A7LAM CSCA ACADEMYCSCA Academy / Physics / Heat
CSCA Physics · Lesson 12 of 16The first law of thermodynamics
Le premier principe de la thermodynamique · المبدأ الأول في الديناميكا الحرارية
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Energy is conserved for heat too. The internal energy of a gas changes when heat flows in or out and when work is done on it or by it.
Key ideas
- Internal energy U is the total energy of the molecules; for an ideal gas it depends only on T.
- First law: ΔU = Q + W.
- Q > 0: heat absorbed. W > 0: work done ON the gas (compression).
- Heat flows by itself only from hot to cold (second law).
Formulas to know
- ΔU = Q + W
- Work at constant pressure: |W| = pΔV
- Heating: Q = cmΔt
Worked example
A gas absorbs 500 J of heat and does 200 J of work on its surroundings. ΔU?
- Signs: Q = +500 J, W = −200 J (work done BY the gas)
- Law: ΔU = Q + W
- Compute: ΔU = 500 − 200
Answer: ΔU = +300 J
Practice questions
1. A gas is compressed with 300 J of work and loses 100 J of heat. ΔU?
+400 J · +200 J ✓ · −200 J · −400 J
ΔU = −100 + 300 = +200 J.
2. Heat needed to warm 2 kg of water by 10 °C (c = 4,200 J/(kg·°C))?
8,400 J · 84,000 J ✓ · 42,000 J · 21,000 J
Q = 4,200 × 2 × 10.
3. An ideal gas expands at constant temperature. ΔU = ?
Positive · Zero ✓ · Negative · Equal to Q
U depends only on T.