The First Law of Thermodynamics for Any Process
Internal energy changes only by heat added and work done, whatever route the system takes. Applying it means being careful about signs and about which quantities depend on the path.
What a learner can do afterwards
- States the first law with a consistent sign convention and applies it to a stated process
- Explains why internal energy is a state function while heat and work are not
- Traces energy round a complete cycle and shows the internal energy change is zero
1 · Read
The first law says the change in internal energy equals the heat added minus the work done by the system. Energy never appears from nothing. Fix one sign rule, with work by the system counting positive, and keep it for every term.
Internal energy is fixed by the current state alone, so it is a state function. Heat and work describe the route taken, so the same endpoints can pair with many values of each.
Squeeze a gas while it loses heat and two effects fight: the squeeze pushes energy in while the leaking heat drains it, and the change in internal energy is the balance. Heating at fixed volume sends every joule into internal energy, since no boundary work happens, while heating at fixed pressure spends part on work. With no heat flow at all, called adiabatic, any work comes straight from internal energy and the temperature shifts.
Round a full cycle the internal energy change is zero, so the net heat in equals the net work out. Trace each leg with its sign and the loop must balance.
Heat in minus work out sets the change inside, and loops always balance to zero.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.