- Recall and apply the first law of thermodynamics: ΔU = q + W
- Interpret ΔU as change in internal energy, q as heat added to system, W as work done on system
- Analyze simple processes (constant volume, constant pressure, isothermal) using ΔU = q + W
- Calculate heat and work for closed systems undergoing heating or mechanical work
- Use terms “internal energy,” “heat,” “work,” “system boundary” accurately
- Explain energy flow and sign conventions in clear academic English
- Describe thermodynamic processes (e.g. isochoric, isobaric, isothermal)
- Interpret formula ΔU = q + W and define each symbol precisely
| English Term | Russian | Kazakh |
|---|---|---|
| Internal energy (U) | Внутренняя энергия (U) | Ішкі энергия (U) |
| Heat (q) | Теплота (q) | Жылу (q) |
| Work (W) | Работа (W) | Жұмыс (W) |
| First law | Первый закон | Бірінші заң |
| Isochoric process | Изохорный процесс | Изохоралық процесс |
| Isobaric process | Изобарный процесс | Изобаралық процесс |
| Isothermal process | Изотермический процесс | Изотермиялық процесс |
Internal Energy (U)
Definition: Total microscopic kinetic & potential energy of molecules
Heat (q)
Definition: Energy transferred due to temperature difference
Sign: Positive when added to system
Work (W)
Definition: Energy transferred when system volume changes against external pressure
Sign: Positive when done on system
First Law
Formula: ΔU = q + W
First Law of Thermodynamics
The energy change of a closed system equals heat added plus work done on it: ΔU = q + W.
Russian: Изменение энергии замкнутой системы равно сумме теплоты, переданной системе, и работы, совершенной над ней.Kazakh: Жабық жүйенің энергиясындағы өзгеріс оған қосылған жылу мен жүйеге жасалған жұмыстың қосындысына тең.
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Work done on system
For quasi-static pressure–volume work: W = –∫p dV (positive when volume decreases).
Russian: Для квази-статической работы: W = –∫p dV (положительно при уменьшении объема).Kazakh: Квази-статикалық жұмыс үшін: W = –∫p dV (көлем қысқарғанда оң болады).
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In any thermodynamic process, the change in internal energy ΔU equals the heat q added to the system plus the work W done on it:
ΔU = q + W
**Sign conventions:** q positive when heat flows into system; W positive when work done on system (e.g. compression).
**Special cases:**
- Isochoric (V constant): W = 0 ⇒ ΔU = q
- Isobaric (p constant): W = –pΔV ⇒ ΔU = q – pΔV
- Isothermal (T constant for ideal gas): ΔU = 0 ⇒ q = –W
Theory Questions
Fill in the Blanks
- The first law: ΔU = ___ + ___.
- For constant volume, W = ___ so ΔU = ___.
- In isothermal ideal gas, ΔU = ___.
- Work done on system in PV work: W = -∫ ___ d___.
- q positive when heat flows ___ the system.
Additional Video Resources:
• First Law of Thermodynamics Explained
• Thermodynamic Processes & Work
Example 1: Isochoric Heating
A gas in a rigid container (V constant) is heated with 200 J of heat. Find ΔU and W.
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Example 2: Isothermal Compression
1.0 mol ideal gas at 300 K is compressed isothermally from 10 L to 5 L. Find W and q.
Investigate heat–work–energy balance in PhET “Thermodynamic Processes”:
In small teams, solve this Quizizz on first law problems:
Solve these structured questions:
- Derive ΔU = q + W for a cyclic process.
- A gas does 500 J work while losing 200 J heat. Find ΔU.
- Compare heat and work in adiabatic vs isothermal processes.
- Discuss how to include electrical work in the first law expression.
- Analyze an engine cycle using first law for each stroke.
- Which process (isochoric, isothermal, isobaric) was clearest in applying ΔU = q + W?
- How do sign conventions affect your problem solving?
- What challenges did you face in calculating work under curves?
- Set a goal to improve your understanding of energy balances in cycles.