Heat transfer

Laws of thermodynamics

introduction to biophysics - precourse

Convection Conduction

Radiation

Physical processes of heat transport

Radiation

Heat transport by conduction

Thermal energy, in the form of

continuous random motion of the

particles of the matter, is transferred by

physical contact between the particles.

Heat energy is transferred from one

material to another by direct contact.

Heat transport– rate of conduction

chc TTL

AK

t

Q

Thermal conductivity (Kc)

J/(msCo) kcal/(mhrCo)

Aluminum 240 206

Air 0.024 0.021

Wood 0.12 0.103

Water 0.57 0.504

Human tissue 0.20 0.173

L

A hot cold

Q – heat transferred in unit time, T – temperature,

A – surface area, L - distance between the hot and

the cold surface, Kc – coefficient of conductivity

Heat transport by convection

Convection refers to the movement of molecules

within fluids (i.e. liquids, gases)

Moving hot molecules carry heat energy with them

Rate of convection convection

TAKt

Qc'

'

K’ –coefficient of convection

Convection cycles

See breeze

Land breeze

A result of different thermal properties of water and rocks –

different specific heat and thermal conductivity

Heat transport by radiation

Thermal radiation is electromagnetic

radiation emitted from the surface of each

object which is due to the object's

temperature

The wavelength corresponding to the peak emission in

various black body spectra as a function of temperature

44

os TTAet

Q

Heat emission

Heat absorption

Ts – temperature of the surroundings

To – temperature of the body;

e – emissivity; 0<e<1

- Stefan-Boltzmann constant 5.67x10-8W/(m2K4)

Effective rate of radiation

Thermodynamics

A study of the conversion of heat

energy into different forms of energy

First law of thermodynamics

important terms:

internal energy

work of a thermodynamic system

Heat

Internal energy

Internal energy is all the energy of a system that is associated with its microscopic components - atoms and molecules

Internal energy includes kinetic energy of translation, rotation and vibration of molecules, potential energy within each molecule and potential energy between molecules.

Any given system in a particular state will have a certain amount of internal energy.

Molecular interpretation of

temperature

T ~ ( ½)(mvrms2)

the absolute temperature of an ideal gas is

a direct measure of

the average molecular kinetic energy

Internal energy of a thermodynamic system

is proportional to the absolute temperature

of the system; as far as internal energy

remains constant –the temperature

remains constatnt

Heat transferred to a system is equal to sum of

work done by the system and the change of its

internal energy.

First law of thermodynamics

Q = U + W

Q – heat absorbed or released by a thermodynamic system

(heat absorbed is positive, heat released is negative)

U – change of internal energy of the system

W – work done by the system or work done on the system

U = Q - W

First law of thermodynamics

A statement of the conservation of

energy as applied to

thermodynamic system

[The total amount of energy in the

universe always remains constant.

However, energy can be changed

from one form to another and be

used for doing work.]

Work of a thermodynamic system

is eqiuvalent to a mechanical work

A

P

V

dy

V+dVVf

ViPdVW

W = P V

In thermodynamics , positive work represents a transfer

of energy out of the system (energy done by the system).

U = Q - W

If we want to increase the internal energy of a

system heat have to be add or work have to be

done on the system.

If we want to decrease the internal energy of a

system heat has to be lost or work have to be

done by the system.

Some special cases of the first law of

thermodynamics.

Constant pressure process

Isothermal process (T = constant)

Constant-volume processes (isometric process).

Cyclical process.

Adiabatic processes.

U = Q - W

Direction of processes in nature

Real processes in nature are irreversible

(processes can occur spontaneously only in one direction)

A process is irreversible if the system cannot be returned to its initial states without any change in the surroundings.

Second law of thermodynamics

Heat will not flow spontaneously from a

colder body to a warmer body.

Heat engine

A heat engine is a device that converts

thermal energy to other useful forms of

energy, such as mechanical or

electrical energy.

TH

TL

W

QH

QL

1. heat is absorbed from a source

at a high temperature

2. work is done by the engine

3. heat is expelled by the engine

to a source at a lower

temperature.

A heat engine carries a substance

(called working substance) through a

cycle in which :

W = QH – QL

Examples of heat engines:

steam engines

internal combustion engines

turbines that generate electricity

Efficiency of a machine ( ) :

Thermal efficiency ( ) of a heat engine is a ratio of the net work done to the heat

absorbed during one cycle.

= (work done)/ (energy consumed)

= W/QH

The working substance is carried through a cyclic process, in which the initial

and final states of it are equal, so ΔU = 0.

From the first law of thermodynamics ΔU = ΔQ – W)

W = Q;

W = QH – QL

= (QH - QL)/QH

Exercise:

A small, gasoline-powered engine of a leaf blower

removes 800 J of heat energy from a high-temperature

reservoir and exhausts 700J to a low-temperature

reservoir (surroundings). What is the engine’s thermal

efficiency?

= (QH - QL)/QH

What is work done by the leaf blower during each working

cycle?

W = QH – QL

Gasoline engine 38 %

Cycling ~ 20 %

Swimming 1 – 4 %

Shoveling ~ 3 %

Efficiency of a machine ( ) :

= (work done)/ (energy consumed)

Second law of thermodynamics

It is impossible to construct a heat engine that,

operating in a cycle, produces no other effect than

the absorption of thermal energy from a reservoir

and the performance of an equal amount of work.

In a thermal cycle, heat energy cannot be

completely transformed into mechanical work.

It is impossible to construct an operational

perpetual motion machine.

The first law of thermodynamics is a

general statement of the conservation of

energy and makes no distinction between

different forms of energy.

The second law shows that thermal

energy is different from all other forms

of energy.

In real processes where heat transfer occurs, energy available for doing work decreases. It is because it is converted into heat.

The conversion of other forms of energy into thermal is called the degradation of energy.

There is always some loss of energy when the process is not reversible: this is entropy.

Entropy

If real processes occur spontaneously, the degree of disorder or chaos in the system increases. An example: ordered atoms in crystals of salt and

disordered molecules in salt solution.

To measure the degree of the disorder of the system a quantity called entropy is introduced. An increase in disorder is equivalent to an increase in entropy. An example: ordered atoms in crystals of salt have

lower entropy than atoms in molten salt.

Entropy

Entropy is a function that helps to account for the flow of heat thermal processes - was originally defined for a thermodynamically reversible process.

For a reversible process at a constant temperature

ΔS = ΔQ/T

In irreversible processes

ΔS >0

Entropy of an isolated system never decreases.

Entropy

Exercise:

What is the change in entropy of ice when

1.00 kg melts to form water at 0oC?

ΔS > ΔQ/T

ΔQ = Lm =333kJ/kg x 1kg = 333 kJ

T = 0oC = 273K

ΔS > 1.220 kJ/K = 1220 J/K

The Second Law of Thermodynamics:

The total entropy of the universe

increases in every natural process

It is impossible to reach a

temperature of absolute zero

The third law of thermodynamics