NDA (II) 2023 General Ability Test · Question 54 of 146

Correct — C, Nichrome. A heating element has to turn as much electrical energy into heat as possible, and by Joule's law the heat produced is proportional to the resistance for a given current, so the material must have a high resistivity. It also has to sit red hot in open air for hours without burning away, so it must resist oxidation at high temperature. Nichrome, an alloy of nickel, chromium, manganese and iron, meets both demands — NCERT lists its resistivity as 100 × 10⁻⁶ ohm metre, roughly six thousand times that of copper, and states that alloys do not oxidise readily at high temperatures, which is exactly why they are used in electrical heating devices such as the electric iron and the toaster.

• (a)Constantan — Constantan is a copper-nickel alloy whose resistivity, 49 × 10⁻⁶ ohm metre in the NCERT table, is only about half that of nichrome. Its distinguishing virtue is that its resistance barely changes with temperature, which makes it the material of choice for standard resistors and thermocouple wire rather than for a heating element.
• (b)Tungsten — Tungsten belongs to the filament of an electric bulb, not to an open heating element — NCERT says it is used almost exclusively for bulb filaments. Its resistivity, 5.20 × 10⁻⁸ ohm metre, is far too low for a heater, and a bulb filament survives only because it works inside a sealed, evacuated or inert-gas-filled bulb.
• (d)Copper — Copper has one of the lowest resistivities of all, 1.62 × 10⁻⁸ ohm metre, so it carries current with hardly any heating. That is precisely why NCERT lists copper and aluminium as the materials for connecting wires and transmission lines — a heater made of copper would waste the current instead of turning it into heat.

Every appliance component is chosen by matching a material property to a job. A conductor for carrying current needs low resistivity, a fuse needs a low melting point, a bulb filament needs a very high melting point, and a heating element needs high resistivity plus resistance to oxidation when hot. Alloys generally have higher resistivity than the metals they are made from, and they do not burn away readily at high temperature, which is why heating elements are alloys rather than pure metals.

The near-miss is tungsten, because students remember it as the metal that glows. It does glow, but only sealed inside a bulb where there is no oxygen to attack it. Exposed to air at red heat, a tungsten coil would oxidise away quickly. Sort the four options by NCERT's resistivity table and the answer falls out — nichrome at 100 × 10⁻⁶ ohm metre is an order of magnitude above constantan and thousands of times above tungsten and copper.

• NCERT Class 10 Science describes nichrome as an alloy of nickel, chromium, manganese and iron.
• Resistivity from NCERT Table 11.2 — nichrome 100 × 10⁻⁶ ohm metre, constantan 49 × 10⁻⁶ ohm metre, tungsten 5.20 × 10⁻⁸ ohm metre, copper 1.62 × 10⁻⁸ ohm metre.
• NCERT's stated reason — the resistivity of an alloy is generally higher than that of its constituent metals and alloys do not oxidise readily at high temperatures, so they are used in electrical heating devices like the electric iron and toaster.
• Tungsten is used almost exclusively for the filaments of electric bulbs, while copper and aluminium are used for electrical transmission lines.

• Picking tungsten because it is the famous glowing metal — it belongs in a sealed bulb, not an open heating element.
• Assuming the best conductor makes the best heater; low resistance means little heat.
• Confusing constantan, an alloy prized for a resistance that hardly varies with temperature, with nichrome.

NDA asks for the material of a heating element, a bulb filament, a fuse wire or a transmission line — sort the options by resistivity, melting point and how readily they oxidise.

• practice — not a real PYQ

  The coils of electric toasters and electric irons are made of an alloy rather than a pure metal mainly because

  • (a)alloys are cheaper than pure metals
  • (b)alloys have higher resistivity and do not oxidise readily at high temperature
  • (c)alloys melt at a lower temperature than pure metals
  • (d)alloys conduct electricity better than pure metals

  Answer(b) alloys have higher resistivity and do not oxidise readily at high temperature — the two properties a heating element needs, and the reason NCERT gives for using alloys in heating devices.

• practice — not a real PYQ

  Which one of the following metals is used almost exclusively for the filament of an incandescent electric bulb?

  • (a)Copper
  • (b)Nichrome
  • (c)Tungsten
  • (d)Aluminium

  Answer(c) Tungsten — its very high melting point lets the filament glow white hot inside the sealed bulb without melting.