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Suppose in a ballon of gas is given an extra bit of heat, so as to getting the ballon's internal energy slightly up and this will work out to achieve a little bulk for the ballon. Though the whole way keeps the pressure fixed, as is described on my text. Now, while the pressure was constant, these little gas molecules had no chance to hit the ballon surface a bit more hard (in unit time), even with all it's accumulated energy. Isn't there a fall of terminology somewhere?

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  • $\begingroup$ Keeping the pressure constant, the volume change causes a change in temperature. That's all. The heat added to the system increases the system's internal energy. To keep the pressure constant, this added internal energy is utilized to do work (for the volume to expand). Then there is no energy left to exert additional force on the balloon. So the overall energy remains unaffected. Since the pressure is constant the heat added will be equal to the enthalpy. $\endgroup$
    – UKH
    May 3, 2016 at 16:23
  • $\begingroup$ The added heat is utilized for volume expansion, ok. Though, a confusion is, then both the internal energy and the pressure suspend equality: as in the initial, so at the conclusion(though in between both pressure and the internal energy faced a sort of changing, right?). Then our combined Boyle-Charles law says what, doesn't the volume also face an invariance? Where the expansion did come from... Still a bit of confusion surrounds the logic... $\endgroup$ May 3, 2016 at 16:53
  • $\begingroup$ Sorry, I do not understand what you mean. $\endgroup$ May 3, 2016 at 22:00

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When the balloon is heated, it increases its volume so to keep the pressure constant. The balloon does work to its surrounding. The heat increases its internal energy as well as is used to do work. You might think that the molecular velocity increases, but at the same time, the volume increases as well, so it will take longer time for molecular to travel to hit the surface. The frequency of impact is reduced so that the pressure is kept constant.

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