First Law of Thermodynamics

The first law of thermodynamics is a bookkeeping rule for energy in a system: any increase in internal energy comes from energy entering as heat, or from a decrease caused by the system doing work on its surroundings. It follows the law of conservation of energy, so energy is not created or destroyed, only transferred or converted between thermal energy and mechanical work. The sign convention here means heat added to the system is positive, while work done by the system reduces its internal energy.

Use this law whenever a gas, liquid, or solid changes temperature, expands, compresses, or undergoes heating while also doing or receiving work. It is especially useful in thermodynamics problems involving engines, pistons, refrigeration, or any process where heat and work both matter.

• Using the wrong sign for work and adding it instead of subtracting it
• Forgetting that Q is positive only when heat enters the system
• Mixing up internal energy change with temperature change without checking the substance or process