# FL: Forms of Energy (Exchange)

### Forms of Energy

$\Delta E_{tot} = \Delta E_{k} + \Delta E_{p} +\Delta U$

##### Definition:

The internal energy, $U$, of a system is related to the molecular energy contained within the materials of the system.

• Molecular energy of motion:
• translational -- fluids (gases and liquids) $\rightarrow \frac{RT}{2}$ per degree of freedom
• rotations -- non-rotationally symmetric fluids $\rightarrow \frac{RT}{2}$ per degree of freedom
• vibrational/phonon -- solids
• Molecular potential energy:
• state of aggregation (phase)
• chemical bonds
##### Note:

For ideal gases $u$ is a function of temperature only. Why?

##### Definition:

Work is typically thought of as "useful" energy. It is a way that energy is exchanged between system/surroundings. It is a path function. (Mechanical equilibrium)

• PV work: $W = \int F_E dx = -\int_{{V_1}}^{V_2} P_E dV$
• "shaft work": work transferred via an object that crosses system boundary
##### Definition:

Heat is is a way that energy is exchanged between system/surroundings via thermal energy. It is a path function. (Thermal equilibrium)

• sensible heat: heat that can be "sensed" (i.e., due to a change in temperature)
• latent heat: heat that is used to change the state of aggregation of a material (i.e., does not change temperature)