Which is hotter, 100℃ water or 100℃ oil?

Temperature is actually a load in mechanics. We call it temperature load, which is mainly caused by the change of internal stress of the structure due to thermal expansion and contraction. I have also written many articles on thermal stress. If you are interested, you can search for them. So, I also have my own answer to this question. In this question, although water and oil are both 100 degrees Celsius, due to the different convection heat transfer coefficients, the amount of heat transferred per unit time is different. In a unit time, the more heat is transferred, the faster the human body will feel hot . So, is it water or oil?

1.What is temperature?

I found a strange question. Although many people answered it, no one clearly stated what temperature is. Maybe they thought temperature is too basic. However, if you don’t know what temperature is, you may not be able to continue with the following heat transfer.

You know, temperature is actually defined by us. In the early days, people felt that high temperatures were hot and low temperatures were freezing. In order to measure, scientists defined a temperature standard. The temperature corresponding to boiling water is 100℃, and the temperature corresponding to ice-water mixture is 0℃. This is the definition of the Celsius temperature scale. The temperature changes throughout the year are our most intuitive feeling, as shown in the figure below.

In the above picture, there is another temperature scale, Fahrenheit scale, which is also a common temperature scale, mainly used by Americans. In addition, in academia, the famous temperature scale is Kelvin scale, also called thermodynamic temperature scale K. In addition to these temperature scales, there are many other temperature scales, as shown in the figure below, and there are conversion relationships between them.

Speaking of the Kelvin temperature scale, we have to mention here that absolute zero is the temperature corresponding to K=0. In fact, this absolute zero has always been a god-like existence, which can only be looked up to but cannot be reached . I once followed a thread on a question and answer session about absolute temperature by a big guy, saying "How to measure absolute temperature?" As a result, a lot of people attacked me. The fact is that since it cannot be achieved, of course it cannot be measured.

Back to the topic, absolute zero is actually the temperature of an object when the particles inside are still. Macroscopically, an object can be still or move. Microscopically, the particles that make up an object are always in motion (pictured above) , and where there is motion there is energy. From a macroscopic perspective, the energy expressed by the movement of particles is the internal energy of the object. One measure of internal energy is temperature. Therefore, temperature indicates the hotness or coldness of an object macroscopically, and indicates the intensity of the thermal motion of the molecules inside the object microscopically.

2. How to measure the time of heat transfer?

Since temperature is a measure of the thermal motion of molecules inside an object, the thermal motion of molecules inside different objects should be similar as long as the temperature is the same. If we use an instrument to measure the 100°C water and oil mentioned by the questioner, the temperature we get is obviously 100°C. However, if you are careful and the thermometer you use is based on the principle of heat conduction, you should find that the speed at which the thermometer heats up is different. In other words, different materials (liquids) transfer heat differently.

A few days ago, I calculated boiling water in a sealed glass bottle. A physical parameter was used in the calculation. The convective heat transfer coefficient of boiling water is 2500w/m2k. This data indicates the heat transferred by a temperature change of 1K per unit area . The convective heat transfer coefficient of air is about 8w/m2k, which is a very small data. This means that at the same 100℃, the speed of temperature rise of objects in water and air will be significantly different. The higher the convective heat transfer coefficient, the faster the heat transfer speed . This coefficient is closely related to temperature and flow rate, and it is not a proportional relationship, as shown in the figure below.

At 100℃, the oil has not yet started to boil. According to the data, the convective heat transfer coefficient of oil is between 50-1500, which is much lower than the convective heat transfer coefficient of water. This means that when the same object is immersed in 100℃ water and oil, the temperature of the object in the water rises faster.

The human body feels hot because after the heat is transferred to the human body, the heat per unit area exceeds the range that the human body can bear, and the local temperature of the human body is too high. Under the same circumstances, the temperature of the human body in 100℃ water rises faster and the heat is transferred faster, so the body will feel pain earlier, and the result is that it hurts more.

3. Summary

Under the same conditions, the convective heat transfer coefficients of water and oil are different. The convective heat transfer coefficient of water is much greater than that of oil. The heat transfer of objects in water is faster, and the temperature rises faster. For example, the human body will feel hotter faster in water, and the result is that 100℃ water is "hotter".