Alevel物理真題答案解析-熱力學簡答題

接觸到Alevel物理熱力學（Thermal physics），會從微觀粒子（particles或molecules）的角度來解釋物質的溫度、壓強、動能、勢能等問題。學生們會覺得微觀的部分不太好理解，上課的時候知識點可以理解，落實到練習題，仍然會有下不了手的感覺。

今天我們整理并講解一下ideal gas（理想氣體）相關的幾道真題及答案。通過這幾題輔助大家思考一下理論知識點和考題的聯系。

試題來源：CAIE A Level物理（9702）近年真題

問題1：What is meant by an ideal gas.

解析：這是一道基礎的定義題，解釋什么是ideal gas。簡單來說只要符合ideal gas equation，pV / T = constant的，就叫理想氣體。不過要同時解釋清楚p、V和T分別代表哪些物理量。

答案：
gas obeys formula pV / T = constant
p is pressure
V is volume
T is kelvin temperature

問題2：An ideal gas is said to consist of molecules that are hard elastic identical spheres. State two further assumptions of the kinetic theory of gases.

解析：問題要求寫兩點關于kinetic theory of gas的假設。假設一共有好多條，大致可以整理成6條，這幾條是后面理解kinetic theory equation的推導和有關理想氣體internal energy相關問題的重要前提。

答案：6選2，優(yōu)先選擇前面幾條答案。
total volume of molecules negligible compared to that of container
no intermolecular forces/ no forces between molecules (except during collisions)
molecules in random motion
all collisions involving molecules of the gas are elastic
time of collision small compared with the time between collisions
large number of molecules

問題3：One of the assumptions of the kinetic theory of gases is related to the volume of the atoms.

解析：本題是由上述6條假設延伸出的問題，要求我們寫有關粒子體積的那一條假設。

答案：assumption: volume of atoms negligible compared to volume of container

問題4：ΔU = (+q) + (+w) ?熱力學第一定律這幾個字母所代表的含義。

解析：ΔU指的是內能（internal energy）的變化，如果internal energy增加，那ΔU是正的；如果internal energy減少，那ΔU是負的。q代表這個系統吸收thermal energy或者釋放thermal energy。w代表work done，做功，是對系統做功還是系統對外做功。

答案：
+ΔU ： the increase or decrease in internal energy of the system
-ΔU ： decrease in internal energy of the system
+q：thermal energy transfer to system
+ w：work done on system

問題5：State what is meant by the internal energy of a system.

解析：Internal energy的定義，指的是組成物質的particles所具有的動能和勢能的總和。U=Σk.e.+Σp.e.

答案：
sum of potential energy and kinetic energy (of particles)
(total) energy of random motion of particles

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