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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Why does the bathroom become hot after a bath?

I have noticed many times that whenever I enter into the bathroom just after someone else bathed the temperature inside it would be high. Is it that the body heat comes out when we pour a lot of ...
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1answer
84 views

How does a MRI use 20.1kW yet creates over 204K BTU/h? [closed]

I've been reading through a manual about MRI operating procedures for a large healthcare provider. The manual (written by Siemens) states that the MRI machines use 9kW in stand-by mode and 20.1kW ...
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1answer
87 views

Why does the blade height goes on decreasing in a multi-stage axial flow compressor? [closed]

The axial compressors used in jet engines have their blade heights in decreasing order in the direction of the flow.
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504 views

Which one conducts heat better: water, or air?

If we were to assume room temperature of 20C, which of the fluids conducts heat better, and why?
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172 views

What cools off faster a square or a disc?

What cools off faster and why, a square (4 right angles, all sides same length)with an area of 100 inches or a disc with an area of 100 inches. The same material and thickness.
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2answers
184 views

Why does the coefficient of performance need to be calculated with $W + Q$?

How much work must a heat pump with a COP of 2.50 do in order to extract 1.00 MJ of thermal energy from the outdoors (the cold reservoir)? The first formula that came into my mind after reading ...
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201 views

Question about heat engine efficiency

I would like to know how much thermal energy is converted to kinetic energy in a steam engine, or a more efficient means if available. I have done some research and found out that Carnot made some ...
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1answer
65 views

What thermal conductivity coefficient should I be using when modeling a heatsink?

Using the finite difference model for heat transfer, I am trying to model the heat flow into a heatsink. However, I am unsure of what k value or coefficient of thermal conductivity to use. Should I be ...
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1answer
125 views

What are the experimental observations behind the first principle of thermodynamics?

As far as I understand it, the first principle of thermodynamics is a mere definition of the quantity “Heat”: $$\text d Q: = \text d L + \text d U.$$ This is somewhat the point of view taken in ...
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2answers
145 views

How do you keep a fridge full of food cooler?

I just graduated and a mate moved into a flat (none of us are physicists by the way). So, were graduates, we got a new flat, and were broke. So, were now having a debate on how to keep a fridge ...
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1answer
87 views

How long will it take to temperatures of two solids to become equal?

If I have two solids with commmon area where they are touching each other, is there any way for calculating the time it will take to them to become equal? For example, I have two blocks of different ...
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139 views

Confusion in fourier's law

My book starts section 2.2 like this: Equation 1.1 is this: $$q_x=-k\frac {\partial T}{\partial x}$$ As you can see in the picture I posted, my book says that one can go from equation 1.1 to ...
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578 views

Dielectric with polar molecules

Suppose a dielectric slab contains polar molecules (which are not further polarisable). When placed in an electric field, (for simplicity, an uniform field), align themselves according to the field. ...
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391 views

Understanding Charles's Law

Charles's law states that, The volume of a given mass of an ideal gas is directly proportional to its temperature on the absolute temperature scale (in Kelvin) if pressure and the amount of gas ...
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1answer
200 views

Deriving work done on a solid

For a homework problem, I've been asked to show that the work done by increasing pressure isothermally on a solid of mass m is: $$W \approx -\frac{m\bar{\kappa}}{2\bar{\rho}}(P_f^2 - P_i^2)$$ where ...
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243 views

A really simple question about heat transfer? [closed]

In rectangular coordinates, the heat conduction vector can be expressed in terms of its components $$\vec{\dot{Q_n}}=\dot{Q_x}\vec{i}+\dot{Q_y}\vec{j}+\dot{Q_z}\vec{k}$$ Please explain the meaning of ...
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1answer
116 views

Second law of thermodynamics

I think this is a simple question. If I have that $E(L)=\tau L$ and we are told that $\tau=BTL$ would this mean that $E=BTL^2$ implies $dE=(2BTL)dL$ or should I sub $\tau$ straight into the second law ...
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3k 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 ...
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2answers
6k 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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44 views

Gas transformation for different molar number

When a gas at normal conditions (1atm, 273K, 22.4L) we say like the $$\frac{PV}{T} = 0.0820...$$ And by the know equation: $$PV = nRT$$ Where R is equals to $0.08205...$ and $n$ is the number of mols ...
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348 views

Does electrically heated water have an adverse effect on hair?

I know I should have asked this question on a different site. but this was the most suitable site available right now for my question. Perhaps after this proposed site goes on Beta, we can move it ...
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2answers
383 views

phase transition by sublimation

Why do sublime solids sublimate in the first place? Is it that their melting point and their liquid's boiling point are the same so that they manage to magically skip the liquid phase? (if so, why ...
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2answers
546 views

Computer cooling with dry ice, ideas and question; thermodynamics

I am designing a cooling system for my computer and had a few questions. So I have a computer water cooling radiator and I want to cool it as much as I can. My first two ideas were an old window A/C ...
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1answer
112 views

Can Joule's First Law of Thermodynamics be Applied to Atomic Charges?

James Joule established that all forms of energy were basically the same and interchangeable. My question is if thas law is relevant in particle physics. Can a positive charge and a negative charge be ...
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1answer
105 views

Is there a relation between supersymmetry and entropy?

Considering that entropy denotes the level of order/disorder in a system, would it be possible for entropy and supersymmetry to exist at the same time? Or, are they entirely unrelated?
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60 views

How hot would a disc have to be to create strong drafts that lift it upward?

