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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22 views

A 50 g Ice Cube with an initial temperature of -10°C is put in 400 g of water at 40°C. Find final temperature

My calculations: $$Q_g + Q_l = 0$$ $$Q_g = -Q_l$$ $$mc\Delta T = -mc\Delta T$$ $$(0.05)(2100)(T_f+10) = -(0.4)(4200)(T_f-40)$$ $$105(T_f+10) = -1680(T_f-40)$$ $$105T_f + 1050 = -1680T_f + 67,200$$ ...
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1answer
19 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
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1answer
17 views

relativistic temperatures energy assumptions

why is the assumption $k_bT>>mc^2$ made for relativistic temperatures, where did the kT come from and how is it related to mc^2?
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2answers
41 views

how can energy conservation equation be different in different situations?

Neglecting potential and kinetic energy: energy conservation equation in a closed system is dU=Q-W and in an open system with steady flow is dU+d(PressureEnergyPV)=Q-W or in other words dH=Q-W. How ...
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0answers
12 views

U and PV changes in thermofluid processes

If we have a steady flow open system, how can we know the change in U and PV? I mean if we input work in the system, how can we know that the work goes to U increase or PV increase? Also, can you give ...
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0answers
19 views

Relation between Canonical and Grand Canonical Ensemble for a two levels system

For a two-level system of classical non interacting distinguishable paricles, labelling energy levels as $+\epsilon, -\epsilon$, in the canonical ensemble I found for the partition function ...
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2answers
30 views

Thermodynamics : Heat transfer : Conduction or convection?

I have an hot object, that is place in a room of 20°C. My goal is to be able to estimate how warm the object will be in T seconds. To be able to do that, I must first understand what is really ...
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1answer
49 views

I can get a heating strip - why can't I get a cooling strip?

A typical heating strip is a flexible silicone material containing a resistive heater. However, I would like the opposite - something I can wrap around my experiment and use in a similar way to the ...
0
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1answer
27 views

First Law of Thermodynamics and Signs

I have a question in my text book about the first Law of Thermodynamics. It gives 3 situations with the systems for each in brackets: 1) Rapidly pumping up a bicycle tyre (air in the pump) 2) ...
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0answers
19 views

Room with open door and heater : use inside boundary conditions or heat source for PDE?

I want to model the stationary temperature in a room (=square) that has one open door (=side) is otherwise perfectly insulated is heated through a (circular) heater that is placed somewhere in the ...
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0answers
19 views

What are the calculations specifically needed for High Altitude Ballooning?

I want to know the lifting gas needed to launch the vehicle including the positive lift I want to predict the possible retrieval site I want to know the burst altitude AND MAYBE ALL WITHOUT ...
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0answers
17 views

Ratio of the change of internal energy under the specified condition given below [on hold]

What will be the ratio of the change of internal energy of a diatomic gas U1 & U2 for a unit rise of temperature in constant pressure and volume respectively.
6
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3answers
53 views

Degrees of freedom and temperature

I quote the following lines directly from the Wikipedia page titled "Heat capacity": "...rotational kinetic energy of gas molecules stores heat energy in a way that increases heat capacity, since ...
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3answers
135 views

Why engines don't melt?

The temperatures of internal combustion engines are well above the melting points of the component materials. For example, temperatures can reach 2200° C whereas the melting point of aluminium is ...
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3answers
51 views

How does frost form above freezing temperature?

This is a follow-up to the best answer linked below: Why does ice form on bridges even if the temperature is above freezing? I've witnessed that phenomenon, recently. The temperature was above zero ...
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1answer
37 views

Is potential energy included in internal energy?

I thought internal energy is only the energy due to molecular motion and chemical bonds, but here it says it's also the potential energy of the system! Is that true?
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1answer
30 views

heat as a low quality energy

In a cyclic process, there is no change in internal energy. So the work done by the system must equal to the heat offered to the system. So if all heat is converted to work, how can heat by a low ...
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1answer
136 views

Specific Internal Energy Decreases with Increasing Pressure?

I was looking at "Properties of Compressed Liquid Water" (Table A-5) in Fundamentals of Engineering Thermodynamics by Moran. Table A-5 shows that the specific internal energy $u$ of compressed liquid ...
2
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1answer
394 views

Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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0answers
39 views

Can the second law of thermodynamics be violated in a small enough system if tried repeatedly enough?

Second law of thermodynamics is observed in the universe because statistics favors it, right? And in large enough system this statistical tendency becomes certainty. Does it also mean that negative ...
2
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8answers
234 views

Ideal gas law: Will the piston move at all?

We have the following experimental setup: Before the experiment starts: $$p_1=p_2; \space V_1=V_2; T_1=T_2+\Delta T$$ The experiment starts and both the containers are heated so that the ...
2
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1answer
31 views

Is saying that “Heat always flow from hotter to colder object in a isolated system” equivalent to the second principle?

In other words, can we make exactly the same predictions as the second principle (that involves entropy) by this phrase without ever introducing entropy ?
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1answer
113 views

On Thermodynamics of Spontaneity of the Reduction-Oxidation Chemical Reactions

According to the first and second law for a closed system containing different chemicals we have \begin{align} &\delta Q - \delta W = dU = T dS - p dV +\sum_i \mu_i d N_i\\ ...
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0answers
61 views

Rate of Ice Formation [on hold]

A tank of water has been outdoors in cold weather, and a slab of ice 3.2 cm thick has formed on its surface (see the figure). The air above the ice is at -18°C. Calculate the rate of formation of ice ...
0
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1answer
21 views

Why use dimensionless heat capacity?

