# Tag Info

## Hot answers tagged thermodynamics

50

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

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 ...

47

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 ...

36

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 ...

34

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 ...

32

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: $$T' = T/\gamma, \quad \gamma = ... 28 One reason you might think T should be measured in Joules is the idea that temperature is the average energy per degree of freedom in a system. However, this is only an approximation. That definition would correspond to something proportional to \frac{U}{S} rather than \frac{\partial U}{\partial S}, which is the real definition. The approximation holds ... 21 You are absolutely right about the dimensional analysis. The use of  \ln T  etc. is always a shorthand for  \ln \left(\frac{T}{T_0}\right)  which is okay to use if for some reason you don't care about  T_0 , i.e. because it cancels out or you are interested in the asymptotic behaviour only. In any expression where you have to take derivatives to get ... 21 Actually, temperature is defined as$$\frac{1}{T} = \frac{\partial S}{\partial E} = \frac{k_B}{\Omega}\frac{\partial\Omega}{\partial E}$$So in order to have zero temperature, you would need a system with either zero multiplicity, which you can't have by definition, or an infinite derivative \partial\Omega/\partial E even though the multiplicity itself ... 17 Nature has no preferences, and therefore entropy tends to increase. Sounds paradoxical? The point is that each microscopic state (describing the exact position and velocity of each atom in the system) is equally likely. However, what we typically observe is not a micro state, but a course-grained description corresponding to incredibly many micro states. ... 17 From a purely temperature point of view, not human perceived level of hotness, it is better to point the fan outward. This is because the fan motor will dissipate some heat, and when the air is blown outwards, this heat goes outside. This is all assuming the room has enough ventillation cracks and the like that the pressure inside is still effectively the ... 16 It's exactly the point of thermodynamics – and statistical physics – that one doesn't have to know the microscopic origin of similar processes if he is only interested in thermodynamic and/or statistical properties. The black body radiation arises from all conceivable interactions between the electromagnetic field and the "black body" – from the electric ... 15 The two "no" answers you've already received are correct for all practical purposes. In real-world cases there can be a difference though. The difference depends on when the refrigerator decides to cycle on and cool. If the fridge cycles on a timer or based on heat energy then there will be a difference due to the added heat capacity. The outside of the ... 15 Lets take each case and make some calculation. So, the first case, waiting for 5 minutes than adding some cold water. Assume the following values: The initial "hot" temperature of the coffee T_H=80^{\circ}C The temperature of the surrounding medium T_m=23^{\circ}C Using Newtons cooling law$$\frac{dT}{dt}=-k(T-T_m)$$and after a simple integration ... 13 This is a fantastic question, and a topic I was very confused about when I first took a class on Radiative Processes. The ultimate answer, as hinted at by @LubošMotl, is anything---if you start with a 'white-noise' of radiation (i.e. equal amounts of every frequency), it will equilibrate with the medium/material into a black-body distribution because of its ... 13 A convenient operational definition of temperature is that it is a measure of the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules. The underlying framework of all matter is quantum mechanical. This means that the Heisenberg Uncertainty principle holds. Even for a single particle the HUP ... 12 It's because the Kelvin scale was and still is defined so that as a measure of temperature difference, one kelvin exactly coincides with one Celsius degree. So the temperature in kelvins was defined as the temperature in Celsius degrees minus A where A=273.15 °C is the temperature of the absolute zero, without any additional multiplicative factor. When ... 12 "Total energy of the Earth" is somewhat of an odd concept, but there's no reason we can't really entertain it. It brings up some genuinely difficult questions. The right way to approach this is to define the system correctly and then identify forms of energy content and flows. Things to "count" in the Earth's energy: Heat content Nuclear energy ... 11 A fundamental principle of thermodynamics is that heat flows from warm places to cold ones, through either convection, conduction or radiation, and it will continue to do so until the temperature equalizes across the system. The stones are colder than the whiskey when you put them in the glass, so as the system heads towards equilibrium, the whiskey gets ... 10 Oh, but the edge of the atmopheres of Jupiter and Saturn (and the others) are fuzzy! Look at these Cassini images from a few years ago, at the CICLOPS website: "Adrift at Saturn" (PIA 07667), "Beyond the Limb" (PIA 10426), "Off Saturn's Shoulder" (PIA 09791) There is so much gas, such strong gravity, that it gets thicker and thicker quite rapidly as you ... 10 The heat loss (power) at a particular temperature is the same. So, No - the cooling needed to maintain the thing cold stays roughly the same. However, the empty fridge has lower total heat capacity. So, it will get warm faster in the absence of power. So, it is worthwhile to fill your fridge and freezer with bottle of water a few days before a big storm ... 10 TL;DR: Whiskey stones work by absorbing heat from the whiskey in an attempt to reach thermal equilibrium1. As Thomas mentioned, ice has three cooling effects: Ice itself takes 2.11 kilojoules of heat per g to have its temperature increased by 1 degree (Celsius). This number is known as "specific heat capacity" Ice takes 334 kilojoules of heat per kg at 0 ... 10 Yes, this is a nice way of thinking about energy. Consider the Christmas toy shown below. The candles create hot air, which rises through the fan blades and spins the angels around. This is a simple heat engine. It only works because there's a temperature difference between the hot air below and the cold air above. For example, you could put the engine in a ... 10 One thing to note is observing something's temperature and thermodynamic notions of temperature aren't exactly the same thing. This is in line with @Mattia 's answer. If a star is receding form you then it will appear cooler because its radiation has been red-shifted. Does this mean that there can be a net flow of heat from us to the star (provided it's ... 10 If the metal pan was cool then you would expect to see water droplets staying in the same place once any original movement had dissipated. You would have a combination of cohesive forces within each water droplet and adhesive forces between the water and metal surfaces. With the metal having a temperature well above the boiling point of water, the water ... 9 No. The rate of cooling must simply match the rate of heating, and heating rate depends only on the temperature difference you want to establish and on the thermal conductivity and surface area of the walls. More stuff in the refrigerator would give it a higher heat capacity, so that it wouldn't warm up so much when the door is opened. However, it will take ... 8 I'm going to suppress the sum over different chemical potentials for simplicity as it doesn't materially affect the argument. If we write U as a function of S,V,N, then extensivity of U is mathematically defined as follows$$ U(\lambda S, \lambda V, \lambda N) = \lambda U(S, V, N)  Physically, this is saying that if you scale the quanities that ...

8

It seems we have reached the point where simple models are no longer satisfying. Rather than posing ad hoc DEs maybe it's time to try an actual physical model. Short of doing a full hydrodynamic simulation (definitely overkill here) we can try what is called a lumped capacitance model where we divide the system up into a number of "lumps" and energy flows ...

8

Temperature cannot be measured in units reserved for energy because, for instance, a grain of sand heated to the temperature as the Sun does not contain the same amount of energy as the Sun. Temperature is the property that, when two bodies in thermal contact have the same value of it, no net heat flows from one body to the other: they are in thermal ...

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