Un fir de alama cu rezistenta R=40 Ω ,rezistivitatea ρ=8×10⁻⁸ W·m si aria sectiunii S=0,1 mm², are lungimea:
a) 25m ; b)50 m; c) 40 m

[tex]\displaystyle Se~da:\\ \\ R=40\Omega\\ \\ \rho=8\times 10^{-8}\Omega\times m\\ \\ S=0,1mm^2=10^{-7}m^2\\ \\ l=?m\\ \\ \\ Formule:\\ \\ R=p\times\frac{l}{S}\\ \\ l=\frac{R\times S}{\rho}\\ \\ \\ Calcule:\\ \\ l=\frac{40\times10^{-7} }{8\times 10^{-8}}=50m\\ \\ \\ Raspuns:b).[/tex]