Lecture 2 The First Law of Thermodynamics (Ch.1)

Lecture 1 - we introduced macroscopic parameters that describe the state of a thermodynamic system (including temperature), the equation of state f (P,V,T) = 0, and linked the internal energy of the ideal gas to its temperature.

Outline:

1. Internal Energy, Work, Heating

2. Energy Conservation – the First Law

3. Enthalpy

4. Heat Capacity

Internal Energy

system U = kinetic + potential

system boundary

“environment”

The internal energy of a system of particles, U, is the sum of the kinetic energy in the reference frame in which the center of mass is at rest and the potential energy arising from the forces of the particles on each other.

Difference between the total energy and the internal energy?

The internal energy is a state function – it depends only on the values of macroparameters (the state of a system), not on the method of preparation of this state (the “path” in the macroparameter space is irrelevant).

U = U (V, T)In equilibrium [ f (P,V,T)=0 ] :

U depends on the kinetic energy of particles in a system and an average inter-particle distance (~ V-1/3) – interactions.

P

V T A

B

For an ideal gas (no interactions) : U = U (T) - “pure” kinetic

Internal Energy of an Ideal Gas

The internal energy of an ideal gas with f degrees of freedom: TNk

fU B2

f 3 (monatomic), 5 (diatomic), 6 (polyatomic)

How does the internal energy of air in this (not-air-tight) room change with T if the external P = const?

PVf

Tk

PVNTkN

fU

BroominBroomin 22

(here we consider only trans.+rotat. degrees of freedom, and neglect the vibrational ones that can be excited at very high temperatures)

- does not change at all, an increase of the kinetic energy of individual molecules with T is compensated by a decrease of their number.

Work and Heating (“Heat”)

We are often interested in U , not U. U is due to:

Q - energy flow between a system and its environment due to T across a boundary and a finite thermal conductivity of the boundary

– heating (Q > 0) /cooling (Q < 0)(there is no such physical quantity as “heat”; to emphasize this fact, it is better to use the term “heating” rather than “heat”)

W - any other kind of energy transfer across boundary - work

Heating/cooling processes:conduction: the energy transfer by molecular contact – fast-moving molecules transfer energy to slow-moving molecules by collisions;

convection: by macroscopic motion of gas or liquid

radiation: by emission/absorption of electromagnetic radiation.

HEATING

WORK

Work and Heating are both defined to describe energy transfer across a system boundary.

The First Law

For a cyclic process (Ui = Uf) Q = - W.

If, in addition, Q = 0 then W = 0

Perpetual motion machines come in two types: type 1 violates the 1st Law (energy would be created from nothing), type 2 violates the 2nd Law (the energy is extracted from a reservoir in a way that

causes the net entropy of the machine+reservoir to decrease).

The first law of thermodynamics: the internal energy of a system can be

changed by doing work on it or by heating/cooling it.

U = Q + W

From the microscopic point of view, this statement is equivalent to

a statement of conservation of energy.

P

V TAn equivalent formulation:

Perpetual motion machines of the first type do not exist.

Sign convention: we consider Q and W to be positive if energy flows into the system.

Quasi-Static Processes

Quasi-static (quasi-equilibrium) processes – sufficiently slow processes, any intermediate state can be considered as an equilibrium state (the macroparamers are well-defined for all intermediate states).

Examples of quasi-equilibrium processes:

isochoric: V = const

isobaric: P = const

isothermal: T = const

adiabatic: Q = 0

For quasi-equilibrium processes, P, V, T are well-defined – the “path” between two states is a continuous lines in the P, V, T space.

P

V T

1

2

Advantage: the state of a system that participates in a quasi-equilibrium process can be described with the same (small) number of macro parameters as for a system in equilibrium (e.g., for an ideal gas in quasi-equilibrium processes, this could be T and P). By contrast, for non-equilibrium processes (e.g. turbulent flow of gas), we need a huge number of macro parameters.

Work

The sign: if the volume is decreased, W is positive (by compressing gas, we increase its internal energy); if the volume is increased, W is negative (the gas decreases its internal energy by doing some work on the environment).

