The first law of thermodynamics

Содержание и основные понятия

The change in the internal energy of the system is equal to the amount of heat transferred to it and the activity of the external forces. Under another arrangement the signs use work done by the gas itself.
GEF · 10 grade
PIF · Baseline
Interactive model
Domestic energy
Heat
gas operation
Adiabata
01
The physical picture
What happens and what boundaries the model has.
Main message
The change in the internal energy of the system is equal to the amount of heat transferred to it and the activity of the external forces. Under another arrangement the signs use work done by the gas itself.
Internal energy is a function of state, and heat and work characterize the process.
Gas does not expand in the isochor process.
There is no heat exchange in the adiabatic process, but temperatures can change because of work.
How to reason
First, name the system and the interaction, then select the values, and only then write down the law. The formula works within the model's assumptions.
02
Laws and values
Symbols should be read as a relationship of values, not as a recipe for substitution.
Δ
U
=
Q
−
A
газа
\Delta U=Q-A_{\mathrm{газа}}
Δ
U
=
Q
−
A
Gases
​
First Act
A
газа
=
p
Δ
V
A_{\mathrm{газа}}=p\Delta V
A
Gases
​
=
p
Δ
V
Work at constant pressure
Review of the situation
When adiabatic compression occurs, external work increases internal energy and gas temperature.
04
Links and application
In the real world
When adiabatic compression occurs, external work increases internal energy and gas temperature.
Model verification
Compare the prediction with observation, evaluate the units, the order of magnitude and the influence of the factors not taken into account. Disparity does not always mean an error of law: often idealization is no longer sufficient.
C
What needs to be understood
Outcome 1
Explain the physical meaning of "First Law of Thermodynamics" through the model, not just the formula.
Outcome 2
Clear and correct use of terms: internal energy, heat, gas operation.
Outcome 3
Indicate the conditions for the applicability of the law, verify units and limit cases.
T
Terms of the topic
The definition should indicate the physical meaning, not a substitute for a domestic synonym.
Domestic energy
The sum of the kinetic energy of the chaotic movement of particles and the potential energy of their interaction.
Heat
Power transmission between systems due to temperature differences.
gas operation
The energy transmitted by the gas environment when the volume changes is equal to the area below the p(V) schedule.
Adiabata
Process line without heat exchange with the environment in the state diagram.
F
Origin and conclusion of the formulas
The fundamental law cannot be "proven" by algebras: for each record, it is stated whether it is a definition, an experimental law, or a model effect.
Δ
U
=
Q
−
A
газа
\Delta U=Q-A_{\mathrm{газа}}
Δ
U
=
Q
−
A
Gases
​
Postulate or Model
First law.
The following is an indication of the definition, experience or law from which this entry arises and where it is applicable.
Q
=
Δ
U
+
A
газа
Q=\Delta U+A_{\mathrm{газа}}
Q
=
Δ
U
+
A
Gases
​
The Law on Energy Conservation approves that the heat generated is a change in the internal energy and the system's work on the environment.
A
над газом
=
−
A
газа
A_{\mathrm{над\ газом}}=-A_{\mathrm{газа}}
A
above
Gas
​
=
−
A
Gases
​
The work of the external forces on the gas is the opposite of that of the gas.
Δ
U
=
Q
−
A
газа
\Delta U=Q-A_{\mathrm{газа}}
Δ
U
=
Q
−
A
Gases
​
Regrouping of the balance sheet gives the first law in the signing agreement.
Conditions:
Locked thermodynamic system without transfer of substance; Q > 0 under heat transfer Agas > 0 when gas is working on external bodies.
Monitoring:
The dimensions of both parts shall coincide; the result shall be checked for sign, order and limit case.
A
газа
=
p
Δ
V
A_{\mathrm{газа}}=p\Delta V
A
Gases
​
=
p
Δ
V
model effect
Working under constant pressure.
The following is an indication of the definition, experience or law from which this entry arises and where it is applicable.
F
=
p
S
F=pS
F
=
pS
The pressure gas p operates on the piston of area S by force pS.
A
=
F
Δ
x
=
p
S
Δ
x
A=F\Delta x=pS\Delta x
A
=
F
Δ
x
=
pS
Δ
x
At constant pressure, the force working on the displacement of the piston Δx is equal to F Δx.
Δ
V
=
S
Δ
x
⇒
A
=
p
Δ
V
\Delta V=S\Delta x\quad\Rightarrow\quad A=p\Delta V
Δ
V
=
S
Δ
x
⇒
A
=
p
Δ
V
Making a piston area for moving it equals changing the volume.
Conditions:
Quasistatic expansion or compression under constant pressure; only pV-work, ΔV = V2 - V1 are considered.
Monitoring:
The dimensions of both parts shall coincide; the result shall be checked for sign, order and limit case.
!
Model boundaries and typical errors
Error: Forget model condition
The formula cannot be transferred to any situation simply because of the overlap of letters. The key condition of this topic is that internal energy is a function of state, and heat and work characterize the process.
How to Check the Mind
The gas does not expand in the isochor process. Then check the unit, vector direction, and result behaviour at zero or very large parameter.
Σ
Conclusion on the topic
The first law of thermodynamics
The change in the internal energy of the system is equal to the amount of heat transferred to it and the activity of the external forces. Under another arrangement the signs use work done by the gas itself.
Internal energy is a function of state, and heat and work characterize the process.
Gas does not expand in the isochor process.
Any numerical result shall be accompanied by a unit, a sign and an explanation of its physical meaning.
The content is consistent with the FRP on Physics of the OS/OSS 2025 and the subject matter of the current lines of the PNP: I. M. Peryškin and co-authors (7–9), G. J. Mäkishev and coauthors (10–11), and with the lines "Engineers of the Future" and V. A. Kasyanov. The original texts are not reproduced, the text of the textbooks is not reproduced.