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 ...
25
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1answer
962 views
Rubber band stretched produces heat and when released absorbs heat.. Why?
I always used to wonder why this happens..
when one stretches a rubberband to nearly it snapping point holding it close to your skin - preferably cheek(helps feel the heat), it emits heat. While ...
23
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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?
22
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6answers
2k views
How do whisky stones keep your drink cold?
From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to ...
21
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1answer
696 views
Energy in bolognese reduction - lid on or off?
Generally, to let my bolognese thicken, I leave the lid off in order to "let water vapor escape." I am however distracted from enjoying the taste because I'm having doubts that my physical reasoning ...
17
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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 ...
15
votes
3answers
456 views
Does the Moon's core still contain significant heat?
On earth, using earth-sheltering techniques can significantly reduce the temperature fluctuations on a structure. Would the same statement be true as well on the Moon? Does the Moon's core still ...
14
votes
1answer
327 views
Is there some way to narrow down the Leidenfrost point for water?
Cooks sometimes use the Leidenfrost effect to estimate the temperature of a frying pan by flicking a few drops of water onto the heated pan. I had no idea, before looking into this, that this could be ...
13
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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
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3answers
415 views
Why are materials that are better at conducting electricity also proportionately better at conducting heat?
It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and ...
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 ...
8
votes
2answers
620 views
Do photons lose energy while travelling through space? Or why are planets closer to the sun warmer?
My train of thought was the following:
The Earth orbiting the Sun is at times 5 million kilometers closer to it than others, but this is almost irrelevant to the seasons.
Instead, the temperature ...
8
votes
3answers
2k views
Why do power lines sag when they are heated up?
I was reading some information about the 2003 power blackout in the Northeastern US.
Beginning early in the afternoon of August 14, 2003 big transmission lines began to fail in First Energy's ...
8
votes
4answers
782 views
What happens when you heat vodka in a microwave?
Since ethanol has a lower dielectric constant than water would the water heat up and boil before the ethanol? Would the water transfer heat to the ethanol and, since ethanol has a lower boiling point, ...
7
votes
1answer
509 views
Is it 11% hotter today than it was yesterday?
Yesterday the temperature outside was 0.5 °C. Today, the temperature is 30 °C. 30 is 5300% more than 0.5, but today is obviously not 5300% hotter than yesterday.
In Fahrenheit, the temperatures are ...
7
votes
4answers
681 views
Why does pizza cheese seem hotter than the crust?
When I eat hot pizza or a melted cheese sandwich, the cheese feels a lot hotter than the crust or bread: in particular, the cheese might scald the roof of my mouth. but the crust will not. Is this
...
7
votes
4answers
245 views
How to guess the content of a christmas present?
Let us assume that the present does not make any recognizable sounds when shaken (meow splat - the present now contains a dead kitten). Let us furthermore assume ...
7
votes
3answers
1k views
Would wearing clothing that is black on the inside and white on the outside keep you cooler?
The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
7
votes
5answers
862 views
How do we perceive hotness or coldness of an object?
Some objects, especially metallic ones, feel cold on touching and others like wood, etc. feel warm on touching. Both are exposed to same environment and are in their stable state, so some kind of ...
7
votes
1answer
111 views
What determines bubble locations in boiling water?
Something a little different to our usual fare. I was boiling a pan of water for cookery the other day, and got to wondering what caused the location of the bubble streams from the bottom of the pan. ...
7
votes
2answers
392 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 ...
6
votes
2answers
3k views
Does tea stay hotter with the milk in it?
A little thought experiment, similar to this one: Imagine you are making a cup of tea when the door bell rings. You've poured the boiling water into a cup with a teabag in it. As you're just about to ...
6
votes
1answer
335 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 ...
6
votes
2answers
149 views
When should I take wine out of the fridge - transient heat transfer problem
I am hosting a dinner tonight for which I'll be serving white wine (Riesling to be more specific). Generally white wine is best served chilled (not COLD!) at around ...
6
votes
1answer
1k views
LED Thermal Modeling (How to solve heat equation with constant heat source)
I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state.
Knowing the LED voltage drop and current, I ...
5
votes
4answers
978 views
Should I heat my room when I'm not here, energy-efficiently speaking?
I was wondering as it's getting cold : is it better for my electricity bill to shut down completely my (electric) heater during day, and to turn it on again when I come home (then it will have to heat ...
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 ...
5
votes
3answers
2k views
Could a sleeping bag be warmer if you are naked inside?
