美国大学物理题 HELP!Problem 4.A proton and an electron are far apart at rest.An external agent brings the electron and proton closer together and then holds them at rest.Which statement is correct about the work done by the external agent?A) T
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美国大学物理题 HELP!Problem 4.A proton and an electron are far apart at rest.An external agent brings the electron and proton closer together and then holds them at rest.Which statement is correct about the work done by the external agent?A) T
美国大学物理题 HELP!
Problem 4.A proton and an electron are far apart at rest.An external agent brings the electron and proton closer together and then holds them at rest.Which statement is correct about the work done by the external agent?
A) The work done is negative.
B) The work done is positive.
C) The work done in this case is zero because the proton and the electron will spontaneously come close to each other even without any external force.
D) None of the above.
美国大学物理题 HELP!Problem 4.A proton and an electron are far apart at rest.An external agent brings the electron and proton closer together and then holds them at rest.Which statement is correct about the work done by the external agent?A) T
先翻译：
问题4.一个质子和一个电子相距遥远,且静止不动.外力（external agent 应该是这个意思吧我不太确定.英语不好泪目.）使电子和质子靠近,而后又使两者静止.关于外力做功哪一种说法是是正确的?
A）做负功.
B）做正功.
C）在这种情况下作功是零,因为质子和电子会自发地来靠近对方,即使没有任何外力作用.
D）上述答案都不对.
选A,做负功.
判断做功的正负基本方法有二：
一,从力的角度看,做功的正负取决于外力和物体的运动方向夹角,若夹角为锐角则正功,夹角为钝角则负功.
二,在受力不明时,应当看能量变化.
某个力做功等于该力对应能量的相反数：正功则表示该力对应能量减小,负功则表示该力对应能量增强（物体下落重力做正功,重力势能减小）
而总能量的变化等于外力做的功：总能量增加,外力作正功；总能量降低,外力作负功.
本题中外力不明,当然选择方法二.在整个过程中,体系内部的能量是动能和电势能,总能量的变化就是外力做的功.
动能：在开始电子和质子静止,最终也是静止,故动能不变；
电势能：由于电子和质子相互吸引,且两者相互靠近,即力方向和位移方向一致,电场力做正功,电势能减小.
故总能量降低,所以外力作负功.
好长,终于写完了~