2

1

),(21

V

VdVVTPW

The work done by an external force on a gas enclosed within a cylinder fitted with a piston:

W = (PA) dx = P (Adx) = - PdV

x

P

W = - PdV - applies to any shape of system boundary

The work is not necessarily associated with the volume changes – e.g., in the Joule’s experiments on determining the “mechanical equivalent of heat”, the system (water) was heated by stirring.

dU = Q – PdV

A – the piston area

force

W and Q are not State Functions

P

V

P2

P1

V1 V2

A B

CD

- the work is negative for the “clockwise” cycle; if the cyclic process were carried out in the reverse order (counterclockwise), the net work done on the gas would be positive.

01212

211122

VVPP

VVPVVPWWW CDABnet

2

1

),(21

V

VdVVTPW

- we can bring the system from state 1 to state 2 along infinite # of paths, and for each path P(T,V) will be different.

U is a state function, W - is not thus, Q is not a state function either.

U = Q + W

Since the work done on a system depends not only on the initial and final states, but also on the intermediate states, it is not a state function.

PV diagram

P

V T

1

2

Comment on State Functions

U, P, T, and V are the state functions, Q and W are not. Specifying an initial and final states of a system does not fix the values of Q and W, we need to know the whole process (the intermediate states). Analogy: in classical mechanics, if a force is not conservative (e.g., friction), the initial and final positions do not determine the work, the entire path must be specified.

x

y z(x1,y1)

z(x2,y2)

dyy

zdx

x

zzd

xy

dyyxAdxyxAad yx ,, - it is an exact differential if it is

x

yxA

y

yxA yx

,,

yxzdyydxxzad ,, the difference between the values of some (state) function z(x,y) at these points:

A necessary and sufficient condition for this:

If this conditionholds:

y

yxzyxA

x

yxzyxA yx

,

,,

,

e.g., for an ideal gas:

dV

V

TdT

fNkPdVdUQ B 2

- cross derivativesare not equal

dVPSdTUd

U

V S- an exact differential

In math terms, Q and W are not exact differentials of some functions of macroparameters. To emphasize that W and Q are NOT the state functions, we will use sometimes the curled symbols (instead of d) for their increments (Q and W).

Problem

Imagine that an ideal monatomic gas is taken from its initial state A to state B by an isothermal process, from B to C by an isobaric process, and from C back to its initial state A by an isochoric process. Fill in the signs of Q, W, and U for each step.

V, m3

P, 105 Pa

A

BC

Step Q W U

A B

B C

C A

2

1

1 2

+ -- 0

-- + --

+ 0 +

T=const

Another Problem

During the ascent of a meteorological helium-gas filled balloon, its volume increases from Vi = 1 m3 to Vf = 1.8 m3, and the pressure inside the balloon decreases from 1 bar (=105 N/m2) to 0.5 bar. Assume that the pressure changes linearly with volume between Vi and Vf.(a) If the initial T is 300K, what is the final T?(b) How much work is done by the gas in the balloon?(c) How much “heat” does the gas absorb, if any?

P

V

Pf

Pi

Vi Vf

K2701mbar1

1.8mbar5.0K300

3

3

ii

ffif

BB VP

VPTT

Nk

PVTTNkPV

(a)

(b) f

i

V

V

ON dVVPW )(

bar625.1bar/m625.0 3 VVP

(c) ONWQU

Jmbarmbarmbar 333 41066.04.05.08.05.0)( f

i

V

V

BY dVVPW

- work done on a system f

i

V

V

BY dVVPW )( - work done by a system

BYON WW

JJJ 445 105.41061.0105.112

3

2

3

BY

i

fiiONifBON WT

TVPWTTNkWUQ

Quasistatic Processes in an Ideal Gas

isochoric ( V = const )

isobaric ( P = const )

021 W

TCTTNkQ VB 02

31221

0),( 12

2

1

21 VVPdVTVPW

TCTTNkQ PB 02

51221

21QdU

2121 QWdU

V

P

V1,2

PV= NkBT1

PV= NkBT21

2

V

P

V1

PV= NkBT1

PV= NkBT212

V2

(see the last slide)

Isothermal Process in an Ideal Gas

1

221 ln),(

2

1

2

1V

VTNk

V

dVTNkdVTVPW B

V

V

B

V

V

f

iBfi V

VTNkW ln

Wi-f > 0 if Vi >Vf (compression)

Wi-f < 0 if Vi <Vf (expansion)

isothermal ( T = const ) :

V

P

PV= NkBT

V1V2

W

2121 WQ

0dU

Adiabatic Process in an Ideal Gas

adiabatic (thermally isolated system)

PdVdTNkf

dUTNkf

U BB 22

( f – the # of “unfrozen” degrees of freedom )

dTNkVdPPdVTNkPV BB PVPdVf

VdPPdV 2

fP

dP

fV

dV 21,0

21

constVPPVP

P

V

V

111

1

lnln

The amount of work needed to change the state of a thermally isolated system depends only on the initial and final states and not on the intermediate states.