I have heard conflicting theories about the best way to keep warm in a sleeping bag in the winter. Some people say you should bundle up in your sleeping bag and another theory says that you should be ...
5
votes
4answers
2k views
What causes hot things to glow, and at what temperature?
I have an electric stove, and when I turn it on and turn off the lights, I notice the stove glowing.
However, as I turn down the temperature, it eventually goes away completely. Is there a cut-off ...
5
votes
4answers
627 views
Sauna thermodynamics
Why does it get hotter (feel hotter) in a sauna when one pours water over the hot stones?
Wikipedia says that the water condenses onto the skin, but the actual air humidity is so low that I doubt ...
5
votes
3answers
333 views
What made the keys in my pocket so hot?
Not sure where to ask this question - thought you guys would probably have the best idea!
Today a single key on my keychain in my pocket heated up so that it was too hot to handle and scalded my leg. ...
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
4answers
198 views
what is the basic form of the 'fire'? [duplicate]
Possible Duplicate:
Is fire matter or energy?
What is the basic form of fire? physics defines every entity by a basic form either solid or liquid or as a gas,
example: water is liquid,
ice ...
5
votes
5answers
634 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 ...
5
votes
2answers
453 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 ...
5
votes
4answers
1k views
Is there an equation for convective heat transfer?
Is there an equation I can use to calculate the temperature (as a function of time) of an object which is gaining or losing heat by convection? Or equivalently, the rate of energy transfer from the ...
5
votes
1answer
140 views
Can an induction coil heat two layers of metal?
Imagine we have an induction coil which is strong enough to heat a sheet of metal. We can put a sheet of ferromagnetic metal close to the coil at distance $h_1$, and it gets heated to temperature ...
5
votes
2answers
968 views
Heat equation on ball - one-dimensional description
I want to solve the transient heat equation on a ball. The boundary condition is the same over the hole outer surface. So this should reduce to a one-dimensional problem in radial direction. However I ...
5
votes
1answer
141 views
Why doe we not get a shock on induction cooker?
In induction cooking, eddy currents in cook-pot cause heat to be produced. But why do we not get a shock?
Also, why is it that current is converted to heat while it has a good conductor(say, steel) ...
5
votes
1answer
44 views
Reflectivity of a glowing-hot metal surface
When a polished piece of metal (or steel in particular) is heated to incandescence, how do its reflective properties change?
Given a mirror-like surface, would the object temporarily cease to act ...
5
votes
1answer
103 views
Which came first, movement or heat?
According to my measly understanding of the universe, when particles hit one another, some of their kinetic energy is transformed into heat. But when we heat particles (for instance, putting a bucket ...
5
votes
1answer
311 views
Ceramic vase filled with boiling water makes a sound. Why?
I had some old flowers in a vase and noticed the water had become really stagnant, causing the whole room to smell like a fart. So I tipped out the flowers and the old water, and poured some boiling ...
5
votes
2answers
255 views
Why does a firebrand glow brightest before it burns out?
I've noticed this in match-sticks, and mango wood. After the flames die out, the brand glows red for a while. The glow is brightest right before it dies out.
My guess is that immediately before the ...
5
votes
0answers
180 views
Where to place a fan in order to cool a room the fastest [closed]
Imagine I have room that is very hot and it is colder outside. I want to cool it down using a fan.
Scenario 1
There is only one door. I have one fan.
Where should I put it and which way should it ...
4
votes
3answers
331 views
Can I take heat from the air and convert it to electricity?
Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house.
...
4
votes
2answers
347 views
CPT and heat equation
I haven't understood this thing: Physics is invariant for CPT trasform...But the Heat or diffusive equation $\nabla^2 T=\partial_t T$ is not invariant for time reversal...but it's P invariant..So CPT ...
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?
4
votes
4answers
301 views
Understanding Heat
Heat or thermal energy as understood is nothing but motion of molecules of the matter. If the molecules are tightly bound (in case of solids), it is to-and-fro molecular vibrations, otherwise it is ...
4
votes
1answer
1k views
Why are lasers inefficient?
Why are lasers inefficient? Is it because of the heat lost during lasing? Why couldn't there be thermocouples or turbines in parts of the cooling circuits to extract something out of that heat?
4
votes
2answers
421 views
Why does a slight drip of water protect pipes from freezing?
I've heard that turning on faucets to a slight drip will prevent pipes from freezing, but I've never understood why this is the case. Can anyone out there help me to understand? Thanks!
