JEE Main 2020 · 2 September · morning · Single correct · +4 / −1
Consider four conducting materials copper, tungsten, mercury and aluminium with resistivity ρ C , ρ T , ρ M and ρ A respectively. Then :
- (a) ρ C > ρ A > ρ T
- (b) ρ M > ρ A > ρ C
- (c) ρ A > ρ T > ρ C
- (d) ρ A > ρ M > ρ C
Answer and solution
Answer (b) ρ M > ρ A > ρ C
Solution
Physics sir · 3 steps
Put the four resistivities in order once: copper lowest, then aluminium, tungsten, and mercury far on top.
- Recall the values. Resistivity is a property of the material alone. These four room-temperature values are worth remembering, since they come up often.
Material Resistivity at 20 °C Rank Copper 1.7 × 10⁻⁸ Ω m lowest Aluminium 2.7 × 10⁻⁸ Ω m second Tungsten 5.5 × 10⁻⁸ Ω m third Mercury 9.8 × 10⁻⁷ Ω m highest - Full order. So ρ_M > ρ_T > ρ_A > ρ_C. Mercury is about 18 times worse than tungsten, because it is a liquid metal with a disordered structure that scatters electrons strongly.
- So. Dropping tungsten from the chain leaves ρ_M > ρ_A > ρ_C, which is option (b).
The idea to keep. Learn the ladder: copper < aluminium < tungsten < mercury. Copper and aluminium are the wire metals, tungsten is the heating filament, and liquid mercury is the poorest conductor of the four.
Answer from the NTA final answer key. Solution by PrepWiser.