[tex]L_1=1 \ J \\ P_1=1 \ W \\ P_2=6 \ W \\ t_1=t_2 \\ \boxed{\Delta L-?} \\ \bold{Rezolvare:} \\ \boxed{ P=\frac{L}{t}} \ \ \ \ \ \ \ \ \Rightarrow t=\frac{L}{P} \\ \Delta L=L_2-L_1 \\ L_2=P_2 \cdot t \\ t=\frac{L_1}{P_1}=\frac{1 \ J}{1 \ W} \\ t=1 \ s \\ L_2=6 \ J \\ \Delta L=6 \ J-1 \ J \\ \Rightarrow \boxed{\Delta L=5 \ J}[/tex]