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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“Heat” is a way of referring to how much energy something has. When something has more energy, that energy “wants” to go to anywhere that has less energy in it, in a manner similar to how water in two tanks connected by a pipe will seek the same level, even if one starts off with a much higher level.
When there’s more energy in one place and less energy in another- like, for example, a stove with a burner set to high and the air above it- the energy will move from one to the other- the burner loses a tiny bit of heat, the air gets some of that heat, which in turn heats up the air next to it, and continues to expand toward the areas with less heat- less energy.
As long as the burner is powered, it’s constantly replenishing that heat, which continues to radiate out to other materials touching it or near it- whether that’s air, a pan with food in it, or your hand.
When you feel “heat” in your hand, what you’re detecting is that the air around your hand has more thermal energy in it, so some of that energy is going from the air to your hand. That raises the temperature of your skin a bit, which your senses detect as “heat”.
Heat is the amount of motion that molecules of a substance have. The motion can be transferred when they touch another collection of particles causing their speeds to even out on average and the temperature to equalize. Electromagnetic radiation like light or radio waves can transfer energy as they hit a material and get absorbed. This excites the molecules in it causing them to move faster. When you feel heat coming from another object it is usually infrared or visible light radiation.
When you feel heat on your hand, you are feeling the temperature of your hand, particularly the skin.
The temperature is basically the average speed of the particles in your hand. Because these particles are moving, they have energy, which we can label thermal energy.
Your hand can be heated or cooled by a few different processes. If you touch something hot, it can conduct thermal energy to you. If you touch something cold, you can conduct thermal energy to it. Because you just touched something, you associate touching with it with the sensation of heat or cold.
When out in the sun, thermal radiation falls on you, adding thermal energy, increasing the temperature. Everything emits thermal radiation, even you. Hotter things emit more thermal radiation.
When you sweat, the faster sweat particles escape – this is evaporation. Because the fastest ones disappear leaving behind slower ones, the sweat cools as it evaporates.
If particles are moving too fast, they cause damage when they crash into each other. This is what ultimately causes burning. Sunburns are different though, but regular burning, sunburns and chemical burns are all related to damage to molecules in your body.
Heat is energy. That’s the simple answer. Unfortunately, a lot of other things are also energy. As far as I know thermal energy transfer is a physics/ chemistry thing. When more energy is stored in an objects atoms they begin to move more and become “hot”. Also, even subzero cold things have thermal energy. To get a better idea you gotta look at Temps in Kelvin. 0° Kelvin is like -460°F and is supposed to be when an object has 0 thermal energy, you can’t go any lower than that. While on the other hand you can go pretty high on the scale until you reach like the absolute energy limit of the universe.
Also, think of electricity, you can get electrical burns/melt skin because bare wires can get really hot and start fires. Electricity is just electrons and they energy from it. I think it’s mainly chemistry but you can look at these things from so many scientific ways. That’s without even going into waves and radiation or any real detail. Even the sun on your skin is photons and the energy they have from explosions in the big ol burny ball.
Tldr: Heat is energy but you can keep going down the rabbit hole.
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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