Thermodynamics notes, Lecture notes of Physics

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Thermodynamics

Definition of internal energy:

"the internal energy is determined by the state of the system and can be expressed as the sum of a

random distribution of kinetic and potential energies associated with the molecules of the system."

• Internal energy helps to distinguish heat and temperature
• Temperature is measure of average K.E of the molecules in an ideal gas. It does not depend on

the number of molecules.

• For an ideal gas, the internal energy is the total K.E of the gas. Clearly, it depends on the number

of molecules.

• Heat is the transfer of energy from one substance to another as result of temperature difference.

The direction of energy flow is determined by difference of temperature, not difference of internal

energies.

75

g

109

Heat

4502

150C

For real gases:

• The case is different in real gases because particles actually do exert forces on each other and

there will be certain P.E associated with the positions occupied by particles

• But positions keep changing so P.E is also distributed randomly
• At a given temperature, total P.E of the gas will remain constant, as temperature changes, P.E

also changes

• Looking at single molecule gives us very little information because its K.E and P.E are changing

all the time

• We say the total K.E and P.E are randomly distributed among the molecules. The graph shows

the distribution of K.E among the molecules. P.E is distributed similarly.

• Since the total K.E and P.E stay the

same at a particular temperature,

randomness is removed when we add

them all together.

For liquids and solids:

• The idea of K.E and P.E can be extended to all states of matter.
• As molecules come closer, forces become stronger, P.E becomes more significant
• In liquids, molecules can slide past each other and their motion is random, so this provides K.E
• The forces are stronger than gases so P.E is more significant
• In solids, motion of particles is their vibrations about fixed positions, so this provides K.E
• The forces are even stronger and rigid so P.E is even more significant

equation

->

(u)

=

(9,)

(w)

Sign

convention:

DU ->

increase

in

internal

energy

(Temp

for

ideal

gas

->

BU

->

decrease

in

internal

energy

(Tempt

for

ideal

gas

->

heat

added to

the

system

->

heatremoved from

the system

w-

work done

of

the

system

• w-

work done By the

system

A cylinder fitted with a piston

System is insulated so no heat can escape or enter the system (adiabatic)

So the equation becomes:

Adiabatic process can also be achieved even if insulation is not perfect

Just move the piston so fast that there is no time for heat to flow in or out

during the process

E.g.: cycle pump, diesel engine

⑭simple

processes

1

M

2/

(((((((((((((((()

Gas

At

=

(

w

An electrical kettle containing water:

Heating element provides heat to the system (+q)

No mechanical work is done (no changes in volume) so (w=0)

(isometric)

So equation becomes:

2

possesses

Bu

=

q,

y-