161

What you are seeing is not actually vapor - vapor is invisible. The mist seen above boiling water, commonly but inaccurately called vapor, is actually made of tiny droplets of liquid water, formed when the vapor cools down and condenses. While the stove is on, the constant influx of vapor from the boiling water keeps the air above it hot, so condensation is ...


68

I think you are speaking of a clinical thermometer which records the maximum temperature it reaches. The thermometer has a narrow kink in the bore near the bulb that causes the mercury thread to break at that point when the volume of mercury in the bulb shrinks (the image you've posted actually shows that). As a consequence the top of the thread does not ...


67

This is more a question of chemistry and biology than physics. Solid objects don't burn (try dropping a lit match on a piece of structural lumber sometime -- it'll just go out). Instead, they release or decompose into flammable gasses on heating, and it's those gasses that burn. Some plants (eg. pines) produce volatile resins that provide an easy ignition ...


56

Being a shiny surface the aluminium sheet reflects radiant heat and reduces the heat loss by radiation by as much as $90\%$. Being impermeable the sheet stops the movement of hot air from the vicinity of the surface of the food into the surrounding by convection currents. This also has the effect of reducing the rate at which water evaporates from the ...


30

Without seeing your experiment we can only speculate, but my guess is that this is due to the convection currents generated by the combustion of the gas. When the gas is burning there is a large volume of hot carbon dioxide and water vapour generated by the combustion, and this flows upwards and around the pan. This has two effects. Firstly it keeps the ...


28

I do exactly the same. It is a very effective way of defrosting food fast. Compared to air water has a much higher heat capacity and a much higher thermal conductivity. That means heat flows from the water into the sausages much faster than it would in air and the water cools less than air would as it heats the sausages. Aluminium foil has a much, much ...


25

That's because it's a maximum thermometer, which works by pushing the liquid past a restriction in the tube, preventing the liquid from returning into the reservoir upon cooling. If you look closely, you might see the separation in the liquid column between the restriction and the reservoir, which may be bigger or smaller depending on the model. You don't ...


23

Water evaporates faster at higher temperatures, so plants will lose water faster in a heat wave than in milder conditions. Drier plants will catch fire easier and burn faster and hotter than moist plants.


19

It's probably because your milk cup is made of a material that is a relatively good thermal insulator. First of all, the microwaves directly heat the milk, and not the cup, as long as the cup is made of material that microwaves pass through without being absorbed. The heated milk, in turn, being in contact with the sides of the milk cup directly heats the ...


9

The sensation of something feeling "hot" is a function of the rate of heat transfer to the skin and the duration of the transfer. The product of the rate and duration of heat transfer is the amount of heat transferred to the skin. The rate of heat transfer of the aluminum foil, being a metal, is relatively high because of its high thermal ...


8

The heat equation, as you've written it, models the flow of energy via thermal conduction (heat) through some region with well defined boundary conditions. You have yet to provide the specifics of the boundary region, so my answer will remain general and vague. The $\alpha$ is the "diffusion coefficient" which is the isotropic form (diagonal terms only) of ...


8

Hotter steam has a diffraction index closer to air than steam which is cooler. As the steam cools the droplets get larger, increasing the diffraction making it appear like there is more, when in fact there is less. Put another way, hot steam scatters light less than cool steam.


8

EDIT – Due to the interest, while my previous answer was OK, I’ve summarised official Australian ABC (public broadcaster) Fact Checks on this subject to provide full and correct detail. The three essentials on how bushfire behaves are: a) weather, b) fuel and c) topography. In Australia, the Bureau of Meteorology produces ‘fire danger ratings’ in ...


6

As discussed here under "Incorporating lateral heat transfer" (disclaimer: my site), if you're considering a 1-D transient heat transfer problem as suggested by the variables $x$ and $t$, then the equation $$k\Delta T(x,t)-h(x)T(x,t)=c\rho\dot T(x,t)$$ represents axial conduction with thermal conductivity $k$, linear lateral heat dissipation (through ...


6

In your problem statement, you assert that the two ends of the bar are both maintained at their respective temperatures of 100C and 0C. This means it is not possible for the cold end to heat up to 100C.