Perhaps this is blindly obvious, but in typical discussions of statistical mechanics (with, say, constant volume) one often finds that, rather than using the heat capacity $$ C_V = \frac{\partial ...
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0answers
17 views

How heated air moves [on hold]

Ok so how does air move when heated up inside a tube or cone? I am working on a wood stove and curious on how I can get air to flow faster or easier into the burn chambers to fuel the wood. Heat ...
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2answers
131 views

Reversible and Irreversible Process

I would like to ask a specific conceptual question which bothers me for quite some time! First of all i do know the difference in between reversible and irreversible processes. What is thought in ...
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2answers
46 views

Intuitive explanation of the shape of the Maxwell-Boltzmann distribution

At higher temperatures (for an ideal gas), the Maxwell-Boltzmann distribution is spread more widely and has a lower maximum. At lower temperatures, the spread is much more narrow and the peak is much ...
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1answer
22 views

Thermofluid mechanics Temperature

A steel alloy specimen (mass 4 kg) at 80 0C is submerged in water at 10 C0. At thermal equilibrium state the temperature of the steel-water system is 40 C0. Calculate the mass of the water used. The ...
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4answers
4k views

Surface Area: Volume and its relation with heat

In biology, it is stated that more surface area and less volume helps for keeping bodies cool. How can one explain this phenomena in terms of physics?
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2answers
181 views

Why doesn't the separation of particles by mass via gravity decrease entropy?

Let's say we've got a fluid of heavy and light particles inside a cubical flask, which is initially shaken up so that the density of heavy particles is uniform everywhere. Let's also say that these ...
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0answers
40 views

Reversing Entropy [on hold]

Of course this sounds ridiculous, but I think I have found Maxwell's Demon. Is anyone interested? I made a webpage explaining my thoughts. Can I post it? Thanks, Rob
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0answers
24 views

Thermal de Broglie wavelength - definition

The thermal de Broglie wavelenght is often defined by the formula $\lambda=\frac{h}{\sqrt{2\pi mkT}}$ but equally frequently is it defined as de Broglie wavelength for a free ideal gas of massive ...
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0answers
19 views

Role of heat flux on bubble size

This is sort of related to this question, in that the question was spawned from the same question on Worldbuilding. Say you have a relatively shallow open pool of water (a pot or pan). When the ...
2
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1answer
6k views

Mixing Water at Different Temperature

If I have cup of water at room temperature (say, $25^\circ$C). What would be the resultant temperature if I pour another cup of same amount of water at $100^\circ$C to it? Is it simply ...
1
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1answer
133 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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2answers
80 views

Is Moon too hot?

I find it puzzling that Moon's maximum "daily" equatorial temperature is almost 400K. Earth's theoretical black body temperature would be 279K at 1AU, and Moon is the same distance from Sun, yet its ...
1
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1answer
307 views

Gas compressed and cooled

If a certain amount of a gas is compressed into a cylinder by the action of a piston that reduces its volume (lets say at a low speed), the kynetic energy of the gas molecules increases, what is ...
5
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3answers
66 views

Do hot metals radiate? (Thermography)

I was looking into thermography which talks about emissivities of metals and other materials. Polished metals which have low emissivity appear to be colder in thermal imaging cameras even if they are ...
6
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2answers
210 views

Does the size of bubbles in water brought to a boil ever vary as a function of the amount of water?

Some discussion surrounding this answer on Worldbuilding got me curious. When water is brought towards a boil, bubbles are formed. Anyone who has ever cooked anything whatsoever knows about this. ...
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1answer
18 views

How would an increase in temperature affect ooblek's (non newtonian fluid) viscosity?

Due to the fact that Ooblek (cornstarch and water), contains so much water and from what I understand it is non newtonian due to the particles suspended in it, would it therefore be correct to say ...
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1answer
26 views

How is conversion inside heat engines of heat (random motion) to work (organized motion) explained in quantum physics?

When heat engines convert heat into work, they change the random motion of millions of particles into motion in a single direction. What is the phenomenon responsible for this alignment of ...
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0answers
22 views

Water in pot with a heat source beneath it to boil it, does the water move?

I mean if there is there heat transfer by convection in the water? since convection requires motion. if it moves, then how can i determine the velocity of the water?
1
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2answers
633 views

Why do electron and hole mobilities decrease with temperature?

From page 35 of "Microelectronics" by Millman Grabel Mobility $\mu$ decreases with temperature because more carriers are present and these carriers are more energetic at higher temperatures. ...
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0answers
27 views

Temperature versus heat for water

What does the kink(abrupt rise and fall) in region of phase change from liquid to vapour state in the following graph indicate? Does it indicate that temperature rise and fall abruptly during phase ...
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0answers
21 views

How to model a cooling system?

Suppose a system with a coolant, a pump, a radiator, and a constant power load object with some constant heat capacity. How can we model the dependence of the load temperature on coolant flow rate (or ...
2
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1answer
182 views

Bernoulli Equation for isentropic ideal gas flow

If one derives the Bernoulli Equation for the isentropic ideal gas flow you get: $$ \frac{1}{2}v^2 + g z + \left(\frac{\kappa}{\kappa -1} \right) \frac{p}{\rho} = const. $$ Two questions: Is ...
2
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1answer
294 views

Calculating phase diagrams (Calphad)

I am calculating the phase diagrams for Li-Mg binary alloy with reference to the following text: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5600701 In the equation used in the above ...
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5answers
524 views

Definition of entropy in thermodynamics

In most textbooks, the definition of entropy in reversible processes on a system $S$ is given simply as $$d S=\delta Q/T.$$ It seems to me this definition is insufficient since it does not specify ...
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1answer
263 views

Do magnetic fields cause ionisation of gases?

I am doing my final year engineering project on Magnetic Field Assisted Combustion and was curious to see what people thought about it. Companies sell rare earth magnetic arrangements to be attached ...