[{"data":1,"prerenderedAt":73},["ShallowReactive",2],{"q-fire-109-1-fire-science-011":3},{"subject":4,"subjectSlug":5,"subjectKicker":6,"subjectShort":7,"question":8,"related":27,"sameNumber":52,"hasEssay":26},"火災學概要","fire-science","火災學 · Fire Science","火災學",{"id":9,"webId":10,"year":11,"session":12,"subject":4,"number":13,"stem":14,"options":15,"answer":20,"answerNote":21,"images":22,"imagesPending":23,"lawTimestamp":11,"explanation":24,"explanationDeep":25,"freq":12,"indexable":26},"fire-109-1-火災學概要-011","fire-109-1-fire-science-011",109,1,11,"某一線徑 2.0 m\u002Fm 之電線 1 km 長之電阻值為 5.657 Ω，熱阻抗為 378。若周遭溫度為 20℃，當此 10 cm 長之電線通過 10 安培時，其芯線溫度（℃）約為多少？",{"A":16,"B":17,"C":18,"D":19},"31","41","51","61","B",null,[],false,"本題考點：電線通電發熱的焦耳定律與熱阻抗的運用,由發熱功率推算芯線溫升。\n【正解理由】依焦耳定律,導體發熱功率 P＝I²R;所生熱量經絕緣被覆與周圍空氣散出,穩態時溫升與發熱功率成正比,比例常數即熱阻抗 Rθ,故 ΔT＝Rθ×P,芯線溫度＝周遭溫度＋ΔT。本題先將每公里 5.657 Ω 換算為 10 公分長的電阻,求出發熱功率後乘以熱阻抗 378 得溫升約 21.4℃,加上周遭 20℃ 得芯線溫度約 41.4℃,四捨五入為 41℃,故選 B。\n【逐項排除】\n(A) 31℃ 對應溫升僅 11℃,約為正解溫升的一半,係熱阻抗或長度換算少乘一次所致。\n(B) 溫升 21.4℃ 加周遭 20℃ 得 41.4℃,與選項相符,為應選項。\n(C) 51℃ 需溫升達 31℃,反推熱阻抗約需 550,與題給的 378 不符。\n(D) 61℃ 需溫升 41℃,約為正解溫升的兩倍,常見於長度換算多取一個數量級或電流未平方所得。\n【演算步驟】R＝5.657 Ω\u002Fkm×(0.1 m÷1,000 m\u002Fkm)＝5.657×10⁻⁴ Ω;P＝I²R＝(10 A)²×5.657×10⁻⁴ Ω＝5.657×10⁻² W;ΔT＝378 ℃\u002FW×5.657×10⁻² W≈21.4℃;T＝20℃＋21.4℃≈41.4℃。\n【記憶點】先算 I²R 的瓦數,再乘熱阻抗得溫升,最後加上環境溫度。","焦耳定律 P＝I²R 是電氣火災所有計算題的共同起點。導體電阻 R＝ρL\u002FA,與長度成正比、與截面積成反比,所以題目給「每公里電阻」時,務必先按實際長度等比縮放——本題 10 公分是 1 公里的一萬分之一,電阻也是一萬分之一,這一步算錯就整題偏掉一個數量級。熱阻抗的角色則可類比歐姆定律:把溫差看成電壓、熱流看成電流,ΔT＝Rθ×P 與 V＝IR 形式完全一致,單位為 ℃\u002FW,數值愈大表示散熱愈差、同樣發熱下溫度升得愈高。\n實際導體還有一層正回饋:銅的電阻溫度係數約 0.00393\u002F℃,溫度上升會使電阻再增大,發熱隨之增加。常溫附近溫升二十餘度時影響僅數個百分點,可以忽略,但在過載或半斷線造成局部高溫時,這個正回饋會加速惡化,是電線起火的重要機制。\n與法規面的安培容量觀念相通:電線的容許電流不是由電線本身決定,而是由被覆材料的容許溫度決定。一般 PVC 被覆長期容許溫度約 60℃、耐熱 PVC 約 75℃、交連聚乙烯約 90℃,配線設計即以「發熱後導體溫度不超過此值」反推可通過的電流,散熱條件差(集中配管、埋設、束線)時熱阻抗上升,安培容量必須折減。\n易混淆對照:題幹的「線徑 2.0 m\u002Fm」是導體直徑 2.0 公釐,截面積約 3.14 mm²,與常寫成「2.0 mm²」的截面積規格不同,兩者混淆會使電阻估算失準。延伸考點包括:同樣的電流下,接觸不良處因接觸電阻遠大於導體電阻而形成局部熱點;半斷線使有效截面積驟減、電阻與發熱急升;串接多個高功率電器造成過載,都是由這條公式推導出的火災成因。",true,[28,32,36,40,44,48],{"webId":29,"stem":30,"number":31,"year":11,"session":12},"fire-109-1-fire-science-010","有水之場所的插頭應加裝何種裝置以防止感電發生？",10,{"webId":33,"stem":34,"number":35,"year":11,"session":12},"fire-109-1-fire-science-012","下列常見之乾粉滅火藥劑敘述何者正確？",12,{"webId":37,"stem":38,"number":39,"year":11,"session":12},"fire-109-1-fire-science-008","1 度電為 1,000 瓦．小時（1 kWh），試問 1 度電約等同多少千卡（kcal）？",8,{"webId":41,"stem":42,"number":43,"year":11,"session":12},"fire-109-1-fire-science-013","下列的滅火設備，何者屬於窒息法滅火原理？",13,{"webId":45,"stem":46,"number":47,"year":11,"session":12},"fire-109-1-fire-science-007","進入石化工廠，車輛會被要求在排氣管加裝滅焰器（flame arrester）以避免火災爆炸之發生，加裝該措施防制火災爆炸之原理為：",7,{"webId":49,"stem":50,"number":51,"year":11,"session":12},"fire-109-1-fire-science-014","火場溫度隨燃燒時間而逐漸升高，其空氣亦漸膨脹，空氣由外界流入該起火房間，同時熱煙霧也會從該起火房間流出。若忽略燃燒分解過程而產生之質量流率，建築物內部空氣溫度為 25℃，起火房間溫度為 800℃，流入起火房間之空氣體積流量為 1 m3\u002Fs，則流出起火房間之熱煙霧體積流量約為若干？",14,[53,57,61,65,69],{"webId":54,"year":55,"stem":56,"number":13},"fire-111-1-fire-science-011",111,"下列何者非為影響閃燃的因素？",{"webId":58,"year":59,"stem":60,"number":13},"fire-110-1-fire-science-011",110,"輻射熱是造成木造建築物延燒原因之一，下列何者正確？",{"webId":62,"year":63,"stem":64,"number":13},"fire-108-1-fire-science-011",108,"室內火災所產生的熱煙層由天花板向下沉降的時間直接影響逃生；以此觀點而論，下列何者較不利於逃生？",{"webId":66,"year":67,"stem":68,"number":13},"fire-107-1-fire-science-011",107,"在高科技無塵室作業中，下列敘述何者不是避免人員帶靜電產生危害之處理方式？",{"webId":70,"year":71,"stem":72,"number":13},"fire-106-1-fire-science-011",106,"下列有關擴散火焰的敘述，何者正確？",1785123162735]