Stored Energy

• Thermal or internal energy is the result of motion of molecules or intermolecular forces.
• Potential energy (PE) is the result of attraction between molecules or the elevation of the system, and it is defined by $PE = mgz$, where $m$ is the mass of the fluid, $g$ is the acceleration of the gravity and $z$ is the elevation above the reference place.

• Kinetic energy (KE) is the energy caused by the velocity of the molecules, and it is defined by $KE = \frac{mV^2}{2}$ where $V$ is the velocity of the fluid stream cross the system boundary.

• Chemical energy is caused by the arrangement of atoms composing the molecules.
• Nuclear (atomic) energy derives from the cohesive forces holding protons and neutrons together as the atom’s nucleus.

Energy in transition

1. Heat Transfer:

   • Heat is how energy moves between things that have different temperatures.
   • Heat always travels from hotter things to cooler things.
   • When we add energy to a system, we call it positive heat transfer.

2. Work:

   • Work is how energy moves between things with different pressures or forces.
   • It always happens in the direction of lower pressure or force.
   • If the result is like lifting a weight, we call it work.
   • Positive work happens when we take energy out of a system.

3. Mechanical Energy:

   • This is the energy that machines use or produce, like turbines, engines, or compressors.

4. Flow Energy:

   • Flow energy is about the movement of fluids (like liquids or gases) into or out of a system.
   • It's the push or pull of the fluid just outside the system.
   • When fluid enters or leaves, there's a kind of work happening, and we call it flow work.

5. Enthalpy (h):

   • Enthalpy is a special property that mixes internal energy, flow work, and other things.
   • It helps us understand the total energy change in a system when there's heat or work involved.