Heat is energy transferred from one system to another by thermal interaction. In contrast to work, heat is always accompanied by a transfer of entropy. Heat flow is characteristic of macroscopic objects and systems, but its origin and properties can be understood in terms of their microscopic ...
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19answers
55k views
Cooling a cup of coffee with help of a spoon
During the breakfast with my colleagues, a question popped into my head:
What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?
A qualitative answer would ...
17
votes
6answers
3k views
Is fire plasma?
Is Fire a Plasma?
If not, what is it then?
If yes why, don't we teach kids this basic example?
UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify ...
5
votes
5answers
631 views
What would ACTUALLY happen to a person jettisoned into space?
[insert obligatory statement of my lack of knowledge in physics]
Alright, so we have all seen the movies where someone gets blasted out of the airlock on their starship, or their suit decompresses ...
2
votes
2answers
202 views
Heat transfer between two surfaces
Suppose I have surface A in contact with surface B, if I apply Fourier's law of heat transfer, which $K$ should I use, $K_a$ or $K_b$?
Essentially asking whether the same block of material heats ...
23
votes
8answers
8k views
Will a hole cut into a metal disk expand or shrink when the disc is heated?
Suppose you take a metal disc and cut a small, circular hole in the center.
When you heat the whole thing, will the hole's diameter increase or decrease? and why?
13
votes
6answers
1k views
What exactly is heat?
Is it energy?
Is it energy per unit volume?
Is it energy per unit time i.e power?
What is it?
11
votes
4answers
6k views
Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?
Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?
It's not just a subtle difference, but significant in my ...
6
votes
1answer
334 views
Microwave oven + water: dielectric heating or ion drag?
When you place a water or food in a microwave oven, it heats.
Which process commits more energy to that: dielectric heating, or ion drag i.e. resistive heating?
AFAIK, in distilled water (which is a ...
5
votes
3answers
444 views
Can temperature be defined as propensity to transmit thermal energy?
I was recently surprised to learn that defining temperature isn't easy. For a long time, it was defined operationally: how much does a thermometer expand. Also surprising, temperature isn't a ...
5
votes
2answers
452 views
What happens if you connect a hot resistor to a cold resistor?
Kind of an extension to this question:
If you heat up an object, and put it in contact with a colder object, in an ideal insulated box, the heat from one will transfer to the other through thermal ...
4
votes
3answers
1k views
Why does hot oil explode when pouring water on it?
I am puzzled to know, What is the reason of hot oil make sound and explode when we pouring water on it?
3
votes
2answers
564 views
How air humidity affects how much time is needed for heating the air?
In cold weathers it is suggested to put a humidifier since the air gets too dry. I wonder how the humidity affects how much time is needed to get the air at a temperature of 20 Celsius degrees? I mean ...
7
votes
2answers
388 views
How much more energy does it take for a human body to heat 0C ice vs 0C water?
I'm trying to determine if going through the trouble of ingesting ice is worth the hassle versus ingesting ice-cold water, but my physics skills are rusty.
If I drink a gram of ice water at ~0C, my ...
5
votes
4answers
1k views
After what speed air friction starts to heat up an object?
I understand that air friction cools off an object at low speeds. For example, if you blow on a spoon of hot soup, it cools off. Or if you swing a hot frying pan in the air, it cools off faster.
But ...
4
votes
4answers
4k views
The difference between heat and temperature
So as I understand it, heat energy of an object is the SUM of all the kinetic
energies of the molecules of the object (upto constant factor).
The temperature on the other hand is the AVERAGE of the ...
3
votes
2answers
177 views
How to describe heat transfer between two solid materials?
A general equation for dealing with heat transfer between one material and a region of insulating material. I've seen basic heat transfer equations for one material, but I'd love to see an explanation ...
3
votes
3answers
259 views
How can my water cool down more quickly?
I have a cup and I can only pour hot water inside, I wanna know whether the heat will dissipate more quickly with more water or less water?
How about the occasion when my cup is well covered?
3
votes
1answer
779 views
Why is the lid of the cookware kept on induction cooker not hot?
Induction cookware cooks food by inducing an electro magnetic field in the ferro-magnetic cookware. Since iron offers a lot of resistance to the current, the current is converted into heat in the ...
2
votes
1answer
137 views
Does an object's color change its rate of cooling?
The motivation for this question comes directly from this thread. The proposition is that the color of something changes how fast it cools (note: specifically the rate of cooling, not taking into ...
2
votes
1answer
223 views
Why does blowing on someone who is wet feel colder than on someone who is dry?
The title says it all. If I'm standing in the wind and I'm wet, I feel much
colder than when I'm dry. This is true no matter how warm or cold the water.
Why is this?
2
votes
2answers
683 views
What is more efficient: Add milk, and then heat up coffee in the microwave, or microwave than milk?
Adding milk first increases the volume to heat and lowers average temerature, but adding it afterwards seems to have similar effects. How can you compare the two?
1
vote
1answer
113 views
Determine the flow and amplitude equation for thermal energy (with Del operator)
It is a question vector calculus and Maxwell's laws. I put it this way. Let's say, we are working in a $3$-Dimensional space ( e.g $x\cdot y\cdot z = 4\cdot3\cdot2$, a certain room/class of that size ...
1
vote
3answers
353 views
Maximum efficiency for a counter-current heat exchanger (double flux controlled motorized ventilation)
I am not sure if I can explain the question correctly because I don't know the name of this mechanism in English.
This is my explanation attempt: In a house, a tube is expelling the air from the ...
1
vote
2answers
171 views
If I put 3 bottles of water next to each other in the fridge, which one is cold first?
I was wondering: If I put three bottles of water next to each other in the fridge, which one is cold first? Does it matter? Is it the one in the middle because it gets refrigerated by the other two?
...
