When I feel heat on my hand, what exactly is it I am feeling? Is there is a “large” amount of certain particle hitting my hand at the same time so it causes a burn? Is it a wave? Is heat just radiation? If so, why do we call it heat?
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Wikipedia says “Heat is energy in transfer to or from a thermodynamic system”. And then it says a lot of other technical things.
But our qualitative perception of heat ties up a lot of separate things. One is the difference between heat energy and temperature: a thing can be of a high temperature, but not have much heat energy, even to the point of seeming cold. One example is the very very thin gas that we find in space within our solar system. Temperature is the average kinetic energy of particles in a system. When you *do* find particles out there, they tend to be going quite fast, and thus have a high kinetic energy. But it still *feels* very cold.
Another thing that often gets bundled together is different means of heat transfer. you mention radiation, which is (usually) infrared photons, that can carry heat to us even through a vacuum; but there’s also conduction, where two objects touch, and the bumping-around molecules in the hotter one can bang into molecules in the cooler one, transmitting energy; and convection, where air (or some other gas) currents going past a hot object can absorb heat, or where a cold object can absorb heat from passing air currents.
So the heat you feel on your hand is energy being transferred to your hand from a body that has a higher temperature than your hand (because its particles have a higher average kinetic energy), and that heat is being transferred via radiation, conduction, or convection.
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