Grzałka elektryczna ma moc 700[W] ile wody o temperaturze poczatkowj 10°C zagotujemy ja w ciagu 20 min.

[latex]Dane:\P=700W\\ left { {{t _{p} =10^circ C} atop {t _{k} =100^circ C}} ight. Rightarrow Delta t=90^circ C\\t=20min=1200s\\C _{w} =4200 frac{J}{kg*^circ C} \\Szuk.\m=?\\Roz.\\Q=W\\Q=C _{w}*m*Delta t\\W=Pt\\C _{w}*m*Delta t=P*t\\m= frac{P*t}{C _{w}*Delta t} \\\m= frac{700W*1200s}{4200 frac{J}{kg*^circ C}*90^circ C} approx 2,23kg[/latex]