WAEC SSCE - Physics - 1992 (Objective)

Question 1 Report

Calculate the length of a constantan wire of cross- sectional area 4 π x 10^-8m² and resistivity 1.1 x 10^-6 Ω m required to construct a standard resistor of resistance 21 Ω. [Take π = 22/7]

0.42m

2.40 m Correct answer

6.25 m

183.75 m

590.00 m

Answer Details

The resistance of a wire is given by the formula: R = (ρ * L)/A Where R is resistance, ρ is resistivity, L is the length of the wire and A is the cross-sectional area of the wire. To find the length of the constantan wire required to construct a standard resistor of resistance 21Ω, we rearrange the formula to get: L = (RA)/ρ Substituting the given values, we have: L = (21 * 4π x 10^-8)/ (1.1 x 10^-6) L = 0.76 meters Since the value of π was given as 22/7, we use the approximate value of π = 3.14 in the calculation. Therefore, the length of the constantan wire required to construct a standard resistor of resistance 21 Ω is 0.76 meters. So, the correct option is (b) 2.40 m.






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