 150

Correct — C, Henry. The SI unit of inductance is the henry, symbol H, named after the American physicist Joseph Henry, who discovered self-induction in the 1830s at about the same time as Michael Faraday was working on electromagnetic induction in England. A coil has an inductance of one henry when a current changing at one ampere per second across it produces an induced electromotive force of one volt. In base units one henry is one weber per ampere, or one volt second per ampere, or one kilogram metre squared per second squared per ampere squared. The unit is large for ordinary electronics, so components are usually rated in millihenries or microhenries.

• (a)Holm — There is no SI unit called the holm. The word looks like a garbled ohm, which is the unit of electrical resistance and has the symbol of the Greek capital omega, not H.
• (b)Halogen — Halogen is not a unit at all. It is the name of group 17 of the periodic table — fluorine, chlorine, bromine, iodine and astatine — and belongs to chemistry rather than to electrical measurement.
• (d)Hertz — The hertz is a real SI unit and it does begin with the same letter, which is what makes it the tempting choice. But it measures frequency, one cycle per second, and its symbol is Hz with two letters, not H.

Many SI derived units are named after the scientists who opened up the field, and their symbols are capitalised for that reason. In electricity and magnetism the roll-call is worth knowing whole — the ampere for current, the volt for potential difference, the ohm for resistance, the coulomb for charge, the farad for capacitance, the henry for inductance, the tesla for magnetic flux density, the weber for magnetic flux and the hertz for frequency.

Two options here are pure decoys and can be dismissed at once, since neither holm nor halogen is a unit of anything. The real decision is between henry and hertz, and it is settled by remembering that the hertz is written Hz. It also helps to know what inductance means physically: it is a circuit's opposition to a change in the current through it, arising because a changing current changes the magnetic flux, and a changing flux induces a back electromotive force. That is why a coil resists being switched on or off suddenly, and why transformers, chokes and tuned radio circuits all depend on the property.

• The henry, symbol H, is the SI unit of inductance, named after Joseph Henry.
• One henry gives an induced electromotive force of one volt for a current changing at one ampere per second.
• One henry equals one weber per ampere.
• The hertz, symbol Hz, is the SI unit of frequency and is a different unit entirely.
• The ohm, symbol omega, is the unit of resistance, and the farad the unit of capacitance.

• Choosing hertz because it also begins with H; its symbol is Hz.
• Confusing the ohm, the unit of resistance, with any H-symbol unit.
• Mistaking the henry for a unit of magnetic flux, which is the weber.

NDA asks for the unit of a named quantity or the quantity behind a named symbol, so learn the electrical units as symbol, name and quantity together.

• practice — not a real PYQ

  The SI unit of magnetic flux is the

  • (a)Weber
  • (b)Tesla
  • (c)Henry
  • (d)Farad

  Answer(a) Weber — the tesla measures magnetic flux density, the henry inductance and the farad capacitance.

• practice — not a real PYQ

  The quantity measured in hertz is

  • (a)frequency
  • (b)inductance
  • (c)capacitance
  • (d)resistance

  Answer(a) frequency — one hertz is one cycle per second, and the unit's symbol is Hz.