Question 1

Why is internal energy called a state function?

Accepted Answer

Internal energy is a state function because its value depends only on the current thermodynamic state of the system (e.g., its temperature, pressure, and volume), not on the path taken to reach that state. This means if a system returns to its initial state, its internal energy returns to its initial value, regardless of the processes it underwent.

Question 2

What is the difference between heat and internal energy?

Accepted Answer

Internal energy is the energy *stored within* a system due to the random motion and interaction of its particles. Heat (Q), on the other hand, is the *transfer of thermal energy* between systems or between a system and its surroundings due to a temperature difference. Heat is energy *in transit*, while internal energy is energy *contained*.

Question 3

Does the First Law of Thermodynamics apply to all types of systems?

Accepted Answer

Yes, the First Law of Thermodynamics is a fundamental principle of energy conservation that applies to all thermodynamic systems, whether open, closed, or isolated, and to all types of processes, provided energy transfers are accounted for correctly as heat or work.

Question 4

Can you summarize the First Law for different processes (isothermal, adiabatic, etc.)?

Accepted Answer

Certainly! For an **isovolumetric** process (constant volume), $W=0$ so $\Delta U = Q$. For an **isothermal** process (constant temperature), $\Delta U=0$ for an ideal gas, so $Q = -W$. For an **adiabatic** process (no heat transfer), $Q=0$ so $\Delta U = W$. For an **isobaric** process (constant pressure), all terms can be non-zero and $W = -p\Delta V$.

The first law of thermodynamics

What you need to know

The first law of thermodynamics