# What is First law of thermodynamics: Definition and 22 Discussions

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body's state, called internal energy.
The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.
For a thermodynamic process without transfer of matter, the first law is often formulated

Δ
U
=
Q

W

{\displaystyle \Delta U=Q-W}
,where

Δ
U

{\displaystyle \Delta U}
denotes the change in the internal energy of a closed system,

Q

{\displaystyle Q}
denotes the quantity of energy supplied to the system as heat, and

W

{\displaystyle W}
denotes the amount of thermodynamic work done by the system on its surroundings. An equivalent statement is that perpetual motion machines of the first kind are impossible.
For processes that include transfer of matter, a further statement is needed: 'With due account of the respective reference states of the systems, when two systems, which may be of different chemical compositions, initially separated only by an impermeable wall, and otherwise isolated, are combined into a new system by the thermodynamic operation of removal of the wall, then

U

0

=

U

1

+

U

2

{\displaystyle U_{0}=U_{1}+U_{2}}
,where

U

0

{\displaystyle U_{0}}
denotes the internal energy of the combined system, and

U

1

{\displaystyle U_{1}}
and

U

2

{\displaystyle U_{2}}
denote the internal energies of the respective separated systems.'

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1. ### Internal energy won't add up to 0 in cyclic process

My problem isn't exactly with calculating the actual changes in internal energy, I'll put those values below. My problem is that I can't get the values to add up to 0, and I don't understand why since for cyclic processes, by definition, ΔU must equal 0. $$ΔU_{AB} = ΔU_{isothermal} = 0$$...
2. ### Adiabatic Expansion of Pressurized Air in a Piston-Cylinder Setup

Task: A thermally insulated pressurized air cylinder, B, was initially placed inside a closed-bottom, circular hollow cylinder A with an inner diameter of 50 cm. Then a tightly fitting, frictionless sliding piston with a mass of 20 kg was installed. Using the outlet valve, the height of the...
3. ### Work greater than heat given to colder reservoir -- impossible?

photo below... Is it possible to make a machine that would take more heat from the hot reservoir to do work than what would hot reservoir give to the cold reservoir(heat)? Apparently, it's impossible because it violates 1st law of thermodynamics. the thickness of the arrows symbolizes the...
4. ### I Question regarding dh, du, cp, cv for ideal gases

Hi, Considering the question bellow from a government work selection process:Check the FALSE alternative on the use of thermodynamic properties. In a cylinder-piston type system, the variation of the enthalpy property (Δh) is usually applied to determine the heat (per kilogram) exchanged with...
5. ### First law of thermodynamics and the work done on a system

The first law of thermodynamics states that the change in the internal energy of a system equals the net heat transfer into the system minus the net work done by the system. In equation form, the first law of thermodynamics is ΔU = Q − W. in the mathematical expression of the first law of...
6. ### How Do You Calculate Heat Transfer in a Thermodynamics Piston Problem?

I'm new to thermodynamics and after some reading I tried to solve the problem below but I have stuck (I think this problem assumes you know only the first law of thermodynamics) You have a piston resting on some stops inside a container filled with water and you want to find out after heating...
7. ### Assumed value of pressure during quasistatic compression

My text (Ian Ford - Statistical physics) describes an ideal gas system in a piston being quasistatically compressed by a piston head of area A under external force f. It assumes the system has a uniform pressure p. All good so far. Then it says: "the force pA equals the applied external force f"...
8. ### Minimum work needed to compensate for a heat leak

Homework Statement A great deal of effort has been expended to find “high temperature superconductors”: materials that are superconductors at temperatures higher than the boiling point of liquid nitrogen (77 K). Most of the older superconductors had to be operated with liquid helium (boiling...
9. ### How much water is converted to ice?

Homework Statement A 0.530 kg sample of liquid water and a sample of ice are placed in a thermally insulated container. The container also contains a device that transfers energy as heat from the liquid water to the ice at a constant rate P, until thermal equilibrium is reached. The...
10. ### Is work a function of state in adiabatic processes?

