IB Physics Entropy and The Second Law of Thermodynamics

bankercordMechanics

Oct 27, 2013 (3 years and 7 months ago)

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IB











Physics

Entropy and The Second Law of Thermodynamics


Second Law of Thermodynamics

A
simple

statement of the Second Law is that
...



...
you cannot have a process whose only effect is to turn heat energy into work
.

Q
1
.

If you could
, what
would this imply?!



E.g.

I
n a power station the steam that has passed through the turbines must then be
cooled down to turn it back into water. Thus it is brought back to its original state so that
the cycle can be completed and started again. The energ
y lost in condensing the steam
is not useful

(it is not transferred to the turbine)
. Clearly a proportion of the original heat
energy is used to do external work but some will always be wasted. This
illustrates how

all heat engines have a maximum efficiency that must be below 100%.

A typical design
for a power station turbine has a maximum possible efficiency of around 60 to 70%

and a
true efficiency of much less
.

The Second Law of Thermodynamics

also suggests that

a true isothermal pro
cess cannot
really happen...

Q
2
.

Explain the above statement.





Energy Degradation

We know that energy can never be created or destroyed. However, energy transfers
occur all the time from one form to another.

Useful forms of energy

that can be
used to do work

are called
high grade

forms of energy.
E.g.
Chemical and electrical energy.

Systems containing high grade forms of energy are
considered to be more ‘ordered’.

Less useful forms of energy cannot easily be transferred and are ca
lled
low grade

forms
of energy. E.g. Internal energy is a low grade form of energy. A large iceberg
has a

huge
amount of internal energy but how could we ever use it to do work? Thus the internal
energy is low grade energy.

Systems containing low grade energy are considered to be
less ‘ordered’.

Ultimately all energy changes lead to the degradation of energy from a high grade form to
low grade, internal energy of the surroundings.

i.e. The systems are becoming less
ordered.

E.g.

All the energy transferred by our bodies from chemical energy in food ends up as
internal energy in the air and our surroundings
(radiation and conduction; doing work
against friction etc).




Entropy

A different way of stating the Second Law of Ther
modynamics is to say that...




...
energy will always spread out.

E.g.

A net flow of e
nergy will flow from

a hot to a cold body but it cannot do the opposite
as that would violate the second law.

The quantity ‘entropy’ is a way of measuring how the energy in a system
is ‘spreading out’
or how the system is becoming less ordered.




i.e.
the less order
,

the more entropy.

E.g. If chemical energy (highly ordered) in a fuel is released as heat by burning,
this increases the internal energy of a room. The entropy of the system has
increased

because internal energy is a disordered form of energy.

If energy will always spread o
ut this also means that the entropy of the universe (or any
theoretical closed system)
must be increasing.

Hence our final definition of the
Second Law of Thermodynamics
...




Note: In some examples it may seem that the entropy is
decreasing
.

However

this will
only be in a local area and the overall amount of entropy will increase
.


Q3.


A petrol engine in a crane, lifting a heavy mass.

a.

Explain why the entropy
of the mass

decreases.

b.

Explain why the total entropy of the
mass, fuel and surroundings
increases.







Q
4
.

A refrigerator.

a.

Explain why the entropy inside the refrigerator decreases.

b.

Explain why the total entropy of the refrigerator and room increases.





Entropy is a way of measuring the degree of disorder in a sy
s
tem.

During any thermodynamic process the total entropy of the sy
s
tem must increase.