m=masa petrolului; m=?
q=puterea calorica a petrolului lampant; q=42MJ/kg=42000000J/kg
m1=masa ghetii; m1=500g=0,5kg
t1=-70°C
t2=0°C (temperatura de topire a ghetii)
η=randamentul procesului; η=40%
Q=caldura generata prin arderea petrolului
Q1=caldura necesara sa incalzeasca gheata de la t1 la t2;
Q2=caldura necesara topirii ghetii la temperatura constanta, t2.
Qt=caldura totala necesara incalzirii s topirii ghetii
c=caldura specifica a ghetii; c=2,09kJ/kg°C =2090J/kg°C
λ=caldura latenta de topire a ghetii; λ=0,334MJ/kg=334000J/kg
Calculam Q1:
Q1=m1c(t2-t1)
Q1=0,5×2090×[0-(-70)]=0,5×2090×70=73150
Q1=72150J=73,15kJ
Calculam Q2
Q2=m1λ
Q2=0,5×334000=167000
Q2=167000J=167kJ
Calculam Qt
Qt=Q1+Q2
Qt=73,15+167=240,15kJ
Calculam Q
η=Qt/Q
Q=Qt/η
Q=240,15/(40/100)=240,15/0,4=600,375
Q=600,375kJ=600375J
Calculam m
Q=mq ⇒ m=Q/q
m=600375/42000000=0,01429≈0,0143
m=0,0143kg=14,3g
