Masses of three conductors of same material are in the proportion of \(1: 2: 3\) their lengths are in the proportion of \(3: 2: 1\) then their resistance will be in the proportion of... (A) \(1: 1: 1\) (B) \(1: 2: 3\) (C) \(9: 4: 1\) (D) 27:6:1

Short Answer

Expert verified
The proportion of their resistances will be \(9 : 4 : 1\).

Step by step solution

01

Write the given information as ratios

The given information states that the masses are in the proportion of \(1:2:3\), and the lengths are in the proportion of \(3:2:1\). Let the masses of the conductors be \(M_1\), \(M_2\), and \(M_3\), and their lengths be \(L_1\), \(L_2\), and \(L_3\). We can represent this information as: \[M_1 : M_2 : M_3 = 1 : 2 : 3\] \[L_1 : L_2 : L_3 = 3 : 2 : 1\]
02

Find the ratio of cross-sectional areas

Since the conductors are of the same material, their volume ratios are equal to their mass ratios: \[V_1 : V_2 : V_3 = 1 : 2 : 3\] Volume is given by the product of cross-sectional area and length: \[V = A \times L\] So, the area ratios are: \[A_1 : A_2 : A_3 = \frac{V_1}{L_1} : \frac{V_2}{L_2} : \frac{V_3}{L_3}\] Substituting the given mass and length ratios: \[A_1 : A_2 : A_3 = \frac{1}{3} : \frac{2}{2} : \frac{3}{1} = 1 : 1 : 3\]
03

Find the ratio of resistances

Using the resistance formula, \[R = \rho \frac{L}{A}\], we can find the ratio of resistances: \[R_1 : R_2 : R_3 = \frac{\rho \frac{L_1}{A_1}}{\rho \frac{L_2}{A_2}} : \frac{\rho \frac{L_2}{A_2}}{\rho \frac{L_3}{A_3}} : \frac{\rho \frac{L_3}{A_3}}{\rho \frac{L_1}{A_1}}\] Since the resistivity \(\rho\) is constant for all conductors: \[R_1 : R_2 : R_3 = \frac{L_1}{A_1} : \frac{L_2}{A_2} : \frac{L_3}{A_3}\] Using the given length ratio and the area ratio we found in step 2: \[R_1 : R_2 : R_3 = \frac{3}{1} : \frac{2}{1} : \frac{1}{3} = 9 : 4 : 1\] So, the ratio of resistances is \(9 : 4 : 1\), which corresponds to the answer (C).

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