10 12 The resistivity of a metal is an important property of wire used in an electric circuit. (a) A student carried out an experiment to determine the resistivity of a type of wire. He used a micrometer to measure the diameter d of the wire. (Source: © Viktor Chursin/Shutterstock) He recorded the following values. d1 / mm d2 / mm d3 / mm d4 / mm 1.40 1.44 1.42 1.41 (i) Calculate the percentage uncertainty in the mean diameter of the wire. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. % uncertainty in mean diameter of wire = ...................................................... Turn over 11 (ii) The student used an ohmmeter to measure the resistance R of a 1.65 m length of the wire. He looked up the resistivity values of some materials. Material Titanium Constantan Stainless Steel Resistivity / 10–7 Ω m 4.2 4.7 6.9 Identify the material of the wire. R = 0.72 Ω (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (b) Nichrome wire is often used in heating elements. Nichrome wire is used to make a coil for a 65 W mains powered heater. The nichrome wire has a resistance per metre of 87.5 Ω m−1 . Calculate the length of wire required. potential difference across the coil = 230 V (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. Length of wire required = ...................................................... (Total for Question 12 = 9 marks)