5 Thermodynamic Cycles and Devices Free Online FlashCards

What is a thermodynamic cycle?

A sequence of processes that returns a working substance, such as a gas or refrigerant, to its starting state.

What energy relation holds over a complete thermodynamic cycle?

Because internal energy is a state property, its net change over a complete cycle is zero: $\Delta U_{\text{cycle}}=0$. Thus, $Q_{\text{net}}=W_{\text{net}}$ under the stated sign convention.

What does an enclosed cycle on a $P$-$V$ diagram represent?

The enclosed area represents the cycle’s net work. A clockwise cycle generally produces net work; reversing the cycle requires work input.

How are heat input, rejected heat, and work related in a heat engine?

A heat engine takes in heat $Q_H$, rejects heat $Q_C$, and produces net work $W_{\text{out}}=Q_H-Q_C$.

How is a heat engine’s thermal efficiency defined?

Thermal efficiency is the fraction of input heat converted into net work: $\eta=\frac{W_{\text{out}}}{Q_H}=1-\frac{Q_C}{Q_H}$.

Why must a cyclic heat engine reject some heat?

A cyclic engine cannot convert all absorbed heat into work. It must reject some heat to a cooler reservoir, as required by the second law.

What is the Carnot limit for heat-engine efficiency?

The reversible Carnot cycle sets the maximum engine efficiency between fixed reservoir temperatures: $\eta_{\text{Carnot}}=1-\frac{T_C}{T_H}$, with temperatures in kelvins.

What distinguishes a refrigerator from a heat pump?

A refrigerator uses work to remove heat from a cooler space; a heat pump uses work to deliver heat to a warmer space. Their useful effect differs.

What is the energy balance for a refrigerator or heat pump?

Over a complete cycle, the heat delivered to the warm region equals the heat removed from the cold region plus supplied work: $Q_H=Q_C+W_{\text{in}}$.

How are refrigerator and heat-pump COPs defined and related?

The refrigerator COP is $\mathrm{COP}_{\mathrm{R}}=\frac{Q_C}{W_{\text{in}}}$; the heat-pump COP is $\mathrm{COP}_{\mathrm{HP}}=\frac{Q_H}{W_{\text{in}}}$. Since $Q_H=Q_C+W_{\text{in}}$, $\mathrm{COP}_{\mathrm{HP}}=\mathrm{COP}_{\mathrm{R}}+1$.

Why can a device’s COP exceed $1$ without violating energy conservation?

COP compares heat moved with work supplied, not work output with heat input. Therefore, it can be greater than $1$.

What are the Carnot COP limits for refrigerators and heat pumps?

For reversible devices, $\mathrm{COP}_{\mathrm{R,Carnot}}=\frac{T_C}{T_H-T_C}$ and $\mathrm{COP}_{\mathrm{HP,Carnot}}=\frac{T_H}{T_H-T_C}$. Temperatures must be in kelvins.