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THERMAL PHYSICS

 

Thermodynamics

 

zeroth law

first law

second law

heat engines & pumps

 

 

 

The Zeroth Law of Thermodynamics

 

 

If two systems are in thermal equilibrium with a third system, they are also in thermal

 

equilibrium with each other.

 

 

To understand this concept we must first appreciate what thermal equilibrium is.

 

Consider a body at a high temperature in contact with a body at a low temperature.

 

Heat is transferred from the high temperature body to the lower temperature body until the temperatures are equalized.


When this equal, constant temperature is reached and maintained, the two bodies are said to be in thermal equilibrium.

 

 

zeroth law of thermodynamics

 

 

Consider three bodies, X, Y and Z.

 

Z is in thermal equilibrium with Y.

 

X is in thermal equilibrium with Y.

 

Then Z is in thermal equilibrium with X.

 

 

To try to visualize this further, consider a hot cup of tea.

 

After about twelve hours, the saucer, the cup and the tea will all be at the same temperature.

 

The saucer is in equilibrium with the cup.


The cup is in equilibrium with the tea.


Therefore the tea is in equilibrium with the saucer.

 


They are each in thermal equilibrium with eachother.

 

 

 

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The First Law of Thermodynamics

 

 

The change in the internal energy* ( ΔU ) of a system is equal to the amount of heat

 

supplied ( ΔQ ) to the system, minus the amount of work( ΔW ) performed by the

 

system on its surroundings.

 

 

* Only internal energy changes can be measured. Absolute values of internal energy are not defined.

This can be stated as the equation:

 

first law of thermodynamics

 

NB

 

ΔQ is positive when heat is absorbed by the system

 

ΔQ is negative when heat flows away from the system

 

ΔW is negative when work is taken out of the system

 

ΔW is positive when work is put into the system

 

 

 

This can be better understood if we consider a mass of gas in a piston arrangement (frictionless piston, an ideal gas in the cylinder).

 

 

frictionless piston with an ideal gas

 

 

Work is done when the volume of the gas changes.


By considering dimensions it can be shown that :

 

work (W) = constant pressure (p) x change in volume (ΔV)

 

dimensional analysis

 

work in terms of pressure and volume

 

 

When heat energy ΔQ is supplied to the gas :

 

 

the temperature of the gas increases

 

work ΔW is done by the gas expanding to move the piston

 

the internal energy of the gas increases ΔU

 

the volume of the gas increases ΔV

 

 

 

heated gas expanding in a piston

 

 

first law of thermodynamics

 

NB

ΔW is negative - work is removed from the system

 

ΔQ is positive - heat is supplied to the system

 

 

 

When no heat is applied and work is done externally by pushing the piston inwards to compress the gas:

 

the temperature of the gas increases

 

work ΔW is done compressing the gas

 

the internal energy of the gas increases ΔU

 

the volume of the gas decreases ΔV

 

 

 

compression of a gas in a piston

 

 

first law of thermodynamics - gas compression

 

 

NB

ΔW is positive - work is done on the system


ΔQ is negative - heat is removed from the system

 

 

 

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The Second Law of Thermodynamics

 

There are a number of statements outlining this law.

 

 

The Clausius Statement

 

No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature.

 

 

The Kelvin Statement

 

No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work.

 

 

A general statement

 

It is not possible to convert heat continuously into work without at the same time transferring some heat from a warmer body to a colder one.

 

 

 

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Heat Engines & Heat Pumps

 

Heat engines are devices that use the flow of energy from a hot source to a cold sink to produce work.

 

 

heat engine process

 

 

The Thermal Efficiency η (eta) is given by:

 

thermal efficiency

 

This also applies to heat pumps.

 

In reality the efficiency of an actual engine (eg a petrol engine) is much less than theory predicts.

 

There are a two main reasons:

 

1) frictional effects

2) heat is absorbed and emitted over a range of temperatures

 

 

Heat pumps are devices that use work to alter the energy flow from cold to hot (eg a refridgerator).

 

 

heat pump

 

 

 

 

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