I'm reading a thermo book and I'm on the chapter dealing with the first law of thermodynamics. The book is discussing functions of state and how the internal energy is a function of state because it has a well defined value at every equilibrium state of the system. It says that the work and...
11. ### Heat absorbed by an ideal gas in a cycle

Homework Statement Homework Equations and the attempt at a solution:[/B] AC is adiabatic and AB is isothermal. Heat absorbed during process AC = 0 (adiabatic). Heat absorbed during process CB = C_p \triangle T=-\frac{\gamma}{\gamma -1} (P_2V_3 - P_2V_2) Heat absorbed during process BA =...
12. ### Finding the work done by a Stirling Cycle

1. Given the following p-V diagram of an ideal Stirling Cycle, determine the theoretical values of W12, Q12, W34 and Q34 in terms of T1, T2, V1, V2, n (the number of moles) and R (the universal gas constant). Determine the total theoretical p-V work W12341 for the full cycle.2. dU = dQ-Pdv3...
13. ### 1st law, 2nd law, entropy by Gyftopoulos / Beretta is confusing

I'm reading Thermodynamics: Foundations and Applications by Gyftoploulos and Beretta, because the authors claim to give a presentation of classical thermodynamics without "... the lack of logical consistency and completeness in the many presentations of the foundations of thermodynamics" [from...
14. ### Heat transferred into a closed system

So i am making a simple demonstration of ideal gas law using a cylinder piston system, heating the system so the piston is pushed up, i wanted to calculate the heat transferred into the system, will it be Cp(Tf-Ti) or Cp(Tf-Ti)+ work done by the system ?
15. ### What is the exit steam velocity?

Homework Statement Steam, at 15 bar and 280oC, enters a nozzle with an initial velocity of 125 m/s. The steam enthalpy at the exit section is 2800 kJ/kg and the heat loss is 25 kJ/kg. What is the exit steam velocity?. Homework Equations 1)Qin - Qout = m [h2-h1+((c22-c12))/2] 2)h = Pv + u...
16. ### Calculate Pressure in piston cylinder after heating

Homework Statement Water is contained in a cylinder fitted with a frictionless piston (figure shows that atmospheric pressure acts on piston from above). The mass of the water is 0.45 kg, and the piston area is 0.186 m2. Initially, the water conditions are 110oC and 90% dryness fraction, and...
17. ### What is the Internal Energy of 158 moles of CO2

Homework Statement A gas bottle contains exactly 158 moles of carbon dioxide CO2. Find the change in the internal energy of this much CO2 when it is cooled from 36C down to exactly 25C at a constant pressure of 1 atm. The gas can be treated as an ideal gas with γ=1.289. The gas constant reads...
18. ### Ideal Gas Expansion State Properties & Exergy Balance

Homework Statement Two well-insulated rigid tanks of equal volume, tank A and tank B, are connected via a valve. Tank A is initially empty. Tank B has 2 kg of Argon at 350 K and 5000 kPa. The valve is opened and the Argon fills both tanks. State 2 is the final equilibrium state. The temperature...
19. ### Work on the Entire System in Thermodynamics

A box is initially at rest on a frictionless surface. David then pushes it and the box moves across the frictionless surface. Let's discuss this process in the context of thermodynamics, especially the First Law. One way to state it is: ΔU=Q−W (where positive W is work done by the system...
20. ### Compressing ideal gas, show no heat exchange

Homework Statement An ideal monatomic gas is contained in a cubic container of size ##L^3##. When ##L## is halved by reversibly applying pressure, the root mean square ##x##-component of the velocity is doubled. Show that no heat enters of leaves the system.Homework Equations ##dU = dQ -pdV##...
21. ### Thermodynamics thought experiment

There is some ideal gas in a container moving with some velocity on a smooth surface and you suddenly stop it( say by using your hands) , will the temperature of the gas increase? It seems to me that since you're suddenly stopping it there is no work done (because of no displacement ) so the...
22. ### How to use the first law of thermodynamics for simple mechanical problems

I'm confused about what exactly is Q and U and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. Then Q=dU+W where dU is change in internal energy. (I'm using the sign convention that heat absorbed...