# Temperature and molecular Kinetic Energy

• ChrisXenon
In summary, the conversation discusses the concept of temperature and how it relates to the average kinetic energy of molecules in a substance. The speaker questions this relationship by giving an example of a jar of water in a moving car. The experts explain that the average kinetic energy only applies in the object's rest frame and that heat is statistically distributed over all particles. They also mention the role of velocity in the car's rest frame.
ChrisXenon
We are told that temperature measures the average kinetic energy of the molecules or atoms in a substance, but this cannot be true.
If I take a jar of water and put the jar in my car and drive it aorund, it won't get hotter, though it's kinetic energy must surely be higher.
Clearly, I've got something wrong here - is it due to the fact that there's a vague notion that this average kinetic energy is vibratory? Or something else?

Thanks

ChrisXenon said:
We are told that temperature measures the average kinetic energy of the molecules or atoms in a substance, but this cannot be true.
If I take a jar of water and put the jar in my car and drive it aorund, it won't get hotter, though it's kinetic energy must surely be higher.
Clearly, I've got something wrong here - is it due to the fact that there's a vague notion that this average kinetic energy is vibratory? Or something else?
You pretty much have it; the vibration (or bouncing around of gas molecules) is random and only relevant for the object's/gas's rest frame. The kinetic energy of bulk motion is separate.

Lord Jestocost
Heat is energy which is statistically distributed over all particles. Maybe, the following (“Thermodynamics and Statistical Mechanics” by Walter Greiner, Ludwig Neise, Horst Stöcker) might be of help:
Thermodynamics and Statistical Mechanics

russ_watters
ChrisXenon said:
If I take a jar of water and put the jar in my car and drive it aorund, it won't get hotter, though it's kinetic energy must surely be higher.

@russ_watters already mentioned the object's rest frame.

Have you considered that the car's velocity in the car's rest frame is zero. Or that the car's velocity relative to another car driving the same direction decreases as the car accelerates from a stop?

BAM! Outstanding. Thanks guys. I'm sorted.

## 1. What is the relationship between temperature and molecular kinetic energy?

The temperature of a substance is directly proportional to the average kinetic energy of its molecules. This means that as the temperature increases, the molecules move faster and have more kinetic energy. Conversely, as the temperature decreases, the molecules move slower and have less kinetic energy.

## 2. How does temperature affect the state of matter of a substance?

Temperature plays a crucial role in determining the state of matter of a substance. At higher temperatures, the molecules have more kinetic energy and are able to overcome the intermolecular forces holding them together, resulting in a change from solid to liquid or liquid to gas. As the temperature decreases, the molecules have less kinetic energy and are more tightly bound, leading to a change from gas to liquid or liquid to solid.

## 3. Can temperature affect the rate of chemical reactions?

Temperature has a significant impact on the rate of chemical reactions. As the temperature increases, the molecules have more kinetic energy and move faster, leading to more frequent and energetic collisions. This results in a higher reaction rate. On the other hand, at lower temperatures, the molecules have less kinetic energy and move slower, leading to fewer collisions and a slower reaction rate.

## 4. How is temperature measured?

Temperature is commonly measured using a thermometer, which typically contains a liquid (such as mercury or alcohol) that expands or contracts with changes in temperature. Other methods include using thermocouples, which measure changes in electrical resistance, or infrared thermometers, which detect the amount of infrared radiation emitted by an object.

## 5. Can temperature be negative?

Yes, temperature can be negative. In the Celsius and Fahrenheit scales, negative temperatures are below 0 degrees, while in the Kelvin scale, 0 Kelvin (absolute zero) is the lowest possible temperature. Negative temperatures are typically found in very cold environments, such as in outer space or in laboratories where substances are cooled to extremely low temperatures.

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