Resistivity, conductivity and temperature dependence of resistance
- The resistance of platinum wire at 0^°C is 2 Ω and 6.8 Ω at 80^°C. The temperature coefficient of resistance of the wire is : Answer: 4. 3×10⁻² ^°C⁻¹ R_t=R₀(1+α t) , so α=R_t-R₀/R₀ t . α=6.8-2/2×80=4.8/160=3×10⁻² ^°C⁻¹ . The answer is 3×10⁻² ^°C⁻¹ . 1
- A copper wire of length 10 m and radius (10^-2/√(π)) m has electrical resistance of 10 Ω. The current density in the wire for an electric field strength of 10 (V/m) is : Answer: 1. 10⁵ A/m ² Current density is J=σ E=E/ρ , so first find resistivity from R=ρ L/A . A=π r²=π·dfrac10⁻⁴π=10⁻⁴\ m² ρ=RA/L=dfrac10×10⁻... 1
- As the temperature increases, the electrical resistance : Answer: 4. increases for conductors but decreases for semiconductors Resistance of a conductor rises with temperature, while that of a semic... 1
- Which of the following graph represents the variation of resistivity (ρ) with temperature (T) for copper ? Answer: 3. graph (3) For a metal like copper, resistivity rises with temperature because lattice vibrations scatter electrons more: ρ=ρ₀[1+α... 1
- The solids which have the negative temperature coefficient of resistance are : Answer: 4. insulators and semiconductors Metals have a positive temperature coefficient of resistance: resistance rises with temperature as... 1
- A carbon resistor of (47±4.7) kΩ is to be marked with rings of different colours for its identification. The colour code sequence will be Answer: 3. Yellow - Violet - Orange - Silver Colour code digits: Yellow 4, Violet 7, Orange 3 (multiplier 10³ ), Silver ±10% , Gold ±5% . 47... 1
- The resistance of a wire is 'R' ohm. If it is melted and stretched to 'n' times its original length, its new resistance will be : Answer: 2. n²R Key idea: melting does not change the volume, so Al stays constant. If length becomes nl , area becomes A/n . Resistance R=ρl... 1