6

What they have done is focus exclusively on the long-time solution when the system has reached "oscillatory steady state." This solution does not feature any exponentially decaying terms in time. So their solution for the temperature is taken to be of the form: $$T(x,t)-T_0=\alpha(x)\cos(\omega t-\phi)+\beta(x)\sin(\omega t-\phi)+\frac{A}{k}(L-x)$$where ...


6

The heat transfer from 'hot' air or water to 'cold' sausages is roughly determined by Newton's law of cooling/heating: $$\boxed{\frac{\text{d}Q}{\text{d}t}=hA[T_{\infty}-T(t)]}\tag{1}$$ where, for heating: $\frac{\text{d}Q}{\text{d}t}$ is the rate of heat transfer into of the body, which determines how quickly the body's temperature rises, $h$ is the heat ...


5

equal amounts of steam and water, both at $100~^\circ\rm C$ I think it boils down (sorry) to the density and this notion of "equal amounts". Water is dense, and a stream of boiling water is going to do a lot of damage to your skin simply because there's so many more molecules hitting you per second than a vapor ordinarily would, as vapor is not very dense. ...


5

The cooling rate will increase as the flow rate is increased. The water in the pipe will heat up as it absorbs heat from the hot body, setting up a temperature distribution in the water within the pipe. Near the inlet it will have the temperature you pump it in with, and the temperature near the outlet will depend on how much heat it has absorbed from the ...


5

Let's say we have a gas enclosed in a container. Supplying heat to it will cause an increase in the (magnitude of?) vibration of gas particles in the medium. Increasing the pressure of the gas in the container (say by decreasing its volume) will increase the extent of vibration too. So what differs pressure from heat? Let's take the simple case of ...


4

I assume this is a thought experiment. Your question is more complicated than you think because of the ill-defined parameters of this situation. For one, the word 'amount' here could be taken to mean mass, volume or number of particles and each would be a different scenario. This question also raises a common myth that most people still believe which is '...


4

When you put the pot on the stove, the heat from the stove is somehow getting to the pot, which gets hot. The pot and the stove are obviously in contact with each other. Therefore conduction plays a role here. If you have an old pot, with a warped bottom, it will heat up slower, because the contact surface between pot and stove is smaller. When you hold ...


4

It would be difficult to construct a mathematical model of heat loss from a dog without some experimental input as there are so many variables (though it wouldn't surprise me to discover some enterprising thermodynamicist had done this) so the best I can do here is make some general observations. If the dog is in the shade then you are correct that provided ...


4

What you see of the "steam" is actually condensed water, i.e., more or less fine drops of liquid water. Turning off the gas means that the temperature of the steam and air column above the pot decreases; therefore, more water condenses. In reality, what you have is less and slowly rising steam—not more—so it starts to condense before it can be ...


4

Your equation is correct. To carry out the changes reversibly (in order to determine the entropy change of the system), you need to separate the two solids and subject each of them separately and reversibly to the same changes in temperature in different alternate processes. Those alternate reversible processes lead to the same formulas you stated.


3

Apply Fourier's equation of heat conduction (here in one dimension, $x$, only): $$\frac{\partial T}{\partial t}=\alpha \frac{\partial^2 T}{\partial x^2}$$ For Steady State temperatures no longer time-evolve: $$\frac{\partial T}{\partial t}=0 \Rightarrow \frac{\partial^2 T}{\partial x^2}=0$$ $$\frac{\partial^2 T}{\partial x^2}=0 \Rightarrow \frac{\mathrm{...


3

Heat is energy transfer from one thing to another due solely to there being a temperature difference between the two. Things do not "contain" heat. The earth's atmosphere does not contain heat. Temperature is a measure of average translational kinetic energy component of a substances internal energy. The three basic types of heat transfer are conduction, ...


3

There is a rate at which an object cools which depends on the temperature difference between system and surroundings. This is Newton's law of cooling. $\frac{dQ}{dt} \propto (T_{system}-T_{surroundings})$ When a process occurs so rapidly that the work done on the system is much larger than the heat transfer between the system and surroundings in that time ...


3

The local pressure could go either up or down when the wind blows, depending on the orientation of the object relative to the wind. But I suspect the total air pressure is irrelevant here, because its influence is negligible compared to the simple process of removal of air that is saturated with water vapour, and its replacement with drier air. After all, if ...


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