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What does 'internal energy' even mean? Baffled me for months

All i know is that it's the thing that defines temperature? But isn't temperature just a measure of kinetic energy? Doesn't this then mean that if you accelerate an air particle up to a high speed that it'll become very hot?
The obvious answer for your question would be that "internal energy" is the energy within something, say for example, a gas. A gas can be considered either a "real" or an "ideal" gas, hopefully you know the difference between them. If you don't, it basically is that a real gas's internal energy (the energy inside that gas) is the sum (the total) of the potential and kinetic energy of it's molecules. For ideal gases, we assume that there are NO forces of attraction between the atoms, so the ideal gas doesn't possess any potential energy, therefore it's internal energy is solely kinetic energy.
Temperature isn't a measure of Kinetic energy. Heat, or an increase in temperature provides energy to molecules which in turn increases their kinetic energy.
Thirdly, in order to get a particle to "high speeds", with particle accelerators, we use an increase in temperature to increase the kinetic energy, which in turn increases the speed of the particles. Therefore your question is almost the right answer, in order to accelerate the particle, the temperature is increased, so it would be very hot.

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