Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Maybe combine with [tag:statistical-mechanics].

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273 views

Why increase pressure to go from liquid to steam?

Assume the following phase diagram $T-H$ of water At a nuclear engineering course, I was told that in order to increase the performance of a pressurized water reactor, one has to increase the ...
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1answer
282 views

Adding heat to a system doesn't equal the work done by gas?

So, I answered a physics question for a class that goes as follows: A gas in a cylinder is kept at a constant pressure of $250000\: \mathrm{Pa}$ while $300\: \mathrm{kJ}$ of heat are added to it, ...
0
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1answer
528 views

How long does the 2nd pot of water take to boil right after the 1st one finishes?

Say I have a pot of water that boils in 20 minutes, at whatever temperature. If I leave the fire on, take the pot off, pour the hot water into a container, refill the pot with tap water and put it ...
6
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1answer
3k views

Physical interpretation of different boundary conditions for heat equation

When solving the heat equation, $$ \partial_t u -\Delta u = f \text{ on } \Omega $$ what physical situations are represented by the following boundary conditions (on $\partial \Omega$)? $u=g$ ...
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0answers
289 views

Constant Pressure lines in evaporation diagram

Assume that there is the following $T-H$ phase diagram with the constant pressure lines. Which is greater preasure? The preasure drops as we move to lower temperatures (i.e. from top to bottom)? ...
6
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2answers
721 views

Why isn't the Earth's core temperature the average of its surface temperatures?

Assuming that the earth is spherical, that its temperature is continuous, and that some other more or less realistic conditions hold, we might think that the Earth's core temperature should be about ...
2
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1answer
99 views

Finding cooling time using an impulse of heat energy

A chip has a thermal resistance of 0.007 K/J s and thermal capacitance of 1.6 J/K. You give it an impulse of heat equal to 100 J. How long does it take for the chip to cool to within 10% of the ...
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0answers
89 views

Calculate heat loss of aquifer with unknown shape

I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for ...
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1answer
448 views

Triple point temperature and freezing point

Why does the triple point temperature have very similar values to the freezing point, in most substances?
0
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2answers
129 views

Help me check units [closed]

I need to check units in this equation, I know, that there's Pa in the numerator, but what about the denominator? $p_1$ - pressure in [Pa] $T_1$, $T_2$ - temperature in [K] $\rho_w$ - density $g$ - ...
3
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2answers
5k views

Why is a degree Celsius exactly the same as a Kelvin?

How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
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4answers
1k views

Can a single molecule have a temperature?

A show on the weather channel said that as a water molecule ascends in the atmosphere it cools. Does it make sense to talk about the temperature of a single molecule?
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0answers
157 views

How long would it take for a container in vacuum to leak half of its air? [closed]

Let's say I know the size of the container, size of the hole the air leaks through, pressure the air is under and temperature of the air if that helps anything. Is it possible to calculate this only ...
6
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4answers
340 views

Physical interpretation of changing variables $S(T,V)$ to $S(T,p)$

I'm confused in this situation. In an example of this book (Example 16.5. Thermal Physics, by Blundell) they ask to prove that $$C_P - C_V = VT\frac{\alpha^2}{\beta_T}\tag{1}$$ but using the fact ...
4
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1answer
859 views

Microwave oven heating time

It's logical to think that the time it takes a microwave to heat the food would be proportional to the mass heated. But since a microwave is based on dielectric heating, I think that if you increase ...
2
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1answer
2k views

Sackur-Tetrode equation - clarification required - problem with units

I'm a 2nd year physics undergraduate and recently I've volunteered to give a short presentation on the Sackur-Tetrode equation derivation and its use at removing the Gibbs paradox. I've looked on the ...
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0answers
383 views

How does the volume change affect an adiabatic process?

A gas consisting of rigid diatomic molecules was initially under standard conditions. Then the gas was compressed adiabatically, 5 times the original volume. Find the mean KE of rotating molecules ...
0
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2answers
156 views

Dividing values with units

I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is ...
0
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1answer
413 views

A Calorimetry Problem

I have a question in calorimetry from an old competitive exam. The question is: The temperature of $100$ grams of water is to be raised from $24 ^\circ$C to $90 ^\circ$C by adding steam to it. ...
2
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2answers
496 views

How would a change in ambient temp affect a radiator?

I'm curious if you have a radiator or say a block of metal (lets say it's copper since it has the highest thermal conductivity) and on one side is a processor producing heat. At idle the processor ...
2
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1answer
638 views

Does increasing the pressure of the gas in a container by pumping gas into it increase temperature?

Basis is the ideal gas law: http://en.wikipedia.org/wiki/Ideal_gas_law This law only states a mathematical relationship between many variables. It does not state what happens if one of the variables ...
2
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3answers
831 views

Question about the proof that heat capacity goes to zero if temperature approaches $0K$

I don't completely understand the proof that is given for the claim that the heat capacity goes to zero, if the temperature approaches $0K$. They do it as follows, if $C_x$ is the heat capacity where ...
4
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3answers
5k views

Why is it faster (as in proportion to volume) to boil 4 cups of water than to boil 2 cups?

I did an experiment where I boiled two cups (500ml) of water in a kettle, and it took 1:30 minutes to reach around 98 C, average. However, when I boiled 4 cups of water, (1L) it only took me 2:30 ...
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3answers
5k views

Isothermal Gas Expansion, a Reversible or Irreversible process?

Consider the following situation. A certain quantity of ideal monatomic gas (say one mole) is confined in a cylinder by a piston and is maintained at constant temperature (say $T_0$) by thermal ...
0
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2answers
212 views

Electric power transmission

If we want to transmit electic current for a long distance, we must minimize a heat that releases because of the resistanse. We cannot make a cable wide because it is expensive and it will be massive. ...
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2answers
234 views

Why aren't the energies of two systems in thermal equilibrium fixed?

