3 The First Law and Energy Balances Free Online FlashCards

What does the first law of thermodynamics express?

Energy can cross a system boundary or change form, but it cannot be created or destroyed.

What components make up a system’s total energy?

The total energy is the sum of internal, macroscopic kinetic, and gravitational potential energy: $E=U+KE+PE$.

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

For a closed system, $\Delta E=Q-W$.

How do heat and work differ from internal energy?

Heat and work describe energy crossing the boundary; neither is stored as a system property. Internal energy is a property of the system’s state.

What does internal energy represent microscopically?

Internal energy accounts for energy at the molecular level, including molecular motion and interactions.

How is enthalpy defined?

Enthalpy is a state property defined by $H=U+pV$, or per unit mass, $h=u+pv$.

Why is the $pv$ term useful for flowing fluids?

The $pv$ term accounts for pressure-related work needed to push fluid into or out of a control volume, so flowing mass transports enthalpy.

What is a control volume?

A control volume is a chosen region in space through which mass may flow.

What can change a control volume’s energy?

Its energy changes through heat and work transfer and through energy carried in or out by flowing matter.

What happens to stored energy in a control volume at steady state?

At steady state, stored energy does not change, so $\frac{dE_{\mathrm{cv}}}{dt}=0$.

What determines an adiabatic turbine’s shaft power under negligible speed and elevation changes?

For an adiabatic turbine with negligible speed and elevation changes, $\dot W=\dot m(h_{\mathrm{in}}-h_{\mathrm{out}})$.

Under the stated sign convention, when are heat and work positive?

Heat transferred into the system is positive, and work done by the system is positive.