Why can tinfoil be touched immediately after coming out of a super hot (hundreds of degrees) oven?
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Temperature and heat are different. The amount of heat energy needed to bring an object up to a specific temperature depends on the material. Tin foil doesn’t need a lot of energy to bring it to the temperature of the oven. When you touch it, the water in your body absorbs a relatively massive amount of that energy and barely raises the temperature of your skin.
To understand this you first have to understand that temperature alone is not a measure of total or available energy. It’s a specific measure of how energetic the molecules at the point of measurement are shaking. A fingerprint sized piece of aluminum foil at 200ºC has less energy than a cup of lukewarm water. Think of it like the energy of a speeding bullet, versus the energy of a freight train inching forward on the tracks. One is fast and energetic, while the other, despite its slow speed, has a massive amount of kinetic energy behind it.
The amount of energy required to heat your skin high enough to burn you is far higher than what is stored in a sheet of hot foil. The foil will transfer its heat quickly and cool off just as quickly, long before your finger feels any dangerous temperature increase.
Heat is energy, specifically thermal energy.
So, we can think of this tinfoil piece as a battery of sorts for thermal energy. Now if you crumpled up the tinfoil as tight as you could we’ll think of this as how big the battery is.
You’ll quickly notice that tinfoil crumple up very, very small, so it’s a small battery.
Now how long will that battery last (be hot). Well this is determined by 2 main factors.
1) How many things are plugged in? This would be most similar to how much surface area is there? Tinfoil has a lot of surface area typically so there are a lot of things plugged in to our small battery.
2) how quickly are those things taking that energy. This is determined by a constant (specific heat) which is typically low for metals like tin. As well as the difference in temperature, that is to say a very hot thing passes energy faster to a very cold thing.
Overall these two things combined with our ‘small battery’ means that the tin foil drops from oven temperature (full battery) down to a safe to touch temperature (let’s say 20% battery) pretty quickly.
DO NOT TOUCH STUFF THAT JUST CAME OUT OF THE OVEN! GIVE IT PLENTY OF TIME TO COOL, THEN WAIT AN EXTRA 30 MINUTES JUST TO BE SAFE!
An object sheds heat based on how wide its surface is and how hot it is compared to its surroundings. An object carries heat based on how big it is and how hot it is.
As such, a twice as thick object will lose heat at the same speed but have twice as much so it takes twice as long to cool.
So let’s look at some hot coffee. With a lid, it takes maybe ten minutes to cool down to a drinkable temperature. A sheet of foil is about 0.63 thousandths of an inch, compared to my six inch tall pool of coffee. As such, our very primitive estimate would indicate that foil cools down in 1/9,500th the time, or 0.06 seconds.
It’s a combination of:
* High heat conductivity (aluminum transfers heat quickly)
* High surface area-to-volume ratio (an object exchanges heat with the environment through that object’s surface, and aluminum foil is almost *all* surface)
* Low mass (the actual amount of “stuff” in a sheet of aluminum foil is very small, so it can’t retain much heat energy)
So as soon as you take it out of the oven, it starts losing the relatively-small amount of heat energy it has very rapidly from the entirety of its surface. Which means that it cools down *super* quickly.
Lots of correct answers here, but for more of an ELI5 explanation:
Think of heat and temperature in an object as a glass of water. Depending on the size and shape of the glass, you will fit very different amounts of water in it!
Temperature would be how “full” the glass of water is, while Heat would be the actual volume of water in the glass. The thin sheet of aluminum foil is very high temperature, but due to how little mass it has, and how little heat aluminum as a material can hold, you can think of it as a tiny thin glass of water that is relatively tall. When two objects touch, their “level” determines how fast the “water” will flow; bigger difference, faster transfer of heat.
So, when your body ( a pretty big and wide glass, but not super high level) touches the foil (tall and very thin) the heat rapidly drains from the foil… But it’s barely a drop in comparison to your body’s volume of water, so the level hardly rises (your temp doesn’t change much).
Now imagine touching a parking lot on a sunny day. It might feel uncomfortable, but not awful ; the “level” of temperature between your hand and the asphalt is not too drastic, so there isn’t a huge heat transfer ( even though there is probably enough heat in that parking lot to vaporize you!)
Lot of good explanations on here already, but to make it more ELI5:
If some hail hits your head at 30MPH, you’ll feel a light sting.
If a truck hits your head at 30MPH, you’re going to the hospital.
They’re both going the same speed, but intuitively, the truck is “heavier”. When it moves at 30MPH, there’s more stuff your head’s gotta stop. Being hit by the truck is like being hit with 100,000 pieces of hail frozen into a single block of ice.
That’s sort of what’s happening with the heat. The aluminium foil only has a tiny, tiny bit of “heat”, and so it’s easy for it to be “stopped”. You feel that “stop” as the foil being slightly hot.
Compare that to a cup of hot chocolate, though. If you leave your hand on the hot mug, you’ll burn yourself! There’s a lot more “heat” being stored, and the mug just keeps on giving that heat. Your hand burns because of this transfer of heat.
This leads to an important take-away: temperature doesn’t tell you how much heat something’s got to give. Rather, it’s a good rule of thumb for everyday life.*
*For the nerds: Objects with both a low thermal conductivity and can store a relatively large amount of thermal energy are rarely a problem on everyday life so generally it’s fine to say “this thing’s at 10 degrees F” and just assume that the heat transfer rate is in “gonna suck” territory.
What burns you is when a lot of energy is transferred to you really quick, you can’t actually feel temperature, you can only feel how energy moves, if you’re in the bathroom the carpet and the floor are at the same temperature, however the carpet feels less cold, because it transfers energy slower (in this case it absorves but that’s a different topic)
Now aluminum transfers energy quick, that’s true, however it’s very light, that means it doesn’t store much energy so if you touch it you will cool it down really fast and the energy it has won’t do much to you.
TLDR; Aluminum foil is very light and as such it can’t store much energy
Thermal mass…. or lack thereof… wrap a steel brick in tin foil and touch it… or a solid piece of Aluminium.
Yeah, but have you noticed that if you quickly ball it up, it does burn?
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