Conceptual Physics

20.7 - Classifying thermal processes

As a heat engine operates, the gas expands or contracts and its pressure changes. A gas can move from one pressure-volume state to

another by different thermal processes. Four thermal processes are commonly studied and used in engines. These processes are

characterized as follows.

1. Does the gas’s volume remain constant? When it stays the same, the process is called a constant-volume process. This is also called
   anisochoric process.


2. Is the gas’s pressure constant? If so, the process is called isobaric.


3. Does any heat flow in or out of the engine during the process? If there is no heat flow, the process is called adiabatic.


4. Does the temperature of the gas stay the same? If it does, the process is called isothermal.

Above, we show each of these processes on a pressure-volume graph. Although we use an arrow to show each process occurring in a

particular direction, the arrows could be reversed without changing the types of the processes. Any of these processes can occur in either

direction.

Gas cycles

Four types of gas processes:

·Constant volume: isochoric

·Constant pressure: isobaric

·Constant temperature: isothermal

·No heat transfer: adiabatic

20.8 - Gotchas

A gas is hotter after an engine cycle is complete. No, the definition of an engine cycle is that the engine returns to its initial state.

Adding heat to an engine immediately causes it to do work. Although that is likely the purpose of the heat transfer, the heat can also increase

the internal energy of the engine. It may be later in the cycle that the engine uses the energy to do work.

Doing work on an engine can cause the temperature of its gas to increase. This is true. It can also cause heat to flow out of the engine as well.

The first law of thermodynamics is the same as the principle of conservation of energy. Essentially, this is true. The first law explicitly factors in

work, a way to add or subtract energy from a system. However, both physics principles are statements of conservation. Energy cannot

magically appear or disappear; it can always be accounted for.

James Joule demonstrated a fundamental relationship between heat, temperature and work. Yes. He showed how both work and heat could

increase the temperature (and energy) of a substance.

20.9 - Summary

The first law of thermodynamics relates heat, work, and internal energy, and is a re-

statement of the law of conservation of energy. It states that the net heat transferred

to a system equals its change in internal energy plus the work done by the system.

A heat engine uses thermal energy to do useful work. An engine contains a working

substance, usually a gas, that goes through a series of thermal processes. During a

thermal process, heat can be transferred to the gas from a hot reservoir, or can flow

out to a cold reservoir, or it could be that no heat exchange takes place. In an

analogous way, during the process, energy transfers can also take place via work:

work can be done on the system, work can be done by the system on its

surroundings, or no work might be done by the system. An engine cycle is a series

of processes that returns the gas to its original temperature, pressure, and volume. During a cycle, because the gas has returned to its original

state and the change in its internal energy is zero, the net work done equals the difference between the heat transferred into the engine and

the heat transferred out.

When the amount of gas is constant, the ideal gas law states that the product of pressure and volume is proportional to the temperature of the

gas.

Four thermal processes that occur frequently in engines are constant-volume, isobaric, adiabatic and isothermal.

First law of thermodynamics

Q = ǻEint + W

Engine cycle

W = Qh – Qc

(^380) Copyright 2007 Kinetic Books Co. Chapter 20