2012广西省桂林市中考物理试题及答案

　26.解：⑴柑橘排水质量：

　　m排=m总-m剩=360g-240g=120 g ····································· (1分)

　　⑵由　ρ=m/V 可得 ····································································· (1分)

　　柑橘的体积：V橘 =V排=m排/ρ水=120 g/1.0 g/cm3=120 cm3 (2分)

　　柑橘的密度：ρ橘=m橘/V橘=114g/120 cm3=0.95g/cm3 ············ (1分)

　　⑶偏小 ··························································································· (1分)

　　27.解：⑴由　P=UI　可得 ······································································· (1分)

　　正常加热状态下的电流：I= P/U = 920W/220V ≈ 4.2A　 ··············· (1分)

　　⑵当开关S闭合、S0断开时，电热饮水机只有R2工作，处于保温状态。

　　由　P=UI　I=U/R　可得 ························································ (1分)

　　电阻R2消耗的功率：P2=U2/R2=(220V)2/1210Ω=40W ····· (1分)

　　当开关S、S0闭合时，电热饮水机处于加热状态。此时R1消耗的电功率为：

　　P1 =P总- P2=920 W- 40W=880 W ··························· (1分)

　　则有：R1=U2/P1=(220V)2/880 W=55Ω ··························· (1分)

　　⑶方法一：电热饮水机处于加热状态时的总电阻为：

　　R总=U2/P总=(220V)2/920W=1210/23Ω≈ 52.6Ω ······· (1分)

　　实际加热功率：P实=U实2/R总

　　=(198V)2/(1210/23)Ω=745.2W ············· (1分)

　　[或P实=U实2/R总=(198V)2/52.6Ω≈745.3 W ]

　　方法二：电路电阻不变，可得：

　　R总=U2/P总=U实2/P实 ···································· (1分)

　　实际加热功率：P实=P额×U实2 /U2

　　=920 ×(198/220)2=745.2 W ···················· (1分)

　　方法三：实际加热功率：P实=P1实+P2实=U实2/R1+U实2/R2 ···· (1分)

　　=(198V)2/55Ω+(198V)2/1210Ω

　　=745.2 W ········································· (1分)

　　28.解：1 m3可燃冰转化生成的甲烷气体完全燃烧放出热量：

　　Q=q甲烷V

　　=3.6×107J/ m3×164 m3=5.904×109 J ··································· (1分)

　　由题知： t=1640min=9.84×104s

　　由　P=W / t　可得 …………………………………………………………(1分)

　　发动机的实际功率：P=Q / t=5.904×109 J/9.84×104s=6.0×104 W (2分)

　　由题知：36km/h=10m/s

　　由　v=s/t 可得 ·········································································· (1分)

　　公交车行驶路程：s=v t=10m/s×9.84×104s=9.84×105 m ················ (1分)

　　车匀速行驶，有：

　　F牵=f=0.05 G车=0.05mg=0.05×6000 kg×10N/kg=3×103N (1分)

　　由W=Fs可得牵引力做的有用功： ·················································· (1分)

　　W有=F牵s=3×103N×9.84×105 m=2.952×109 J ··················· (1分)

　　发动机效率：

　　η=W有 /Q=2.952×109 J/5.904×109J=50% ······················· (1分)

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