Resistivity  Practice QuestionsResistivity (these calculations are for those at AS level) 24 Taking the resistivity of platinoid as 3.3 x 10^{7} m, find the resistance of 7.0 m of platinoid wire of average diameter 0.14 cm. 25 The resistance of the ohm is very approximately that of a column of mercury 1.06 m long and of uniform crosssection of one hundredth of a cm^{2}. Find the resistivity of mercury. 26 What length of German silver wire, diameter 0.050 cm, is needed to make a 28 resistor, if the resistivity of German silver is 2.2 x 10^{7} m? 27 The maximum allowable resistance for an underwater cable is one hundredth of an ohm per metre. If the resistivity of copper is 1.54 x 10^{8} m, find the least diameter of a copper cable that could be used. 28 A block of carbon, 1.0 cm by 2.0 cm by 5.0 cm, has a resistance of 0.015 between its two smaller faces. What is the resistivity of carbon? 29 A uniform strip of eureka (resistivity 50 x 10^{7} m) has a resistance of 0.80 per metre and is 0.25 cm wide. What is its thickness? 30 A wire of uniform crosssection has a resistance of R . What would be the resistance of a similar wire, made of the same material, but twice as long and of twice the diameter? 31 A wire of uniform crosssection has a resistance of R . If it is drawn to three times the length, but the volume remains constant, what will be its resistance? 32 A simple way to make a resistor of high resistance is to draw a line with a graphite pencil across a sheet of insulating material. One such line has a resistance of 10 M and is 80 cm long. If its width is 1.8 x 10^{2} cm, what is its average thickness? (Resistivity of graphite = 6.3 x l0^{5} m.) ANSWERS 24) 1.50 25) 9.4 (9.434) x lO^{7}m 26) 25m 27) 014cm 28) 6.0xl0^{5}m 29) 0.025 cm 30) ^{1}/_{2} R 31) 9R 32) 2.8 x 10^{4} cm

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