[tex]dane:\\R = 5 \ \Omega\\t = 30 \ min = 30\cdot 60 \ s = 1800 \ s\\U = 15 \ V\\szukane:\\W = ?\\\\Rozwiazanie\\\\W = U\cdot I \cdot t\\\\ale \ \ I = \frac{U}{R}, \ zatem\\\\W = \frac{U^{2}\cdot t}{R}\\\\W = \frac{(15 \ V)^{2}\cdot1800 \ s}{5 \ \Omega}\\\\W = 81 \ 000 \ J = 81 \ kJ[/tex]
Odp. Praca prądu elektrycznego W = 81 kJ.
