Energy

The capacity to do work or the ability to cause changes.

Total energy is denoted by $E$.

$$E = U + \frac{1}{2}mC^2 + mgz$$

Here:

• $U$ - internal energy
• $\frac{1}{2}mv^2$ - kinetic energy of the system.
• $mgz$ is the potential energy of the system.

Microscopic Energy

The energy possessed by individual particles or molecules within a system. Associated with the motion and interactions of these particles at the atomic and molecular level. Denoted by $U$. $$

Examples:

• Thermal energy from random molecular motion in steam
• Chemical energy stored in fuel molecules

Internal Energy

Sum of all microscopic energies of a system.

Macroscopic Energy

The energy associated with the bulk properties of a system. Such as its temperature, pressure, and volume. It is the energy that can be measured and observed at the macroscopic scale.

Examples:

• Kinetic energy of a moving car
• Potential energy of water behind a dam

Heat & Work

Has only meaning in transitions. Can be identified only when crossing the system boundary.

Called path functions as they depends on the path taken.