Reversed heat engines (A-level only) Flashcards – AQA A-level Physics (7408)

During each cycle of a reversed heat engine, what energy transfers occur?

It takes in heat $Q_C$ from the cold space, receives work $W$, and delivers heat $Q_H$ to the hot space.

What energy-conservation equation relates the heat transfers and work in a reversed heat engine?

$Q_H = Q_C + W$

Why does a reversed heat engine require work input?

It transfers heat against the natural direction of heat flow.

How do the purposes of a refrigerator and a heat pump differ?

A refrigerator removes heat from a cold space to keep it cool; a heat pump delivers heat to a hot space to keep it warm.

What is the coefficient of performance (COP) of a reversed heat engine?

The desired heat transfer divided by the work input.

What is the COP of a refrigerator in terms of work input $W$, heat $Q_C$ removed from the cold space, and heat $Q_H$ delivered to the hot space?

$COP_{\mathrm{ref}}=\frac{Q_C}{W}=\frac{Q_C}{Q_H-Q_C}$

What is the COP of a heat pump in terms of work input $W$, heat $Q_C$ removed from the cold space, and heat $Q_H$ delivered to the hot space?

$COP_{\mathrm{hp}}=\frac{Q_H}{W}=\frac{Q_H}{Q_H-Q_C}$

What is the COP of an ideal reversible refrigerator operating between absolute temperatures $T_C$ and $T_H$?

$COP_{\mathrm{ref}}=\frac{T_C}{T_H-T_C}$

What is the COP of an ideal reversible heat pump operating between absolute temperatures $T_C$ and $T_H$?

$COP_{\mathrm{hp}}=\frac{T_H}{T_H-T_C}$

What temperature unit must be used in the ideal reversible COP expressions?

Kelvin.

How are the heat-pump and refrigerator COPs related for the same device operating between the same spaces?

$COP_{\mathrm{hp}}=COP_{\mathrm{ref}}+1$

Why can a coefficient of performance exceed one?

The desired heat transfer is not supplied by the work input alone.

How does a real device’s COP compare with the ideal reversible COP?

The real device has a lower COP.