Second Law and engines (A-level only) Flashcards – AQA A-level Physics (7408)

What energy transfers occur during a heat-engine cycle?

The engine takes in heat $Q_H$ from a hot source at $T_H$, delivers work $W$, and rejects heat $Q_C$ to a colder sink at $T_C$.

What energy balance does the First Law give for one complete heat-engine cycle?

$Q_H = W + Q_C$

Why does energy conservation alone not rule out complete conversion of heat into work by a cyclic engine?

The energy balance permits $Q_C = 0$ and $W = Q_H$.

What does the Second Law rule out for a cyclic heat engine?

An engine whose only effect is to take heat from a single source and convert it entirely into work.

What is the efficiency equation for a heat engine in terms of work $W$, energy received from the hot source $Q_H$, and energy transferred to the cold sink $Q_C$, all measured over the same cycle?

$\eta=\frac{W}{Q_H}=\frac{Q_H-Q_C}{Q_H}$

What is the maximum theoretical efficiency of a heat engine operating between absolute temperatures $T_H$ and $T_C$?

$\eta_{\max}=\frac{T_H-T_C}{T_H}$

What temperature unit must be used in the maximum heat-engine efficiency formula?

Kelvin.

What kind of heat engine approaches the maximum theoretical efficiency?

An ideal reversible engine.

How does increasing the temperature difference between the source and sink affect a heat engine’s theoretical efficiency limit?

It increases the limit.

Why are practical heat engines less efficient than the theoretical maximum?

Real processes are irreversible.

What are two examples of irreversibility in practical heat engines?

Energy dissipation through friction and heat transfer across finite temperature differences.

How do unwanted heat losses affect the useful work obtained from a heat engine’s supplied energy?

They reduce the useful work obtained.

How does combined heat and power produce useful outputs from an engine?

It uses the engine’s work, commonly converted to electricity, and recovers heat that would otherwise be rejected for heating.

How is the overall energy utilisation of a combined heat and power system calculated?

$\frac{\text{useful work} + \text{useful heat}}{\text{energy supplied}}$

Does recovering rejected heat increase an engine’s work output or work efficiency?

No. The recovered heat provides useful heating; it is not converted into additional work.

Does combined heat and power remove the Second Law limit on work production?

No. The Second Law limit on work production still applies.