Resistivity and conductionEdexcel International A Level Physics: Flashcards

Exam questions on Resistivity and conduction

1. A technician has a 2.0 m length of constantan wire of diameter 0.50 mm. The resistivity of constantan is 4.9 × 10⁻⁷ Ω m.
   The technician needs a resistance of 12 Ω from the 0.50 mm diameter constantan wire. Calculate the length of wire required.2 marks
2. A copper wire of cross-sectional area 1.5 mm² carries a current of 3.0 A. It is connected in series with a strip of doped silicon, a semiconductor, of the same cross-sectional area, in which the number density of conduction electrons is much smaller than in copper. For copper, the number density of conduction electrons is 8.5 × 10²⁸ m⁻³. The elementary charge is 1.60 × 10⁻¹⁹ C.
   Use I = nqvA to explain why an insulator has a much greater resistivity than copper.2 marks
3. A student determines the resistivity of a nichrome wire as part of a core practical. She measures the diameter of the wire at several points and finds a mean of 0.32 mm. She then measures the resistance R of different lengths l of the wire and plots a graph of R against l. The graph is a straight line through the origin with gradient 14.0 Ω m⁻¹.
   Describe how the student should measure the diameter of the wire accurately.3 marks