# Tag Info

21

Typically: $\rm d$ denotes the total derivative (sometimes called the exact differential):$$\frac{{\rm d}}{{\rm d}t}f(x,t)=\frac{\partial f}{\partial t}+\frac{\partial f}{\partial x}\frac{{\rm d}x}{{\rm d}t}$$This is also sometimes denoted via $$\frac{Df}{Dt},\,D_tf$$ $\partial$ represents the partial derivative (derivative of $f(x,y)$ with respect to $x$ ...

15

The reason is the same as why a metal pipe feels colder than wooden plank at the same temperature: thermal conduction. The heat from your tongue (including the moisture) is absorbed faster than your body can replenish it. This has the effect of freezing your saliva in the tongue's pores to the metal surface (which itself isn't too smooth at small scales). ...

10

The low-entropy initial state of the universe is an open problem without a satisfactory answer. Your question is the first time I've heard the suggestion that the initial state should have been a crystal; you remind me that the quark-gluon plasma, which was the state of the universe while it was too hot for nucleons to be stable, has been shown to be a ...

8

You might get an order of magnitude estimate as follows. We make the rough assumption that everything ends up in its vessel as a monoatomic ideal gas - actually it will be a plasma, with a thermal energy per mole of $\frac{3}{2}\,R\,T_{final}$, where $T_{final}$ is the thermodynamic temperature of the plasma. Neglecting heats of vaporisation (we assume ...

3

First, I want to say that different people use different notation and I welcome any comments. I also feel as if I am about to enter a minefield. Here the answer is made up with examples of use of $d$, $\partial$ and $\delta$. I would say for $d$ that $dV \over dx$ would be the total derivative in one dimension for $V(x)$ where the potential $V$ is a ...

3

The answer depends on many factors, but here are the basic bits of physics that play: The power and wavelength of the laser The reflectivity of the surface (function of wavelength of the laser) The size of the focal spot The thickness of the sheet The thermal conductivity of the sheet The reflectivity of the copper is a particularly important one. If you ...

3

Joule's law, and thermodynamics in general, is a model of the classical world. Here, classical should be interpreted as non-quantum-mechanical. Thermodynamics is the study of large collections of particles and their collective behavior. No microscopic model is assumed, and one tries to extract as many (non-trivial) features as possible based on purely ...

2

The formula is actually better written $$\Delta S = \frac{Q}{T}.$$ That is, the change in entropy associated with the flow of heat is inversely proportional to the temperature at which the heat flow occurs. Note that $Q$ is already a change itself: it is not a state variable, but rather something more like $\Delta W$. Physically, this is because adding ...

2

That's a very hard question to answer with the appropriate level of detail! Very broadly speaking in an ideal metal all atoms are forming a perfectly regular crystal lattice. Conduction band electrons can move freely around these atoms, which makes it easy to pass a current trough the metal. In a (theoretical) metal with perfect crystal lattice the ...

1

In the early Universe, entropy is preserved (dS=0). This comes out of the equations of general relativity, but it can be also understood by thinking in terms of classical dynamics: the Universe is a closed system, no heat is exchanged when expanding, so its entropy must not variate.

1

Ovens in the kitchen are not suppose to heat above 450°F. Any type of glass can handle that temperature, if the change in temperature is not too fast. For safety, I would think that tempered glass is used on the ovens.

1

I think you have two questions here, namely What is heat? Is it a fundamental property of stuff? I'll answer both at once. In modern physics we view heat as an average property of a system containing many particles. Roughly speaking, the heat of box is a measure of microscopic energy transfer between the box and its surroundings. More precisely, we use ...

1

You have come to the right place, and your question will be answered. But, be warned, you may not like the answer. You can not talk about the heat of anything. No object, system, or molecule can be said to contain this much heat, at all. A physicist states this as the following: Heat is not a state variable. What you can talk about is called internal ...

1

The answer lies in the fact that, in graphene, there is an effective long range interaction mediated by the inverse biharmonic operator (which in 2D goes as $x^2\ln(x)$ and is extremely long-ranged) coupling the gaussian curvature at any two points on the sheet. Due to this, any static ripples or thermally produced dynamic ripples interact at arbitrary ...

1

It's a sum over states. The partition function can also be written as \begin{align} Z = \sum_\alpha g_\alpha e^{-\epsilon_\alpha/kT} \end{align} where $\alpha$ is an index which labels levels, $\epsilon_\alpha$ is the energy of level $\alpha$, and $g_\alpha$ is the degeneracy of level $\alpha$. This follows from the sum over states by noting that in that ...

1

When opening the bottle in space, all the air that was initially in it will flow out due to the pressure difference. The inside of the bottle will then become approximatelly vacuum, so when you open it on Earth air will flow in it again. (Unless it's not sturdy enough (for example a plastic bottle), in which case it will be compressed/crumpelt before you ...

1

A formula such as this (being a analog to parallel electrical resistors, as pointed out in the comments) can get a little more complicated when different areas are involved. However, I think that the author of your formula restricted the discussion to one single heat transfer area in order to avoid dealing with that. The heat transfer rate originally starts ...

1

Since $n=\left(n_1,\,n_2,\ldots\right)$ with each $n_i\in\{0,\,1,\,2,\,\ldots\}$, then you can write the sum as $$\sum_{n_\alpha}=\sum_{n_1\in\{0,\,1,\ldots\}}\sum_{n_2\in\{0,\,1,\ldots\}}\cdots\sum_{n_m\in\{0,\,1,\ldots\}}$$ while your product is, $$\prod_\alpha e^{n_\alpha \beta}=e^{n_1\beta}e^{n_2\beta}\cdots e^{n_m\beta}$$ Thus, we have $$... 1 The specific heat of a molecule depends on the number of degrees of freedom the molecule has. There are several degrees of freedom available: translation (3), rotation (3), vibration (depends on the number of bonds in a molecule) and electronic modes. Now, for something that is monatomic, you have 3 translational modes (x,y,z directions), zero rotation ... 1 Well it all depends on what assumptions you are able and willing to make. For a start you can work out how much of the laser light is reflected. The reflected power will be something like (for normal incidence)$$\frac{P_{r}}{P_i} = \frac{(\eta_m - \eta_{vac})^{2}}{(\eta_{vac} + \eta_m)^2}, where $\eta_{vac}=377$ Ohms and $\eta_m$ is the impedance of the ...

Only top voted, non community-wiki answers of a minimum length are eligible