021 Q 21WdU

to calculate W1-2 , we need to know P (V,T)

for an adiabatic process

2

1

),(21

V

V

dVTVPW

011

P

P

V

V P

dP

V

dV

V

P

V1

PV= NkBT1

PV= NkBT21

2

V2

Adiabatic Process in an Ideal Gas (cont.)

V

P

V1

PV= NkBT1

PV= NkBT21

2

11

12

11

1121

11

1

1

),(2

1

2

1

VVVP

dVV

VPdVTVPW

V

V

V

V

1+2/31.67 (monatomic), 1+2/5 =1.4 (diatomic), 1+2/6 1.33 (polyatomic)(again, neglecting the vibrational degrees of freedom)

constVPPV 11

An adiabata is “steeper” than an isotherma: in an adiabatic process, the work flowing out of the gas comes at the expense of its thermal energy its temperature will decrease.

V2

Problem

Imagine that we rapidly compress a sample of air whose initial pressure is 105 Pa and temperature is 220C (= 295 K) to a volume that is a quarter of its original volume (e.g., pumping bike’s tire). What is its final temperature?

2211

222

111

VPVP

TNkVP

TNkVP

B

B

1

2

1

2

12

1

1121

2

11

2

112 T

T

V

VT

T

VPTNk

V

VP

V

VPP B

constVTVT 122

111

KKKV

VTT 51474.12954295 4.0

1

2

112

For adiabatic processes:

Rapid compression – approx. adiabatic, no time for the energy exchange with the environment due to thermal conductivity

constTP /1also

- poor approx. for a bike pump, works better for diesel engines

Non-equilibrium Adiabatic Processes

- applies only to quasi-equilibrium processes !!!

constTV 1

2. On the other hand, U = Q + W = 0

U ~ T T – unchanged

(agrees with experimental finding)

Contradiction – because approach

#1 cannot be justified – violent

expansion of gas is not a quasi-

static process. T must remain the

same.

(Joule’s Free-Expansion Experiment)

constTV 11. V – increases

T – decreases (cooling)

The Enthalpy

Isobaric processes (P = const):

dU = Q - PV = Q -(PV) Q = U + (PV)

The enthalpy is a state function, because U, P, and V are state functions. In isobaric processes, the energy received by a system by heating equals to the change in enthalpy.

Q = H

isochoric:

isobaric:

in both cases, Q does not depend on the path from 1 to 2.

Consequence: the energy released (absorbed) in chemical reactions at constant volume (pressure) depends only on the initial and final states of a system.

H U + PV - the enthalpy

The enthalpy of an ideal gas:(depends on T only)

TNkf

TNkTNkf

PVUH BBB

1

22

Q = U

Heat Capacity

T

QC

The heat capacity of a system - the amount of energy

transfer due to heating required to produce a unit temperature rise in that system

C is NOT a state function (since Q is not a state function) – it depends on the path between two states of a system

T

V

T1

T1+dT

i

f1 f2 f3

The specific heat capacitym

Cc

( isothermic – C = , adiabatic – C = 0 )

CV and CP

dT

PdVdU

dT

QC

V = const

P = const

VV T

UC

the heat capacity at constant volume

the heat capacity at constant pressure

PP T

HC

To find CP and CV, we need f (P,V,T) = 0 and U = U (V,T)

dT

dVP

V

U

T

U

dVV

UdT

T

UdU

dT

PdVdUC

TV

TV

PTVP dT

dVP

V

UCC

CV and CP for an Ideal Gas

nRf

Nkf

C BV 22

CV = dU/dT

3/2NkB

5/2NkB

7/2NkB

10 100 1000 T, K

Translation

Rotation

Vibration

CV of one mole of H2

R

NkP

NkP

dT

dVP

V

UCC B

B

PTVP

( for one mole )0 nR

f

Nkf

NkCC BBVP

12

12

For an ideal gas

TNkf

U B2

# of moles

For one mole of a monatomic ideal gas: RCRC PV 2

5

2

3