Physics, molecular potential energy

Some books make it very clear that with the increase of gas volume, the molecular potential energy increases; Some books don't think so. What happened?

The main arguments of "the gas volume increases and the molecular potential energy increases" are: the distance between gas molecules is large, and the intermolecular interaction is attractive; As the volume increases, the distance between molecules increases, and gravity does negative work, so the molecular potential energy increases. However, this statement is not reliable: at the same time, there are some intermolecular forces between molecular pairs in gas that show attraction Q, and there are also a few intermolecular forces that show repulsion Q. Because Q is often much larger than Q, the latter is not necessarily a secondary factor.

In fact, when the gas volume increases greatly at a certain temperature, the molecular potential energy may increase or decrease slightly, depending on the gas type and temperature.

Joule made an experiment in 1845: there is almost no heat exchange between gas and the outside world, and the temperature of gas is almost constant during its free expansion (no external work). This means that the internal energy of a certain amount of gas is almost only related to temperature, but not to volume. This means that the molecular potential energy of gas is almost independent of volume.

Joule and William? Tang Musun made a more accurate experiment in 1852 (page 163 of Thermology, edited by Li Chun et al., People's Education Publishing House, 1978 edition). The experimental results show that the temperature of the gas drops slightly (for example, it drops by 65438 0℃ during an expansion process in which the sum of the product PV of the gas and the internal energy U remains unchanged). It varies according to the kind and temperature of the gas used. Let's analyze what this experiment shows. In the experiment that the temperature drops by 65438 0℃, according to Claperon equation PV = NRT, even if it doesn't drop, PV will drop slightly. It is noted that (Pv+U) remains unchanged in the experiment, which means that U has increased, which means that "the volume increases, the temperature decreases, and the internal energy increases". This shows that the molecular potential energy increases when the gas volume increases. In the experiment that the temperature rises by 65438 0℃, even if the Claperon equation PV = NRT is not followed, the PV will rise slightly. It is noted that (Pv+U) remains unchanged in the experiment, indicating that U decreases, indicating that the volume increases and the temperature increases, but the internal energy decreases.

In short, when the gas volume changes greatly, the total molecular potential energy of the gas changes little; It is possible that the total molecular potential energy of gas increases or decreases slightly with the increase of volume.

Because the molecular potential energy of gas has only a weak relationship with volume, the assumptions of ideal gas model can include the following assumptions: the molecular potential energy of ideal gas does not change with volume, or the internal energy of a certain number of ideal gases is a function of temperature.

Title: As shown in the picture, container A, container B and the pipes connecting them are all insulated. Initially, container A is filled with gas with temperature t, and container B is in a vacuum state. Open the valve k, and the gas flows from the container A to the container B at the temperature of T'.

(a) If the gas in the container is an ideal gas, there must be t = T=T'

(b) If the gas in the container is an ideal gas, there must be t > T>T'

(c) If the gas in the container is actual gas, there must be t = T=T'

(d) If the gas in the container is actual gas, there must be t > T>T'

Figure 13- 1 1

Solution: In the process of gas flowing from A to B, there is no external work, and there is no internal energy transfer between gas and the outside world, so the internal energy of gas remains unchanged.

The total molecular potential energy of an ideal gas is constant in the process of state change, so the internal energy of the ideal gas is constant, that is to say, the total kinetic energy of molecules is constant, while the number of molecules is constant, so the average kinetic energy of molecules is constant, and thus the temperature is constant. At this point, option (a) can be affirmed and option (b) can be denied.

Because the total molecular potential energy of the actual gas may increase, remain unchanged or decrease during the above changes, when the internal energy of the actual gas remains unchanged, there is the possibility that the total molecular kinetic energy will decrease, remain unchanged and increase, and there is also the possibility that the temperature will decrease, remain unchanged and increase. Options (c) and (d) can be rejected.

Of the four options in this question, only (a) is correct.