[tex]dane:\\P = 2 \ kW = 2\cdot1000 \ W = 2 \ 000 \ W\\W = 500 \ J\\szukane:\\t = ?\\\\\\P = \frac{W}{t} \ \ /\cdot t\\\\P\cdot t = W \ \ /:P\\\\t = \frac{W}{P}\\\\t = \frac{500 \ J}{2000 \ W} = \frac{500 \ J}{2000\frac{J}{s}}\\\\\underline{t = 0,25 \ s}[/tex]
