If you are near something hot, say an open flame, you can feel the heat radiating from the fire. I have a juvenile understanding of thermodynamics, but I understand that there is energy radiating from the heat source which is what I’m feeling.
By contrast, if you hover your hand near an ice cold drink, you can FEEL the cold before even touching the glass.
How is this possible since my hand is seemingly radiating more energy than the drink is? Is what I’m feeling just the energy leaving my hand and entering the drink?
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Equilibrium is what nature aims for. The simplest explanation is that there is a temperature difference between your warm hand and the cold (or hot) drink and so to reach equilibrium the temperature in your hand that touches the drink will either raise or drop to compensate for the difference.
You are exactly right. When you feel temperature of something you are feeling the flow of heat. The ice is cold and so absorbs heat from the air making it cold. The cold air is absorbing heat from your hand. Like wise with hot things. They heat up the air and the air is heating your hand. It’s a little different for things like hyper and hypothermia but in general this is how it works when your body is at a normal healthy temp.
>you can FEEL the cold before even touching the glass.
This may be too short an answer for eli5, but the phenomenon of feeling cold radiate off a cold beverage, or a window in the winter, is simply due to the conduction of the heat in the air towards the cold body. The cold drink and the (relatively) warm air exchange energy to create an equilibrium – which will be warmer than the drink’s initial temperature and colder than the air’s initial temperature. Thus, compared to the air far away from the class (that can’t directly conduct heat to the glass) the air immediately surrounding the perimeter of the glass is colder. In reality, cold cannot radiate, and even though air conducting near a cold body may mirror heat radiation – it’s a different process. Although a good chunk of the heat you feel from a fire is also conductive through the air
It would be more accurate to say that your hand is feeling the cold air around the drink. It’s not like there’s cold radiating from the drink glass. Your hand is continually radiating heat, but that’s not really something that you feel in normal circumstances.
But you can feel when something is colder than your hand, such as cold air.
Two things, really your skin feels drops or increases in your skin temperature, you get that because your skin, which is heated by your body, either loses heat or gains heat.
For gaining heat, it’s easy, something much hotter than you can radiate heat. But that’s really not the full story, you are always radiating heat away, and the environment is always radiating it back. If the walls are 70°F, they are radiating almost as much heat into your skin as your skin radiates away so it feels neutral. With cold things they don’t radiate back, so you feel cold because you radiate your heat away with nothing coming back.
For cold specifically, you also feel the cold air, a glass of cold water chills the air near it which falls down, if you’re close enough you can also feel the cold air near the glass.
You’re just feeling the convective action of the cold air against your skin. When you sense the radiant heat of a fire, the temperature difference is significant, around 1000 °F.
When the temperature of everything around you is pretty close to the temperature of your skin, you don’t lose or gain a lot of heat through radiant heat, because you radiate about as much as you absorb. The ice is only a few tens of degrees colder than you are, so you wouldn’t notice.
It does cool off the air around it, so if you get close, your skin will heat up that air, and convection will cause it to circulate as it warms up, being replaced with more cold air, and carrying away enough heat that you can feel it.
You’ve predictably gotten a few answers claiming that we can’t sense temperature – only the flow of heat. This isn’t true. It’s become one of those science memes that at this point serves more to hinder understanding than to facilitate it.
Homeostasis requires absolute sensing of body temperature, which IIRC is done by the hypothalamus. If you sit in a 100F hot tub for long enough, your body will stabilize at that temperature. There’s no more heat flow between you and the water, but you’ll still feel uncomfortable and hot as hell.
A cursory reading of how [thermoreceptors](https://en.wikipedia.org/wiki/Thermoreceptor#Mechanism_of_transduction) are thought to work indicates that they do sense absolute temperature, as well as relative temperature. [Another paper](https://www.sciencedirect.com/science/article/pii/S096098220600354X) on the topic. Your skin senses “heat flow” insofar as the magnitude of the heat flow is what determines the temperature of your skin cells (as well as the thermoreceptors). But it’s not correct to say that you can only sense heat flow.
Heat flux certainly explains why some surfaces feel colder or warmer than others despite being at the same temperature, but that’s not literally what your body is sensing. It can detect both absolute and relative changes in temperature.
As for the glass – a cold glass doesn’t just exist in a vacuum – although it can, and in that case it wouldn’t feel cold when you brought your hand near it. The glass is cooling the air around it, which increases its density, which causes it to flow down along the sides of the glass and warm air to take its place. This is convection and is happening constantly until the temperature stabilizes. The cold air is what you’re feeling.
Your body can feel both the gain and loss of heat.
If you’re in a hot environment, you’ll feel your body absorb the heat from the air or whatever it is that’s hot.
And if you’re in a cold environment, you’ll feel your body getting colder as a result of it losing heat to the environment.
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