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Question

(a) Give two examples of substances which are good conductors of electricity. Why do you think they are good conductors of electricity ?
Calculate the resistance of a copper wire $$1.0 \,km$$ long and $$0.50 \,mm$$ diameter if the resistivity of copper is $$1.7 \times 10^{-8} \,\Omega m.$$


Solution

(a) Silver and copper are good conductors of electricity because they have free electrons available for conduction.

(b) $$l = 1 \,km = 1000 \,m$$
$$r = \dfrac {d}{2} = \dfrac {0.5}{2}mm = 0.25 \,mm = 0.25 \times 10^{-3} m$$
$$\rho = 1.7 \times 10^{-8} \,\Omega m$$

$$R = \rho \dfrac {l}{A} = \rho \dfrac {l}{\pi r^2}$$

$$R = 1.7 \times 10^{-8} \times \dfrac {1000}{3.14 \times (0.25 \times 10^{-3})^2} = 86.6 \,\Omega$$

Physics

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