gen-define-the-first-law-of-thermodynamics-and-give-one-short-wo-4981

Define the first law of thermodynamics and give one short worked example.

# First Law of Thermodynamics **Definition:** Energy cannot be created or destroyed, only converted from one form to another. For a system, the change in internal energy (ΔU) equals the heat added to the system (Q) minus the work done by the system (W): $$\Delta U = Q - W$$ ## Worked Example A gas in a cylinder absorbs 500 J of heat and expands, doing 200 J of work on the piston. What is the change in internal energy? **Solution:** - Q = +500 J (heat absorbed) - W = +200 J (work done by the gas) $$\Delta U = Q - W = 500 - 200 = 300 \text{ J}$$ The internal energy of the gas increases by **300 J**. This means that of the 500 J of heat energy absorbed, 200 J went into mechanical work (pushing the piston), while 300 J remained in the gas as increased internal energy (higher temperature).