• Think of it as "Energy Balance of a System."
• The system's energy can change if:
  • Work is done by it or on it.
  • There are temperature differences between the system and its surroundings.

• For IB Diploma: Q = ΔU + W
  • Q = energy transferred
  • ΔU = change in system's internal energy
  • W = work done by the system

🎈 Real-World Example: Imagine a balloon. If you heat it (Q is positive), it expands and does work to push the surrounding air out of the way (W is positive). If the balloon’s air heats up too, its internal energy rises (ΔU is positive).

• Clausius (For IB):
  • +Q = Heat goes from surroundings to the system
  • +ΔU = System's energy increases
  • +W = System does work on surroundings
• IUPAC Convention (Be aware!): ΔU = Q + W
  • Different signs for work, but don't get tricked - both lead to the same answers.

🍳 Example: Cooking an egg. The pan gets hot (Q from stove to pan is positive), the pan's energy increases as it heats up (+ΔU), and it cooks the egg by doing work on it (+W).