In the derivation of the Boltzmann distribution they consider a system $A$, enclosed by a diathermal wall in a heat reservoir $R$. Then they calculate the probability that the system $A$ is in an ...
2
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1answer
62 views

Heat deposition at optical focus in a clear medium

This is an applied physics question for an engineering problem. The well known experiment we have all done--burn paper and wood with a magnifying glass and the sun's rays. My question is, if one were ...
4
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0answers
163 views

Lattice model completely constrained by boundary data

I am dealing with a lattice model that has the peculiar property that if I specify all the spins on the boundary, by local conservation laws, the whole lattice configuration (throughout the whole ...
0
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1answer
294 views

Does brown but transparent swimming pool water heat significantly faster than western style highly chlorinated pools?

Eastern European swimming pools are often brown tinted water. i was told it was the color of the chemical to keep the pools clean, but who knows. These pools did not smell unsanitary and may have even ...
6
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3answers
16k views

How does an earthen pot keep water cool?

I understand that evaporative cooling takes place thanks to small pores contained in the pot and that allow some water to go through and evaporate. However I couldn't understand clearly whether water ...
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3answers
4k views

Air pressure in balloon

I have to calculate the air pressure inside of an hot air balloon. After some searching I found out that I can use the ideal gas law: PV = nRT (from Wikipedia) So to get the pressure in the balloon I ...
7
votes
1answer
943 views

Is it 11% hotter today than it was yesterday?

Yesterday the temperature outside was 0.5 °C. Today, the temperature is 30 °C. 30 is 5300% more than 0.5, but today is obviously not 5300% hotter than yesterday. In Fahrenheit, the temperatures are ...
7
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4answers
676 views

Definition of the entropy

In physics, the word entropy has important physical implications as the amount of "disorder" of a system. In mathematics, a more abstract definition is used. The (Shannon) entropy of a variable $X$ is ...
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0answers
95 views

Can one get clear ice crystals from a dirty suspension?

Euteictic freeze crystallization is a method where an electrolytic solution is cooled and separated into a stream of (relativly) clean, pure ice and a salty brine. I know anectdotally of wine ...
9
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2answers
806 views

Mechanical Equivalent of Heat

Recently I have been looking up James Joule's experiment regarding the mechanical equivalent of heat. After viewing some drawings of the apparatus, I assumed that the lines holding the weights would ...
2
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1answer
204 views

Thermodynamics, PV diagrams?

My teacher told me that the total amount of work done on or by a gas can be represented by the area enclosed in the process in a PV diagram. This is only valid for non isothermic processes, right?
5
votes
1answer
319 views

First law of thermodynamics?

The first law says that the change in internal energy is equal to the work done on the system (W) minus the work done by the system (Q). However, can $Q$ be any kind of work, such as mechanical work? ...
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4answers
671 views

The Preference for Low Energy States

The idea that systems will achieve the lowest energy state they can because they are more "stable" is clear enough. My question is, what causes this tendency? I've researched the question and been ...
2
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2answers
55 views

Gas Circulation Using Pressure Difference

Dear all, see attached picture Please, is it possible to have the gas recirculated from the gas phase to the liquid as described in the diagram assuming the gas is not soluble in the water. These ...
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0answers
424 views

Explain to me how this water cooler works?

Goodday all, I was recently reading up on a few projects that might be of interest to me when I found "CPU Bong water coolers", there isn't much online on them so I figure I would ask y'all. If you ...
6
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1answer
200 views

Reflectivity of a glowing-hot metal surface

When a polished piece of metal (or steel in particular) is heated to incandescence, how do its reflective properties change? Given a mirror-like surface, would the object temporarily cease to act ...
2
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4answers
8k views

What is the effect of an increase in pressure on latent heat of vaporization?

What is latent heat of vaporization ($L_v$) in the first place? Wikipedia seems to indicate that it is the energy used in overcoming intermolecular interactions, without taking into account at all any ...
3
votes
1answer
488 views

Error in Sear's and Zemansky's University Physics with Modern Physics 13th Edition (Young and Freeman)?

I was reading up on the Ideal Gas Equation in University Physics with Modern Physics by Young and Freeman when I chanced upon a seemingly illogical mathematical equation. Can anyone rectify this ...
0
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4answers
8k views

How do you determine the heat transfer from a P-V diagram?

I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me. I am currently studying the First Law of Thermodynamics, which includes the p-V ...
0
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2answers
12k views

Does more reflective aluminum foil make a room cooler compared to less reflective foil?

Aluminum foil is said to be not absorbing light at all. It reflects light. So, does it mean that a more shiny aluminum foil will reflect more light and thus make the room more cooler as compared to ...
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1answer
497 views

What is tricritical point?

Critical point is the transition temperature of a second order phase transition. But what does tricritical point mean? WIki says that a tricritical point is a point in the phase diagram of a system at ...
8
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4answers
2k views

How can I understand a Vortex Tube and its efficiency?

A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon. Both ...
5
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1answer
4k views

Why does compressing a piston increase the internal energy?

When we compress a piston, its total internal energy increases, however I don't understand why. As the piston compresses, the temperature should change, as the total energy density increases. As a ...
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1answer
412 views

Michaelis-Menten derivation for 2 enzyme substrates

We know that the Michaelis-Menten derivation for the following reaction: $E + S \rightleftharpoons ES \rightarrow E + P$ However, what if the reaction took place in a different scenario whereby: $E ...
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3answers
547 views

Mathematical proof of non-negative change of entropy $\Delta S\geq0$

I understand that we can prove that for any process that occurs in an isolated and closed system it must hold that $$\Delta S\geq0$$ via Clausius' theorem. My question is, how can I prove this in a ...