I read this article on xkcd.xom, from its spin-off blog "What If?" and that answer made me wonder. It was mentioned in that article that a very hot but indestructible box would eventually be able to ...
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1answer
534 views

Best way to move teaspoon to dissolve sugar in a cup of tea

I know that there is asked questions about something like cooling down cope of coffee..etc, but this one is little different. My friend was watching TV program of English eating etiquette, and they ...
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154 views

What is evidence for an irreversible change?

Knowing some about thermodynamics and reactions, I do understand how it can be shown that a change is reversible. But irreversible? Why can't it be that a change that was deemed irreversible thousands ...
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1answer
155 views

If human energy use were to increase by 2.3% per year, would we use all solar energy by 1400 years?

Assuming 2.3% per year exponential growth of human energy consumption (so, roughly speaking, that corresponds to multiplying consumption tenfold every century), it's argued that human annual energy ...
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370 views

Temperature and density

As temperature rise the density become lower,When temperature goes down, density is higher but in higher temperature the body become bigger so why density become lower?
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240 views

Work done by introducing a spin in supersposition into a Magnetic Field

A spin is created in a superposition of up and down states. A magnet is moved very slowly, towards the spin. What is the work done by the magnet. It may be helpful to imagine that the magnet is ...
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1answer
7k views

How much work is needed to compress a certain volume of gas?

I want to know the formula (and what does the symbols stand for) for how much work is needed to compress a certain volume of gas?
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1answer
184 views

Does the molten salt used at projects such as Gemasolar need to be replaced after some cycles?

The project at Gemasolar uses molten salt to store heat. This heat is then used with water to drive steam turbines. Over time does this kind of repeated heat/cold cycle make the molten salt a less ...
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1answer
419 views

How does the entropy of a system change when the amount of substance changes?

I am thinking of a closed system consisting only of an ideal gas. Suddenly, the amount of substance of the gas changes. No other property of the system shall change. How does this affect the entropy ...
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1answer
171 views

Is a low-current electrical arc harmful to humans?

I've heard that electrical flux non-destructive particle testing machines are considered safe because they use less than 2 amps. I have seen an arc created between two objects do considerable damage, ...
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1answer
86 views

System moves away from equilibrium $\rightarrow$ it has energy added?

Suppose there is an isolated system $A$ at time $(-\infty, t_1)$, whose entropy is $S=S_{max}$, i.e. it is at thermodynamical equilibrium. Between moments $[t_1, t_2)$ the isolation is violated and ...
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1answer
372 views

Energy versus free-energy diagram

Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please? From Borrell ...
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1answer
677 views

Temperature change effected by electric heater [closed]

A 40-gallon electric water heater has a 10kW heating element. What will the water temperature be after 15 min of heating if the start temp is 50F degrees. There must be an equation. I can't find it ...
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1answer
868 views

Calculate time to heat my swimming pool

I have a swimming pool of 5300Liters. I have a heating element from a washing machine taking 1min17s to heat a bucket (10L) of water from 22 degrees to 30 degrees celcius. the water in my pool is 19 ...
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1answer
602 views

Heat & thermodynamics question based on heat loss [closed]

A Sphere A is placed on a smooth table.Another sphere B is suspended as shown in the figure.Both the spheres are identical in all respects.Equal quantity of heat is supplied to both spheres.All ...
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1answer
63 views

If I take a bottle of air into space, and open it, then close it and come back to earth and open it one last time, what would happen?

I was reading this post, but no one seem to go further as say what would happen if you brought the bottle back to Earth? Guess what another thing I'm wanting got know is, what would now be in the ...
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1answer
22 views

find total change in temperature of a person being struck by lighning [closed]

A lightning flash releases about 1010J of electrical energy. If all this energy is added to 50 kg of water (the amount of water in a 165-lb person) at 37∘C, what are the final state and temperature ...
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1answer
44 views

What is the density operator for an isothermal–isobaric ensemble (T,p,N)?

In the microcanonical ensemble $(E,V,N)$, the density operator is $$\hat{\rho}=\frac{\delta(\hat{H}-E\,\hat{I})}{Tr(\delta(\hat{H}-E\,\hat{I}))}$$ Where $\hat{H}$ is the Hamiltonian of the system and ...
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1answer
56 views

A simple experiment and the Maxwell-Boltzmann distribution

Consider two containers separated by a removable wall, each side of which is a perfect mirror for the gas in the respective container. Also the walls of the containers are ideal mirrors. In each ...
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1answer
44 views

If entropy of a closed system decreases by a certain amount, why does not the entropy of the surroundings increase by the same amount? [duplicate]

As said by the Second law of thermodynamics, Energy spontaneously disperses from being localized to being spread out if not hindered from doing so. Now, for systems other than isolated one, and ...
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1answer
37 views

Can diffusion,osmosis be explained by second law of thermodynamics?

Second law of thermodynamics states: Energy tends to disperse from localized to more spread out form, if not hindered from doing so. So, can the two processes diffusion,osmosis be explained by ...
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1answer
90 views

Applying Euler Homogenous Function Theorem to $df$ or $f$

I am currently reading Demirel's Nonequilibrium Thermodynamics in order to learn more about entropy and nonequlibrium thermodynamics However I and stumped on how the author's do the following step: ...
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1answer
29 views

Entropy maximum postulate and reversibility

I'm studying Thermodynamics on Callen's book and he introduces entropy through the entropy maximum postulate, namely: There exists a function (called the entropy $S$) of the extensive parameters ...
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1answer
86 views

Do physicists use agent based models?

I am hoping that this is a simple and specific question. I just wanted to know whether physicists from any branch of physics use agent based models as a tool in their research? If so, then in which ...
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2answers
31 views

translational energy levels

My textbook says translational energy level is so closed, and it basically covers the whole EMR spectrum, so any temperature beyond absolute zero is high enough for molecules to have translational ...