题目

It was a dark night. Thick, dark, gray clouds had made sure that there was no starlight or moonlight to _______ me as I walked out on my porch. Still, I kept the porch light off because I didn’t want to be _______ by bugs and moths on this _______ summer evening. I stood there for a while letting my eyes adjust to the lack of _______.It was then, though, that I saw them. First one blink of light at the far end of the meadow followed by another and then another. I _______. I recognized them at once as my firefly(萤火虫) _______, my lightning bug buddies who always _______ this time of the 热熔胶在室温下为固体，加热到一定程度时变为较强粘性的液体，冷却后，将物体牢牢地粘在一起．如图甲是热熔胶枪的外形，其内部有2只相同的PTC加热元件，电路如图乙．单只PTC元件的电阻与温度的变化如图丙所示．扣动扳机能使固体胶条向前挤压，联动开关S同时倒向A、B，枪口冒出胶液，松开扳机S同时倒向B、C．（1）松开扳机时为预热状态，当温度为80℃时，电路中的电流是多少？（2）当温度为180℃时，胶液有最佳粘性，已知一根胶条的质量为40g，若熔化1kg胶需要吸收6.0×105J热量，试计算在正常情况下使用完一根胶条至少要多长时间？（3）试根据PTC特性定性分析该装置能使温度稳定在180℃附近的道理．