Why can't a heat engine operate by obeying only the First Law of Thermodynamics?

The First Law permits complete conversion of heat into work, but this would violate the requirement that some energy must always be rejected to a cold reservoir. Without rejecting heat, the engine cannot operate cyclically.

State the Second Law of Thermodynamics as it applies to heat engines.

A heat engine must operate between a hot source (high temperature reservoir) and a cold sink (low temperature reservoir); it cannot convert heat entirely into work.

If a heat engine could convert all absorbed heat into work with no heat rejected, which law of thermodynamics would this violate?

It would violate the Second Law of Thermodynamics, which requires that some heat must always be rejected to a cold reservoir.

In a heat engine, what does QH represent and where does it come from?

QH represents the heat absorbed by the engine from the hot reservoir (high temperature source).

What is the work output W in a heat engine?

W is the useful mechanical work produced by the engine, derived from part of the heat absorbed from the hot reservoir.

What happens to QC in a heat engine?

QC is the heat rejected by the engine to the cold reservoir (low temperature sink); it is the portion of absorbed heat not converted into work.

AQA A-Level Physics 7408

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Engineering Physics

Second Law and engines

5 things to know
Paper 3, section B

Overview

Why a heat engine must reject heat, thermal efficiency, and the maximum theoretical efficiency.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

How is the efficiency of a heat engine defined and calculated?

Numbers

What sets the maximum theoretical efficiency, and what temperature scale must be used?

Key term

Why must a heat engine reject heat, and where does QH come from?

Numbers

What would happen to the maximum theoretical efficiency of a heat engine if the cold reservoir temperature could be reduced to absolute zero?

Process

In a CHP scheme, how is the heat that would otherwise be wasted (QC) used?

Questions from this subtopic

Each one with the model answer underneath.

1
Why can't a heat engine operate by obeying only the First Law of Thermodynamics?
Model answer
The First Law permits complete conversion of heat into work, but this would violate the requirement that some energy must always be rejected to a cold reservoir. Without rejecting heat, the engine cannot operate cyclically.
2
State the Second Law of Thermodynamics as it applies to heat engines.
Model answer
A heat engine must operate between a hot source (high temperature reservoir) and a cold sink (low temperature reservoir); it cannot convert heat entirely into work.
3
If a heat engine could convert all absorbed heat into work with no heat rejected, which law of thermodynamics would this violate?
Model answer
It would violate the Second Law of Thermodynamics, which requires that some heat must always be rejected to a cold reservoir.
4
In a heat engine, what does QH represent and where does it come from?
Model answer
QH represents the heat absorbed by the engine from the hot reservoir (high temperature source).
5
What is the work output W in a heat engine?
Model answer
W is the useful mechanical work produced by the engine, derived from part of the heat absorbed from the hot reservoir.
6
What happens to QC in a heat engine?
Model answer
QC is the heat rejected by the engine to the cold reservoir (low temperature sink); it is the portion of absorbed heat not converted into work.

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