V水=30L=30dm^3=30000cm^3
m水=pV=1g/cm^3*30000cm^3=30kg
Q=mc&t=30kg*4.2*10^3j/(kg*C.)*(40C.-20C.)=2520000J
P烧水=P额*70%=2kw*70%=1.4kw=1400w
t=Q/P=2520000j/1400w=1800s=30min
W耗电=pt=2kw*0.5H=2kw*h
V水=30L=30dm^3=30000cm^3
m水=pV=1g/cm^3*30000cm^3=30kg
Q=mc&t=30kg*4.2*10^3j/(kg*C.)*(40C.-20C.)=2520000J
P烧水=P额*70%=2kw*70%=1.4kw=1400w
t=Q/P=2520000j/1400w=1800s=30min
W耗电=pt=2kw*0.5H=2kw*h