# Tag Info

### Why is it so much harder to keep the same room cool than to keep it warm?

I am asking why the cool room warms up faster (loses the cold after you turn off the AC) then the warm room loosing cool down (lose the heat after you turn off the heater). That's not a universal ...
• 33.6k

### What does $C^{XY}_{\ell}$ mean when we weasure $a_{\ell m}$ in the sky?

It seems to be a mix between the 2 observales but are the 𝑎𝑋ℓ𝑚 and (𝑎𝑌ℓ𝑚)∗ are measured like in the first case in eq(1) with eq(2) ? Yes, for example say $X$ was temperature, then $a_{\ell m}^X$...
• 33k

### Slope of constant pressure line on $T$-$S$ plot

$$U=T\,\mathrm{d}S-P\,\mathrm{d}V+\mu\,\mathrm{d}N$$ We want $\frac{\partial T}{\partial S}$. We know $T=\frac{\partial U}{\partial S}$, so $\frac{\partial T}{\partial S}$ is the second derivative of ...
• 795
1 vote
Accepted

### Equilibrium temperature and phase changes

The simplest way to handle this, or even more complex combinations, is to proceed in two steps: Pick a temperature and fraction solid/liquid for the final composition. This choice will almost ...
• 10.1k

### The entropy given by stefan Boltzmann's law looks remarkably similar to the volume of the sphere; $S(T)=\frac{4}{3}\sigma T^3$

It's a coincidence, as the lack of $\pi$ indicates. The entropy per surface of a blackbody in $D$-dimensional space is $\frac{D+1}{D}\sigma T^D$. (You can deduce it e.g. by generalizing this.) By ...
• 21.7k

I don't know if it could be of use for your problem, but the ideal gas with a homogeneous gravity field is well known classical problem of statistical physics. It induces a non-homogeneous ...
• 515

### What exactly is the specific heat of a gas?

specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one Celsius degree and there is a pressure dependence. Please see Does specific heat change with ...

### How long can you survive 1 million degrees?

An environment of 1 million Kelvin would irradiate the human body (which is about $2\,\, m^2$) with around $10^{17}$ Watts per the black body law: $$Q=\sigma_{SB}AT^4$$ If we estimate the human body ...
• 2,337
1 vote

### Can low temperature plasma exist?

If the density is extremely small, the recombination rate of free electrons with ionized atoms (or just protons) can exactly balance the thermal ionization rate even at reasonably low temperatures. It ...
• 3,221

### How can the universe be hot or dense in the first moments after the big bang when it has no matter?

Fundamentally, "matter" is manifestation of energy excitations of a universal, all-encompasing substrate (the vacuum or "aether" or spacetime - properly understood, these all refer ...
• 4,283
Accepted

### How can the universe be hot or dense in the first moments after the big bang when it has no matter?

In Physics, hot is an adjective meaning at high temperature. Heat is a different concept. In the case of ordinary matter, the temperature can be associated with atomic speeds. However, it is possible ...
• 23.2k