# Trying to better understand temperature and entropy

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• Jaccobtw
In summary: So, when you consider a huge gravitational system, the energy and entropy will be very large, but the temperature and entropy density will be small (but nonzero). For example, if you have a huge sphere of gas, inside the sphere you have a very large entropy and energy, but the entropy density and energy density are both small. On the other hand, if you have a small sphere of gas, the entropy and energy will be small, but the entropy density and energy density are relatively large. In summary, the total amount of energy in the universe is constant, but the average energy (temperature) seems to decrease as the universe expands. This means that the entropy of the universe can only increase, as stated by the Second Law of Thermodynamics
Jaccobtw
TL;DR Summary
From my understanding, we can think of entropy as how spread out energy is within a system. The more spread out, the more entropy. The more condensed, the less entropy.
If you were to condense all the energy in the universe into a point, wouldn't the temperature be very high, yet the entropy be very low? Also if you were to spread out all of the energy in the universe, wouldn't the temperature be near zero and the entropy be very high? And this makes entropy units - J/K - make sense. Because the total amount of energy in the universe is constant (2nd Law Thermodynamics), yet the average energy (temperature) seems to decrease, this would mean that the entropy of the universe can only net increase. Is this a correct understanding of entropy or am I off? Thanks for your help.

Jaccobtw said:
If you were to condense all the energy in the universe into a point, wouldn't the temperature be very high, yet the entropy be very low?
Temperature at that point would be very high, but everywhere else it would be zero. The entropy density at that point would be very high, but everywhere else the entropy density would be zero. The entropy, that is entropy density integrated over whole universe, would therefore be low.

In thermodynamics it's very important to distinguish extensive quantities from intensive quantities. Extensive quantities increase with the size of the system, examples are entropy and energy. Intensive quantities do not increase with the size of the system, examples are temperature, pressure, entropy density and energy density. In fact, all intensive quantities can be understood as kinds of "densities". Pressure is a "density" of force (but force per area, not force per volume), while temperature is related to the energy density (but the exact relation, in general, is not so simple).

Last edited:
Jaccobtw and topsquark

## 1. What is temperature and how is it measured?

Temperature is a measure of the average kinetic energy of particles in a substance. It is typically measured using a thermometer, which uses the expansion or contraction of a liquid or gas to indicate the temperature.

## 2. How does temperature affect entropy?

Temperature and entropy have a direct relationship - as temperature increases, so does the entropy of a system. This is because as temperature increases, the particles in a substance have more energy and are able to move more freely, increasing the disorder or randomness of the system.

## 3. What is the difference between absolute and relative temperature?

Absolute temperature is measured in Kelvin (K) and is based on the theoretical concept of absolute zero, where all molecular motion stops. Relative temperature is measured in degrees Celsius (°C) or Fahrenheit (°F) and is based on the freezing and boiling points of water. Absolute temperature is always higher than relative temperature.

## 4. How does the second law of thermodynamics relate to temperature and entropy?

The second law of thermodynamics states that the total entropy of a closed system will never decrease over time. This means that as temperature increases, so does entropy, as the system becomes more disordered.

## 5. Can temperature and entropy be used to predict the direction of a chemical reaction?

Yes, temperature and entropy can be used to predict the direction of a chemical reaction. In general, reactions will proceed in the direction that increases entropy, which usually means an increase in temperature. However, there are other factors that can also influence the direction of a reaction, such as the concentration of reactants and the presence of a catalyst.

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