# Tag Info

384

I did the experiment. (dipping wins) H2O ice bath canning jar thermometer pot of boiling water stop watch There were four trials, each lasting 10 minutes. Boiling water was poured into the canning jar, and the spoon was taken from the ice bath and placed into the jar. A temperature reading was taken once a minute. After each trial the water was ...

122

Stirring will win, hands down, every time. This is why physicists need to talk to chemists once in a while. As Georg correctly remarks, the latent heat of vaporization of water is enormous - but he's wrong about waving the spoon; stirring is the champion here. Why? Temperature is really the average kinetic energy of the molecules in the bulk substance, ...

84

Air molecules $(\require{mhchem}\ce{N2_}$ and $\ce{O_2})$ have an average velocity of around $500\text{ m/s}$, varying some depending on the temperature. This means that a nice $5\text{ m/s}$ wind is a hundred times slower, and the energy represented by wind is 10,000 times smaller than the thermal energy. Therefore, wind does not have considerably more ...

77

There's a lot of detail you could go into with regard to this question, as is done in the other answers and comments, but I think the answer itself is pretty simple. Imagine a surface that just barely surrounds your body, as if you shrink-wrapped a body in plastic. By the law of conservation of mass (valid in non-relativistic physics), the only way your body ...

60

Short answer: The thermometer measures actual temperature (which is the same for both), while your hand measures the transfer of energy (heat), which is higher for the pot than the air. Long answer: Keyword: Thermal Conductivity The difference is a material-specific parameter called thermal conductivity. If you are in contact with some material (gas, ...

56

The reason is because the heat loss occurs mostly in the windows and the fenestration. The idea is that you would like the incoming air to be heated up. Also, it creates an air curtain that prevents more heat from being lost through this exposed areas. The final reason is to make the temperature of the room more or less uniform. If the heaters were placed at ...

51

Ice cubes have three distinct cooling effects: The cube, initially at sub-zero temperature, absorbs some heat to reach fusion point (0⁰C). The cube absorbs more heat to switch phase: it takes some energy to turn 1 kg of ice at 0⁰C into 1 kg of liquid water at 0⁰C. The water absorbs some heat to become warmer than 0⁰C. The three effects occur more or less ...

50

You blow away the flame from its fuel source. If you would blow less hard the flame might burn harder because more air is supplied to the flame (similar to a Bunsen burner). Because normally the flame of a candle gets its oxygen through a convectional airflow generated by the heat of the flame. The reason why the flame is blown away from the candle is ...

49

Partly practical, the wall under the windows isn't useful for anything else. We had a house where the heaters were placed in the middle of the only empty walls, so nowhere you could put furniture, bookcases, etc. Before double glazing there would be a draft from the windows so the idea was to heat this incoming air by having a radiator immediately below the ...

45

No. Boiling doesn't mean that the water will cook anything. If you have boiling water at 30°C you could touch it (if we forget that it's at really low pressure) and nothing would happen. Boiling is not what cooks, but temperature. In fact, if you want to purify water at high altitudes, you need to boil water more time because it will be at lower ...

45

It's obviously not a sharp cut-off, but as a general guide sound waves cannot propagate if their wavelength is equal to or less than the mean free path of the gas molecules. This means that even for arbitrarily low pressures sound will still propagate provided the wavelength is long enough. Possibly this is stretching a point, but even in interstellar gas ...

44

How to cool a cup of coffee with the help of a spoon. Hmm... Empty the cup using the spoon, discarding the hot coffee. Strike the cup with the spoon, shattering it and forcing it to release the hot coffee. Drink the coffee with the spoon. Use the spoon to carve a cup-shaped hole in a large block of ice, put the cup in there. Put the spoon in the coffee, ...

