3 The First Law of Thermodynamics Free Online FlashCards

What defines a closed system?

A closed system contains a fixed amount of matter: energy may cross its boundary, but mass does not.

What is the first-law energy balance for a closed system?

With heat positive into the system and work positive when done by it, the first law is $\Delta E=Q-W$.

Which energy changes make up $\Delta E$?

The total energy change is $\Delta E=\Delta U+\Delta KE+\Delta PE$, combining changes in internal, kinetic, and gravitational potential energy.

What signs represent heat removed and work done on a system?

Heat entering the system is positive, so heat removed gives $Q<0$. Work done by the system is positive, so work done on it gives $W<0$.

How does internal energy differ from heat and work?

Internal energy is a state property. Heat and work are energy-transfer modes, not properties stored in the system, and their amounts generally depend on the process path.

How is quasi-static moving-boundary work calculated?

For a quasi-static process with pressure acting at a moving boundary, work done by the system is $W_b=\int_{V_1}^{V_2}p\,dV$.

What are the signs of boundary work during expansion and compression?

Expansion gives positive boundary work because the system does work; compression gives negative boundary work.

What energy-balance simplification applies to a rigid tank?

A rigid tank has no boundary work because its volume is fixed. With no other work and negligible kinetic and potential energy changes, $\Delta U=Q$.

What does the first law imply for an adiabatic process?

An adiabatic process has no heat transfer, so $Q=0$ and $\Delta E=-W$. Work done on the system can increase its energy.

What relationship holds between net heat and work over a cycle?

A cyclic process returns the system to its initial state, so its net energy change is zero and net heat equals net work: $Q=W$.

Name three forms of work that can cross a closed-system boundary.

Examples include moving-boundary work, shaft work, and electrical work.

What happens to internal energy during insulated stirring?

With negligible heat transfer and negligible kinetic and potential energy changes, work input increases internal energy. Under this convention, work done by the system is negative.