⚖️ First Law of Thermodynamics

Chapter: Thermodynamics · Level ★★★ · Time: 30 min

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After this worksheet you can

Understand: The First Law StatementUnderstand: Internal Energy (U)Understand: Sign ConventionsUnderstand: Special Cases

📚 Quick Recap

The first law of thermodynamics is really just energy conservation applied to heat and work — energy can change form or move around, but it can never be created or destroyed.

🧠 Section A · Concept Check ● BEGINNER 4 × 1 = 4

1A gas absorbs 1000 J of heat and does 400 J of work. Find delta_U.

2A gas has 250 J of work done on it with no heat exchange (adiabatic). Find delta_U.

3What does 'isothermal' mean, and what does it imply about delta_U for an ideal gas?

4In an isochoric (constant volume) process, a gas absorbs 400 J of heat. Find delta_U.

🧮 Section B · Problem Solving ● INTERMEDIATE 4 × 3 = 12

5A gas releases 150 J of heat while doing 50 J of work on its surroundings. Find delta_U.

6Why can't a machine produce more energy output than the energy put into it, according to the first law?

7A balloon expands, doing 120 J of work on the surrounding air, while absorbing 120 J of heat. Find delta_U.

8What happens to a gas's internal energy during rapid adiabatic compression (Q=0, work done ON the gas)?

🚀 Section C · Challenge ● CHALLENGE 2 × 2 = 4

9Why is 'internal energy' different from 'heat'?

10A system undergoes a full cycle and returns to its starting state. What must be true about its total delta_U over the cycle?

💭 Reflection — the most useful thing I learned:

A ___/4 B ___/12 C ___/4 Total ___/20 Teacher's Signature Parent's Signature

✂ answer key — fold or cut before handing out

1 = 600 J 2 = Q=0, W=-250, delta_U=0-(-250)=250 J 3 = Constant temperature; it implies delta_U=0 for an ideal gas 4 = Since W=0, delta_U=Q=400 J | 5 = Q=-150, W=50, delta_U=-150-50=-200 J 6 = Energy cannot be created; the first law requires that all energy changes be fully accounted for by heat and work exchanged 7 = delta_U=120-120=0 J 8 = It increases, since compression adds energy to the system with no heat escaping | 9 = Internal energy is the system's total stored energy at any moment; heat is energy specifically in transit due to a temperature difference 10 = It must equal 0, since internal energy is a state function that returns to its original value