Energy is conserved: change in internal energy equals heat added plus work done on the system.
Practise Thermodynamics & Energetics All Thermodynamics & Energetics topics
The first law states that the change in internal energy of a system equals the heat added to it plus the work done on it, written as delta U equals q plus w, with w defined as work done on the system. It is a formal statement of conservation of energy applied to thermodynamic systems. In an isolated system no heat or work crosses the boundary, so the internal energy stays constant. The law makes it impossible to build a perpetual motion machine that creates energy from nothing, which is known as a machine of the first kind. Every energy balance in chemistry, from calorimetry to power generation, relies on this principle.
Key ideas: Apply the first law as an energy ledger: state which way each of q and w is counted, then sum. For a cycle, delta U is zero so net heat equals net work out. For an adiabatic process delta U equals w, and for a constant-volume process delta U equals q. Always check units before adding q and w.
Common mistakes and connections: The sign convention of w causes most errors, since some textbooks define w as work done by the system, which gives delta U equals q minus w. State your convention. The law says nothing about direction, so it cannot explain why heat flows from hot to cold, and that gap is filled by the second law.
The change in internal energy is q + w: adding 3 kJ of heat and doing 2 kJ of work on the system raises U by 5 kJ. Energy is conserved. The interactive lesson shows this as a live animation or chart.
If a gas absorbs 100 J of heat and has 40 J of work done on it, its internal energy rises by 140 J.
Energy is conserved: change in internal energy equals heat added plus work done on the system.
First Law of Thermodynamics is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Heat, Work in Chemistry, Internal Energy.
The next topics in Thermodynamics & Energetics are Enthalpy and Exothermic Reactions.
Yes. The interactive lesson includes a self-check question on First Law of Thermodynamics, and the Thermodynamics & Energetics quiz draws on every topic in this area.
First Law of Thermodynamics is topic 9 of 91 in Thermodynamics & Energetics.