41

This is a very good question. Einstein himself, in a 1907 review (available in translation as Am. J. Phys. 45, 512 (1977), e.g. here), and Planck, one year later, assumed the first and second law of thermodynamics to be covariant, and derived from that the following transformation rule for the temperature: $$T' = T/\gamma, \quad \gamma = ... 40 Essentially, losing of weight occurs by means of burning fuels precisely like your car does when it burns petrol and emits exhaust gases. The only difference is that for humans that fuel is to be found in the form of sugars. The fat is what you want to get ultimately rid off, of course, but sugars are more easily processed and so this is what you are ... 39 Since this is a physics forum I assume the OP is interested in a quantitative answer in terms of the efficiency of the system and how it differs based on the relative positioning of heat sources and heat sinks. The math required to analyzed such a system is too much for me to manage right now, but I believe the following principles apply and are objectively ... 37 This is a very interesting question with a very interesting answer. The key lies in the reason for the stretchiness of the rubber band. Rubber is made of polymers (long chain molecules). When the elastic band is not stretched, these molecules are all tangled up with each other and have no particular direction to them, but when you stretch the elastic they ... 35 When you exercise, you "burn" more glucose, the simplified reaction for which (from Wikipedia) is: {\rm C_6H_{12}O_6 + 6~O_2 → 6~CO_2 + 6~H_2O} So when you exhale, the carbon in the carbon dioxide, and the hydrogen and the oxygen in the water vapor, came from the glucose being burned, thereby removing that mass from the body. 32 Take the spoon. Use it to push the coffee over. It will spill out into a large puddle. This has maximal surface area and will cool VERY quickly. 32 We can only approach absolute zero asymptotically because we can't suck heat out of a system. The only way we can get heat out is to place our system in contact with something cooler and let the heat flow from hot to cold as it usually does. Since there is nothing colder than absolute zero, we can never get all the heat to flow out of a system. We can ... 31 I'll try to give an answer in purely classical thermodynamics. Summary Heat is a way of accounting for energy transfer between thermodynamic systems. Whatever energy is not transferred as work is transferred as heat. If you observe a thermodynamic process and calculate that system A lost Q calories of heat, this means that if the environment around ... 30 One reason you might think T should be measured in Joules is the idea that temperature is the average energy per degree of freedom in a system. However, this is only an approximation. That definition would correspond to something proportional to \frac{U}{S} rather than \frac{\partial U}{\partial S}, which is the real definition. The approximation holds ... 28 With respect to the content in the cup, all Your hampering with the spoon is irrelevant. Cooling of a hot coffee is achieved by vaporisation of water. At temperatures between 100 and say 50 °C the vapor pressure is so big, that the heat carried away by convection of the hot (and much less dense than air!) vapor dominates all other heat transfer ... 26 Instead of a circular hole, let's think of a square hole. You can get a square hole two ways, you can cut it out of a complete sheet, or you can get one by cutting a sheet into 9 little squares and throwing away the center one. Since the 8 outer squares all get bigger when heat it, the inner square (the hole) also has to get bigger: Same thing happens ... 26 It depends on what you mean by efficiency. Suppose you want to heat your house. An electric heater like you're considering would do this by converting electrical energy directly into heat. Pretty much all the electrical energy does get converted to heat, as you suggest. The energy used to get a certain amount of heat into the house is simply equal to ... 26 I can't believe the density or material of the spoon hasn't been considered. If the spoon is very dense you can take it and wave it in the air in a 15° arc and say, "Dear waiter, if you don't put some cold milk in my coffee I will hit you between the eyes with this abnormally dense spoon". On the other hand, if its made of gold or silver you hold it ... 26 The separation does not violate the 2nd law of thermodynamics, because the oil and water phases being separate is a lower energy state. The water molecules strongly interact with each other, forming hydrogen bonds. The protons of water are shared between two oxygen atoms of two different water molecules, forming a constantly changing network of molecules. ... 25 The gas molecules in your bottle of air aren't just sitting still, they're moving around in random directions. From memory, the speed of oxygen and nitrogen molecules at room temperature is around 500 meters per second. When the bottle is closed, the air molecules hit the walls and lid of the bottle and bounce back, so the air stays in the bottle. If you ... 25 As others have pointed out, in metabolism you breathe in oxygen and exhale carbon dioxide with the net reaction being$$CH_2O + O_2 \to CO_2 + H_2O The carbon dioxide gets exhaled, and the water is lost through some combination of other things. Let's think about whether this is a reasonable way to explain weight loss. A breath is maybe one liter of ...

23

David Zaslavski's answer is correct and complete. But I want to propose a different way to look at the problem. Think of the disc that was cut out, and imagine that you heat it too, exactly as you heat the plate. After heating, the disc will fit in exactly to the hole, just as if it was first heated and then cut out. Therefore, the hole will expand.

23

Arnold Neumaier's comment about statistical mechanics is correct, but here's how you can prove it using just thermodynamics. Let's imagine two bodies at different temperatures in contact with one another. Let's say that body 1 transfers a small amount of heat $Q$ to body 2. Body 1's entropy changes by $-Q/T_1$, and body 2's entropy changes by $Q/T_2$, so ...

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